Chemical Determination of Tryptophan Study of Color-Forming Reactions of Tryptophan, p-Dimethylaminobenzaldehyde, and Sodium Nitrite in Sulfuric Acid Solution JOSEPH R. SPIES AND DORRIS C. CHAMBERS Allergen Research Division, Bureau of Agricultural and Zndustrial Chemistry, Agricultural Research Administration, U.S. Department of Agriculture, Washington, D. C.
The paper describes the optimum conditions for carrying out the color-forming reaction between tryptophan, p-dimethylaminobenzaldehyde, and sodium nitrite in both 13.2 N and 19 A‘ sulfuric acid, and gives several procedures for the quantitative determination of tryptophan.
P
ROCEDURES based on the colors formed by oxidation of the condensation products of tryptophan with various aldehydes have long been used for the qualitative and quantitative determination of tryptophan. These methods have included the use of glyosalic acid (1,8), formaldehyde (16, 21), benzaldehyde (4,5, 13, 141, vanillin (10, 13, 14), salicylaldehyde (5, 13, 14, Zl),and p-dimethylaminobenzaldehyde (7, 15). The early history of the development of methods for the quantitative estimation of tryptophan in proteins is described by llitchell and Hamilton (12). Among the many procedures proposed, the p-dimethylaminobenzaldehyde colorimetric method is still used. Rohde (1.5) in 1905 first observed that tryptophan gave a blue color n ith p-dimethylaminobenzaldehyde and in 1913 Herzfeld ( 7 ) first used this reaction for the quantitative estimation of tryptophan in proteins. Further discussion of the determination of tryptophan in proteins by this and other niethods is deferred to a later paper of this series. In procedures (5,9, 11, 18, 19) recently described for the determination of tryptophan in proteins, strong hydrochloric acid n as used as solvent for p-dimethylaniinobenaaldehyde and also for the test solutions. Because of corrosive and irritating fumes, the use of hydrochloric acid is objectionable. Therefore, experiments were designed t o substitute sulfuric acid for hvdrochloric acid as did Tillmans and .41t (20) who, hon-ever, used formaldehyde. p-Dimethylaminobenzaldehyde was considered preferable to formaldehyde because it is easily purified and is stable in the solid state. Early in this study it became apparent that a critical investigation of the reactions involved m s needed. The original objective of this work was to determine optimum conditions for obtaining maximum color n hen the condensation product of tryptophan and p-dimethylaminobenzaldehyde, in sulfuric acid solution, was oxidized with sodium nitrite. I t was then planned to adapt these conditions to the determination of tryptophan, starting with intact proteins as has been done bv others. I t was found that maximum color was attained in 12 to 13.2 S acid (throughout this paper the term “acid” refers to sulfuric acid; reference to other acids is appropriately indicated). This concentration was not strong enough to determine tryptophan starting with intact proteins, but 19 A- acid was found suitable for this purpose. This paper, therefore, describes the optimum conditions for carrying out the color-forming reaction between tryptophan, p-dimethylaminobenddehyde, and sodium nitrite in both 13.2 iV and 19 AT acid. Several procedures for the quantitative determination of tryptophan, based on these studies, are also described. The reactions were studied in two steps. The combination of tryptophan and p-dimethylaminobenzaldehyde t o form a colorless condensation product was called reaction I, and develop-
ment of the blue color by oxidation of this compound lyith sodium nitrite was termed reaction 11. CHOICE OF WAVE LENGTH
TTave lengths from 590 t o 600 mp gave minimum transmittancy iTith the tryptophan-p-dimethylaminobenzaldehydecolor de0.60
i
I
0.55
0.50
0.45
0.40 h
Y ._
2 0.35
c
‘2 0.30 3
I
\ 0.25
r TQYPTOPHAN
0.20
t
0.15
---
24 HRS
- - -- --- 4 8 H R S , -I HR
i
- 048 - C O d R 7
7
--“A
> BECYY4\1, ,048 1b1 SLANK * I
COLEVAh,
IC I
0.10
I
0.05
300
350
400
450 500 550 Wave Length, mp
600
650
700
Figure 1. Absorption Curves of Tryptophan-p-Dimethylaminobenzaldehyde Color Obtained from Free Tryptophan A , B , C. Color developed by Procedure B, using 100 micrograms of tryptophan per test. Reaction I proceeded 1, 24, and 48 hours, respectively, before color developed. Transmittances determined with Beckman quartz spectrophotometer, using 10-mm. cuvette. D . Color developed by Procedure B using 70 micrograms of tryptophan per teet. Reaction I proceeded 1 hour before color developed. TransmittancieB determined with Coleman Model 11 spectrophotometer Blank solutions contained 30 m g . of p-dhethylaminobenzaldehyde per test, 0.1 m l . of 0.04%sodium nitrite added
V O L U M E 20, N O . 1, J A N U A R Y 1 9 4 8 0.75,
31
I
I
Table I. Effect of Acid Concentration on Rate of Reaction I and on Amount of Color Formed at 25" C.
I 0.70 -
Normal-
ity of Acid 10.1 12 13.2 14.1 15.1 16.2 17.2 19.1
065-
I
0601
, 0.55
Time i n 3 l i n u t e s 15 30 60 120 240 c'c hlaximum Color Formed . . . . . . . . . . 38 58 80 89 4.1 6 1 21 48 69 86 96 100 3.1 10 35 63 83 42 96 100 3.1 12 42 75 89 96 96 96 4.1 21 .56 82 89 92 92 96 3.1 33 67 82 89 89 89 89 3.1 42 75 82 86 86 86 86 . . . . . . . . . . 82 82 82 82
0
1
5
420
1440
92 100
83 100 100 96 92 89 86 82
100
...
,..
... ... ...
I 0.50
-
against a hlank solution containing 30 mg. of p-dimethylaminohr~nzaltkhyde,arid curve F was obtained il-ith a blank solution tliffcring only in that it contained 100 micrograms of tryptophan and no p-dimethylaminobenzaldehyde. Points on the curve r ~ a dfiw xere checked after the second curve was obtained to show that, no change occurred during the time required for the I.e:itiiiiy~.
21
c 2
-
0 45-
B S
8
040-
The coniposition of the blank solution exerted a profound ffwt on the density values a t wave lengths ranging from 320 t o 500 nifi and from 610 t o 700 my. However, the two curves coinvided at the critical wave length of 590 mp. The coincidence of thv t n o curves at this point is important because the tryptophan solutions t o be analyzed sometimes contain some foreign color \\ hich can be eliminated by adding the test solution t o the blank. Thio condition is usually encountered in analyzing proteins. I t I- concluded that a Tvave length of 580 t o 600 nip may be used for inmsurenient of the tryptophan-p-dimethylaminobenzaldehl-de color developed after reaction I had proceeded 1, 24, or 48 hours i n 19 S acid with the blank solution containing either p-dimethylaniinobenzaldehyde or tryptophan.
035-
t
030-
026-
CO'CS
020-
IN G L C N K
0 15
0 10
-
300
--
350
400
-
___-450 500 Wave Length,
EFFECT OF SULFURIC ACID CONCENTRATION ON REACTION I
550
go0
650
700
IIIM
Figure 2 , Absorptioii of Tryptophan-p-Dimethylaniinohriizaldeh>de Color from Free Tryptophan Transmittanciea determinrd w i t h B e c k m a n spectrophotometer, using 10-mm. cuyettes
veloped under optimum conditions in 13.2 N acid according t o Procedure A. (Procrdures A to H are described below.) Wave-lengt h-density curves of the p-dimethylaminobenzaldehydc-tryptophan color developed under optimum conditions in 19 .\- acid are shon-n in Figures 1 and 2. Curves .4 and D n-ere obtained with the Beckman and Coleman spectrophotometers, respectively. The region of maximum density ranged from 580 to 610 mp with the Coleman and from 590 to 600 nip with the Beckman instrument. I n general, the shapcs of the two curves correspond RS well as would be expected, considering the differences in band widths. Curves A, B, and C, obtained with the Beckman spectrophotometer, compare the wave-length-density relationships of the color developed after the tryptophan-p-dimethylaminobenzaldehyde complex had stood a t 25' C. in the dark for 1, 24, and 48 hours, respectively, before addition of sodium nitrite. Curve C is shon-n from 320 to 380 mp only, because over the rest of the spectruiii it practically coincided with curve B. The shape of these curves corresponded closely from 380 to 690 ]nu. llaximuni density for the color developed after reaction I had proceeded for 1 hour was 590 to 610 mp and 580 to 600 my for the color when developed after reaction I had gone for 24 to 48 hours. The unexplained departure in the shape of curve C from that of curves A and B in the spectral region from 320 t o 380 nip is of interest, but it has n o bearing on the analytical method. The effecr oi the coniposition of the blank solution on the viavelength-dciicity curvcs is shown in Figure 2. Curves E and F Tvere obtained on the same colored solution with the Beckman spectrophotometer. Curve E was read
The effect of the concentration of acid on the degree of color obtained with a fixed quantity of tryptophan and p-dimethylaminobenzaldehyde was determined in a preliminary study of the kinctics of reaction I. The kinetic study was based on the observation that rcaction I does not take place if sodium nitrite, in the concentrations used, is added t o the acid solution of pdiniethj-laniinobenzaldehyde just before addition of the tryptophan. If the nitrite solution i8 added a t any interval before reaction I is completed, no further condensation will occur, but color will be formed by oxidation of that portion of tryptophan and p-dimethylaminobcnzaldehyde already reacted. The relative rates of reaction I and the relative degrees of maximum color obtained in various concentrations of sulfuric acid are shoivn in Table I . The acid concentrations used n-ere from 10 t o 19 and time intervals were from 1 t o I440 minutes. Each test cont,ained 100 micrograms of tryptophan and 30 mg. of p-dimethylaminobenzaldehyde in 10 ml. of sulfuric acid. The only variable in this series of tests was the concentration of acid used. The following example shows how tests were carried out in 13.2 A; acid. Sample: 23.8 .V acid, 5.5 ml.; p-dimethylaminobenzaldehyde, 30 mg. in 1.0 ml. of 2 N acid; water, 2.5 ml.; tryptophan, 100 micrograms in 1.0 nil. of distilled water; sodium nitrite, 0.1 ml. of 0.0670 solution in water. The acid solution, p-dimethylaniinobenzaldehyde, and water were mixed and cooled to 25' C. I n the zero time test the sodium nitrite \vas atldtd to the mixture followed by the tryptophan within 10 seconds. The transmittancy of this solution was read after it had stood for 30 minutes in the dark following addition of the sodium nitrite. I n other tests the tryptophan was added and the solutions were kept a t 25" in artificial light for the indicated time interral. The sodium nitrite was then added and transmittancies were read after standing 30 minutes in the dark. Blank: 23.8 N acid, 5.5 ml.; p-dimethylaminobenzaldehyde, 30 mg. in 1.0 ml. of 2 N acid, water 3.5 nil.; sodium nitrite, 0.1 ml. of 0.06% solution, All tests were conducted similarly except that the proportions of 23.8 S acid and xvater wcre varied t o give
ANALYTICAL CHEMISTRY
32 the desired acid concentrations. All blank solutions had the same acid concentration as the corresponding test solutions. For comparative purposes all transmittancies were converted to micrograms of tryptophan from curve A, Figure 3, and then expressed in Table I as per cent of the maximum color obtained in the series of tests. The lowest per cent transmittancy obtained in this series of tests was lo%, which was considered to represent 100% of the color. As an example with 19.1 .JJ acid the lowest transmittancy obtained was 15% which represented 82% of the maximum color obtained with 13.2 Nacid. The rate of reaction I was more rapid in higher than in lower concentrations of acid, but more intense maximum color nas obtained in the lower concentrations of acid than in the higher concentrations. Thus in 16, 17, and 19 AJ acid the reaction appeared t o be complete in 30 minutes (results presented below show that there is slightly further reaction after 30 minutes in 19 -V acid), whereas in 12 and 13.2 AT acid the reaction required 4 hours for completion. However, maximum color was obtained in 12 and 13.2 N acid and the relative degrees of maximum color in 14.1, 15.1, 16.2, 17.2, and 19.1 S acid represented 96, 96, 89, 86, and 82%, respectively. I n 10 S acid 7 hours xere required to attain maximum color which, however, xyas only 92y0 of that obtained in 12 and 13.2 AJ acid. The condensation product was stable during 24 hours a t 25" C. except in 10 ,V acid. Nineteen normal acid is near the upper limit of concentration that can be used safely; some charring occurred in a test made with 22 A: acid. Tillmans and Alt (do) used 66 to 67 weight yo (21.3 N ) sulfuric acid in their method with formaldehyde. OJTIMUM CONCENTRATIOS OF p-DIMETHY LAMINOBENZALDEHYDE
The optimum quantity of p-dimethylaminobenzaldehyde required for reaction I in 13.2 .V and 19 S acid was determined. 1.70
3 1.60
c
Q
*
F 5 1.50
M
2 1.30
I 1.20
I
5
Figure 3.
I
I
20 Time, minutes
25
I
10
15
I.
30
Pseudo First-Order Nature of Reaction I
Table 11. Effect of Concentration of p-Dimethylaminobenzaldehyde on Amount of Color Formed in 13.2 and 19 N Acid Quantity of DAB per Test, hlg. 1.0
2.5 5.0
10 20
30 60
Micrograms of Tryptophan 50 100 Kormality Normality Kormality 13 2 19.0 13.2 19.0 13.2 19.0 % l l a x i m u m Color Formed 32 88 32 82 36 84 56 96 56 96 61 96 so 96 78 100 82 100 92 100 94 100 96 100 100 100 100 100 100 100 100 100 100 100 100 100 92 96 92 96 96 96 25
.
Quantities ranging from 1 to 60 mg. per test n-ere used with each of 25,50, and 100 micrograms of tryptophan. Tests in 13.2 N acid were carried out by Procedure A a t room temperature. Composition of blank solutions Fas the same aa that of test solutions, except that 1 ml. of water was substituted for the 1 ml. of tryptophan solution. Color was developed with 0.1 ml. of 0.06% sodium nitrite solution. Transniittancies were read 30 minutes after addition of the sodium nitrite. Tests in 19 A: acid were carried out by Procedure B except that p-dimethylaminobenzaldehyde was added in 4 S acid solution in such a way that the p-dimethylaminobenzaldehyde solution plus the 4 N acid equaled 1 ml. Color was developed mith 0.1 ml. of 0.06% sodium nitrite solution and transmittancies Fere read 40 to 60 minutes after addition of the sodium nitrite. For comparative purposes all transmittancies were converted to micrograms of tryptophan from curve .4,Figure 3, for tests in 13.2 N acid and curve B, Figure 3, for tests in 19 >V acid and then expressed in Table I1 as per cent of the maximum color obtained in each series of tests. Results in Table I1 show that in 13.2 h' sulfuric acid 20 and 30 mg. of p-dimethylaminobenzaldehydegave maximum color with all concentrations of tryptophan, while in 19 N acid 10, 20, or 30 mg. per test gave maximum color. Sixty milligrams per test gave slightly less color than the maximum in both concentrations of acid. The reason for this effect is not knonn. Thirty milligrams per test were adopted for this work. OPTIMUM CONCENTRATION OF SODIUM NITRITE FOR REACTION I1
The kinetics and end point of reaction I1 were studied with both 13.2 S and 19 X acid. The data in Table I11 show the rate and end point of reaction 11, when 100 micrograms of tryptophan .and molar ratios of sodium nitrite to tryptophan ranging from 0.25 to 16.0 per test in 13.2 S acid were used. The rate of reaction I1 increased with increasing concentration of sodium nitrite. Maximum color was obtained in 60 And 180 minutes with molar ratios of sodium nitrite of 2 and 4, respectively. A curious plateau occurred lvhen molar ratios of sodium nitrite were 8 and 16. The reaction appeared to be complete in each case in 3 minutes with color representing 86 and 7970 of the maximum respectively. Then, horn-ever, a gradual increase in color occurred until after 180 minutes the color nas 96 and 927, of the maximum, respectively. I t is apparent that in 13.2 A- acid the greatest sensitivity was obtained in 60 to 180 minutes nhen the molar ratio of sodium nitrite was 2. The color of the test solution under these conditions is blue with a violet tinge. The data in Table IV show results of a similar kinetic study using 100 micrograms of tryptophan in 19 A* acid. Maximum color was attained in 30 minutes with a molar ratio of sodium nitrite to tryptophan of 1.0 to 1.5. The anomalous plateau indicating apparent rapid completion of reaction I1 in 3 minutes when the molar ratio of sodium nitrite was 4, 8, and 16 was observed. The reaction appeared t o be complete for 60 minutes but then a further intensification of color occurred between 60 and 180 minutes. On the basis of these results 0.1 ml. of 0.04% sodium nitrite reacting for 30 minutes in 19 N acid was adopted for tests on solutions containing only tryptophan. The pure blue color formed under these conditions is stable for at least 3 hours and s h o w only slight deterioration in 24 hours. TTisually,the quality of the color developed was influenced by the quantity of the oxidant used and the time of standing after addition of the oxidant. Oxidation with progressively greater quantities of sodium nitrite gave progiessively deeper violet tinges t o the color. The violet shade of the color tended to increase on standing, particularly in tests in which ewess oxidant was present. Violet tinges were more apparent when tests were observed in ordinary artificial light than when fluorescent light was used. Violet tinges were less apparent when color was developed in 19 -J- acid than when 13.2 S was used. The wave length of maximum color decreased slightly when an excess of sodium nitrite was used and also after test solutions stood for 5 days.
33
V O L U M E 2 0 , NO. 1, J A N U A R Y 1 9 4 8
was greatest a t 25'. If the degree of color obtained a t 25" represents 1 0 0 7 the relative Time, Minutes Sodium Nitrite sensitivities a t 25', 45", and 85' Added ~0 were 100, 85, and 6170, respecMolar ratio a %b % Maximum Color F o r m e d C tively. According to these data, 5 7 3 4 4 ... ... 3 3 3 3 4 3 0.0 56 61 61 61 56 22 40 50 59 4 33 0.0085 28 45 0.25 the tryptophan-p-dimethyl74 67 77 77 72 55 77 65 35 42 59 4 48 0.017 0.50 79 92 aminobenzaldehvde complex was 89 . . 86 73 77 86 83 48 57 65 4 0.027 0.80 83 89 92 96 96 89 53 63 77 83 69 86 4 0.034 1.0 stable a t 25" C. for at least 24 86 92 96 ... 92 96 72 83 86 77 89 3 61 0.051 1.5 96 89 100 100 96 96 67 77 89 89 83 92 3 0.068 2.0 hours and a t 45' for at least 5 96 89 96 100 92 92 77 83 89 89 86 92 3 0.14 4.0 hours; but a t 85' it was stable 96 86 92 96 86 86 79 86 86 86 86 3 83 0.27 8.0 92 83 89 79 92 79 79 79 79 77 79 79 0.54 3 16.0 for only 15 minutes. ~ ~ o lratio: a r sodium nitrite The effect of temperature on tryptophan ' the late and sensitivity of reacb % concentration of sodium nitrite solution added to each test in 0.1 ml. of water solution. c All tests were conducted similarly according to Procedure A a t 24O to 2 6 O C. except t h a t transmittancies mere tion I in 19 S acid was not deread before a n d a t indicated intervals after addition of sodium nitrite. F o r comparative purposes all transmittancies were converted to micrograms of tryptophan from curve A , Figure 3, a n d then expressed as per cent of termined, but probably both maximum color formed. T h e lowest transmittancy obtained in this series of tests was 9 which represented rates of condensation and de100% of t h e color. 100 y of tryptophan used in each test. struction of tryptophan would be greater in 19 S than in 13.2 N Table IV. Kinetics of Reaction I1 i n 19 N Acid with Varying Quantities of Sodium Nitrite acid at the higher temperatures. Time, Minutes Sodium Nitrite optimum temperature of Added 0 1 2 3 5 7 10 15 30 60 180 1440 7200 2 5 O C. ivas chosen for reaction I in Molar ratio5 %b % Maximum Color Formede both13.2AVand19Aracidbecause 0.0 .... 7 . . . . 53 . . .6.1. '68 . . . . . . 7 . . . 14 27 0.25 0.0085 7 41 71 75 i i ' i i ' 79 . . . 77 77 of the greater stability of the 93 90 96 96 06 7 68 77 82 87 90 90 93 0.50 0.017 tryptophan-p - dimethylamino100 100 96 90 90 93 96 96 100 0.034 7 75 84 87 1.0 96 93 96 100 96 96 96 100 100 6 84 90 93 0.051 1.5 benzaldehyde complex, greater 96 96 96 100 96 90 93 96 96 0.068 5 84 90 93 2.0 93 sensitivity of the reaction, and 93 93 93 96 96 90 0.14 5 87 90 93 93 93 4.0 93 93 93 93 96 96 90 93 93 8.0 0.27 5 90 93 93 greater convenience of operation 16.0 0.54 5 90 90 90 90 90 90 90 90 90 93 93 87 at this temperature. a hIolar ratio: sodium nitrite. tryptophan The importance of temperab % concentration of sodium nitrite solution added to each test in 0.1 ml. of water solution. ture controlis apparent from data All tests were conducted similarly according to Procedure B a t room temperature (23" t o 26' C.) except' t h a t transmittancies were read before and a t indicated intervals after addition of sodium nitrite. F o r comparative in Table V. Thus an apprecipurposes all transmittancies were converted t o micrograms of tryptophan from curve B, Figure 3 and then expressed as per cent of maximum color formed. Lowest % transmittancy obtained in this series of tests'was 15, which able error might be made if transwas considered to represent 100% of the color. 100 y of tryptophan used in each test. mittancies obtained in tests conducted a t summer temperatures (30" t o 35" C.) were converted From results shown in Tables I11 and IV, it might be considered t o weight of tryptophan from a standard curve prepared a t necessary t o use a constant molar ratio of sodium nitrite and winter temperatures of 20" to 25' C. Hon-ever, less extreme tryptophan. But, practically, this is not necessary, as is shown variations in temperature would not be important if maximum by a later experiment (Table VIII) in which standard curves accuracy is not necessary. were obtained with both the constant quantity of sodium nitrite The color of the tests at 85' C. was reddish violet in 15 mindetermined as optimum for 100 micrograms of tryptophan and a utes, changing to pinkish violet 1%-ithincreased time a t 85"; a t constant molar ratio of sodium nitrite t o tryptophan. Results 45" a violet tinge appeared in the blue color of the tests. obtained viith 10 t o 120 micrograms of tryptophan were nearly RATE OF REACTION I AND STABILITY OF TRYF'TOPH4Nidentical hy both methods. Table 111. Kinetics of Reaction I1 i n 13.2 N .4cid with Varying Quantities of Sodium Nitrite
(i
EFFECT O F TEMPERATURE OY REACTION I
The effect of temperature on the rate and sensitivity of reaction I was determined bv a study of the kinetics of the reaction a t 25", 45", and 85" C. with 13.2 N acid (Table V). Procedure. Tests were set up according t o Procedure A. The sulfuric acid, Tvater, and p-dimethylaminobenzaldehyde solution were mixed and allowed to stand a t the temperature of the test for about 5 minutes. The tryptophan solution was then added, and the reaction was allowed t o proceed in the dark. After the desired time interval test solutions were cooled 1 minute in tap 4 ater and the sodium nitrite solution was added (0.1 ml. of 0.068y0). Transmittancies were read after standing 30 minutes in the dark. The same transmittancy was obtained when the solution was kept a t 45" C. for 30 minutes after addition of the sodium nitrite as when it was kept a t room temperature. .4t 85", however, a transmittancy of 39y0 was obtained when the solution was kept a t 85" for 30 minutes after adding the sodium nitrite as compared to a reading of 23Y0 when the solution was first cooled to room temperature and kept a t that temperature during color development. The solution of p-dimethylaminobenzaldehyde used for the 25' and 8.5' tests was freshly prepared, and that for the 45' tests was 2 days old. The maximum color for each temperature was obtained a t 25", 45", and 85" when reaction I proceeded 3.5 hours, 2 hours,
and 15 minutes, respectively.
The sensitivity of the reaction
p-DIBlETHYLAMIhOBENZALDEHYDE COYDEVSiTIOY PRODUCT AT 25' C.
The rate of reaction I and the stability of the tryptophanp-dimethylaminobenzaldehyde condenqation product in 13.2 N
Table V. Time, Min. 0 1 0
5 10 15 30
Effect of Temperature on Rate and Sensitivity of Reaction I in 13.2 N Acid 250 i 0.10 0
11
c.
Temperature 450 =I= 0 . 5 0 c. "0 ;\laximum Color Formed6
..
83'-85'
.. ..
24
34
34
48
61 79 89
69
58 59 61 61 58
A-
LL
51
..
79
C.
60 82 Hours 2 96 85 53 3 SF, 53 85 100 3.5 85 5 100 85 24 100 a For comparative purposes all transmittancies mere converted to micrograms of tryptophan f r o m curve A, Figure 3, and then expressed in this table as Yq of maximum color formed in all tests of the series. T h e lowest transmittancy obtained was 9 which was considered to represent 100% of the color. 100 y of tryptophan used in each test.
..
ANALYTICAL CHEMISTRY
34 Table VI. Rate of Reaction I and Stability of Tryptophanep-Dimethylaminobenzaldehyde Condensation Product in 13.2 and 19 N Sulfuric Acid at 25' C. Time of Reaction I , hIin. 0 '0 , 3 1
'
13 Z L V a
To l l a x i m u m Color Formed 19 - V b 19 .Ye 7
3
19
... 11 ...
:2
96 97 99 99
...
100
96
100
io6
ioo
100 100
100 100
49
. . ..
... ...
98 99
99 100 100
I Y
e-
Hours 2 3
4
5 6
7
10 24
100 100
...
100
96 96
...
...
...
...
...
Days 2 ... 100 99 99 3 ... 97 100 100 93 5 93 93 5 Procedure A was used except t h a t reaction I was allowed to proceed f o r indicated time interval instead of 4 hours. For comparative purposes all transmittancies were converted t o micrograms of tryptophan from curve A, Figure 3, and then expressed in column 2 as % of maximum color formed. Lowest transmittancy obtained in this series of tests was 9, which was considered t o represent 100% o f t h e color. lOOy of tryptophan used in each test. b Procedure B was used except t h a t reaction I was allowed t o proceed for indicated time interval. For comparative purposes all transmittancies were converted t o micrograms of tryptophan from curve C, Figure 3, and then expressed in column 3 as yo of t h e maximum color formed. Lowest transmittancy obtained in this series of tests was 13.0 which was considered equivalent t o 100% of the color. lOOy of tryptophan used in each test. C A procedure similar t o E was used except t h a t t h e solution contained 1007 of tryptophan per 10 ml. of test solution. 10-ml. aliquots were with. drawn a t desired intervals. a n d color developed according t o Procedure E. Results with tryptophan sample I are shown in column 4 a n d sample I1 in column 5. For comparative purposes all transmittancies were converted t o microerams of trVDtODhan from curve C. Figure 3. and then exoressed in respe6ive columns 88' % of maximum color Tormed. Lowest transmittancies obtained in this series of tests u a s 13.1, which n a s considered t o represent 100% of color.
...
of tryptophan (100 micrograms per test). To determine how closely reaction I follows the first-order reaction equation, the log of the concentration of unreacted tryptophan was plotted against reaction time using data from Table VI, column 2. The values for the tryptophan reacted in a given time mere obtained from curve A, Figure 4. These values were corrected on the basis of the observation that a t zero time 5 micrograms of tryptophan appeared to have reacted. This value was obtained from the transmittancy shown when the sodium nitrite was added to the test before the addition of tryptophan, as discussed above. The amount of reacted tryptophan, therefore, was corrected in proportion to the amount of unreacted tryptophan that remained when sodium nitrite was added. Thus, a t time intervals of 5, 10, 15, 22, and 30 minutes the values of tryptophan reacted were 34, 51, 62, 73, and 81 micrograms, respectively. To obtain the quantity of unreacted tryptophan, these values were subtracted from 97, 98, 98, 99, and 99 micrograms to give 63, 47, 36, 26, and 18 micrograms of unreacted tryptophan, respectively. The log of the values plotted against time from 10 to 30 minutes, as shown in Figure 3, falls on a straight line. Reaction I, therefore, may be regarded as a pseudo first-order reaction. STABILITY OF TRYPTOPHAN IN 19 S ACID
The stability of free tryptophan in 19 S acid at 25' C. in the dark is shown by results in Table VII. On the basis of the colorimetric test, no loss of tryptophan occurred in 2 hours but a gradual loss then took place. I n 48 and 120 hours the loss !vas
and 19 S acids at 25' C. Ivere determined and pertinent data are shonn in Table TI. The initial rate of reaction I in 19 ic' acid (column 3) was faster than that in 13.2 N acid (column 2). Approximately one half of the tryptophan combined in 0.5 minute in 19 AVacid, but 10 minutes mere required for an equal degree of reaction in 13.2 A- acid. However, maximum color developed in 6 to 7 hours in 13.2 S acid and in between 6 to 24 hours in 19 N acid. Data in columns 4 and 5 show a comparison of the two tryptophan samples I and 11, respectively, used in this study. I n comparing these samples 19 N sulfuric acid was added directly t o a mixture of solid tryptophan and solid p-dimet hylaminobenzaldehyde a t 25' C. (Procedure E). I t is apparent t h a t the tn.0 samples of tryptophan possessed identical degrees of purity as revealed by this test. Reaction I was complete in 3 to 4 hours by this procedure, as compared t o 6 to 24 hours when tryptophan !vas added in aqueous solution. The tryptophan-p-diniethylaminobenzaldehydecompound was stable in 13.2 -1' acid at least 24 hours and for 2 t o 3 days in 19 N acid a t 25" C. The deterioration in color-producing capacity amounted to not over 575 in 5 days. ORDER OF REACTION I
The structure of the condensation product or pioducts of tryptophan and p-dimethylaminobenzaldehyde has been studied by Fearon ( 5 ) and Ghigi (6). No effort was made in this study to elucidate this problem further, but reaction I is probably a second- or third-order reaction. However, under the conditions of the test, reaction I probably would behave as a first-order reaction because the molar concentration of p-dimethylaminobenzaldehyde (30 mg. per test) was 410 times greater than that
1.001
I
1
10
20
; '
1
30
40 y
Figure 4.
50 60 70 80 of Tryptophan
90 100
110
120130
Standard Curves for Tryptophan
A . 13.2 N sulfuric acid determined by Procedure A B, C, D. 19 N aulfuric acid, d i t e r m i n e d by Procedures B, E, and D, reapectively
V O L U M E 2 0 , N O , 1, J A N U A R Y 1 9 4 8 Table VII.
35
Stability of Free Tryptophan in 19 N Sulfuric Acid at 25" C. in the Darka
5 Transmittancy
Time, Hours 0.1 0.5
Loss of Tryptophan,
580 inp 590 mp 600 nip % 13.7 13.1 13.2 0 0 13.1 13.0 13.0 1 .o 13.2 13.1 13.1 0 13.3 0 2.0 13 8 13 3 3 13 9 13.9 13 9 4.0 3 14 0 13.9 6.0 13 9 6 14 8 14.8 12 14 9 6 14 8 14.9 24 14 9 8 15.6 15 3 15 6 48 12 16.8 16 8 12u 16 9 16 18 2 18.9 18 0 264 28 23.0 23.7 504 22.9 a Proredur?. 145.6 ml. of 19 Nsulfuric acid added t o 1.456 me. of trvntophan. This solution was kept a t 25' C. in t h e dark. A t indicaGd inte%vala 10-ml. aiiquots of solution were withdrawn and added t o 30 mg. of DAB. Tryptophan determined by Procedure C. Transmittancies m a y be converted t o weight of tryptophan from curve C, Figure 4. ~~~
~
~~~
~
~~
~
8 and l2C; compared with 0 and 5 7 , respectively, when the tryptophan was bound with p-dimethylaminobenzaldehyde as shown in Table VI. The loss of free tryptophan was 28Yc in 21 dayq. rls shown in Table VII, the wave length of maximum color formed with p-dimethylaminobenzaldehyde and the aged tryptophan solutions shifted from 590-600 mp to 580 mp after 264 and 504 hours' standing. These results show that, under the conditions of the analytical methods described below, practically no tryptophan is destroyed by the acid before it combines t o form the relatively more stable trypt ophan-p-dimethylaminobenzaldehydecompound. PREPARATION OF STANDARD CURVES
Standard curves for 5 to 120 micrograms of tryptophan w r e determined by several representative procedures which embody optimum conditions for reactions I and 11. A straight line mas aln-ays obtained when the log of the per cent transmittanq- was plotted against n-eight of tryptophan, as shoivn in Figure 4. The excellent conformity to Beer's law over a range of transmittancies from 10 t o 887, is thus demonstrated.
the slightly greater heat produced on adding the tryptophan in alkaline rather than in water solution to the test. Procedure D is used in determining the tryptophan content of alkaline hydrolyzates of proteins, as will be described in a later paper. The curves shown in Figure 4 are indicative of the relative sensitivity of the various procedures, because all tests Tere made under comparable conditions with the same cuvettes and n i t h pure tryptophan samples. Results shown in Figure 4 were all obtained with the constant optimum quantity of sodium nitrite (determined for 100 micrograms of tryptophan) for all concentrations of tryptophan. From results shown in Tables I11 and IV it might be expected that it would be necessary to use a constant ratio of sodium nitrite to tryptophan. However, as shown in Table VIII, the transmittancy values obtained, with either the constant molar ratio or constant optimum quantity of sodium nitrite, are in excellent agreement. Therefore, because of greater convenience, the constant optimum quantity of sodium nitrite, as shown in Tables I11 and IV, was used for all quantities of tryptophan over the range of the test. EFFECTOFADDEDSUBSTANCES
Indole, Skatole, and Tryptamine. The effects of indole, skatole, and tryptamine on the analysis of tryptophan, are shown in Table IX. The appearance of each test, as Tell as the wave length of masimum color, is also shown. Indole is without significant effect on the test, as shorn by a transniittancy of 36,0Yc for 50 micrograms of tryptophan alone and 36.1y0 when the test contained 50 micrograms of indole plus 50 micrograms of tryptophan. Skatole, 50 micrograms, gave a traiismittancy equivalent of 23 micrograms of tryptophan. The tryptophan equivalence of skatole, 50 micrograms, and tryptophan, 50 micrograms, combined !vas 80 micrograms, an increase of 7 micrograms over t h e sum of the tn-o substances when determined alone. Tryptamine, 50 micrograms, gave a transmitt,ancy equivalent to 68 micrograms of tryptophan. On a molar basis 50 micrograms of tryptamin are equivalent to 60.4 micrograms of tryptophan. Therefore, mole for mole tryptamine produces about 1370 more color than does tryptophan. When tryptamine and tryptophan n-ere combined, the transmittancy vias 9.0%, which is equivalent to about 130 micrograms of tryptophan. But the sum of the tn-o determined separately as 118 micrograms. So here, as n-ith skatole, the effectiveness in rolnr production is
Curve A, Figure 4, shows the transmittancy-concentration relationship obtained with 13.2 N acid. by Procedure A. Although Procedure A gave maximum sensitivity, most of the subsequent tests were made in 19 N acid, because this concentration was regularly used in later studies on the determination of tryptophan in proteins. Curve B, Figure 4, was obtained with 19 N acid by Procedure B. This procedure is very convenient because it requires only 1.5 hours. To obtain curve C, Figure 4, reaction I was carried out in 19 N acid for 7 hours as described in Procedure E, in which tryptophan (sample 11) Table VIII. Comparison of Transmittancies and p-dimethylaminobenzaldehyde were (Constant optimum quantity of sodium nitrite a n d a constant optimuin ratio of s o d i u i ~nitrite a n d added to the acid solution in the solid trvotoDhan) .. state. A curve practically identical t o Procedure B Procedure .$ C was obtained by Procedure C, in Constant molar Constant molar constant Constant which tryptophan was added to the ratio. sodium ratio, sodium quantity nitrite to nitrite t o test in water solution and reaction I of sodium tryptophan, O f nitrite, tryptophan, nitrite, was allowed to proceed for 24 hours. 1.2:l 0.1 ml. Trypto2:l 0.1 ml. TryptoCurve C is used as a standard curve 0.1 nil. of 0.04% phan phan 0.1 ml./t of O.OSSC, in the determination of tryptophan solution test". testa, solution per T e s t , per Test, 70 %T* %Tb % C;T6 70 T b starting with intact proteins, as will be Y Y 5 0.0034 90 8.i a 0.002 89 89 described in a later paper. 10 80 ' 10 0.004 82 82 0.0068 80 Curve D, Figure 4, was obtained 74 0,010 15 69 0.006 74 71 15 by Procedure D. Tryptophan was 67 67 0.008 64 62 0,014 20 20 0.01 61 57 55 25 0.017 25 added to the test in 1 X sodium ' 30 0.012 54 51 49 0,020 30 hydroxide solution, and reaction I was 45 0,024 45 40 0.016 46 43 35 carried out for 1 hour. Procedure D 0.020 38 37 0,027 40 50 39 40 0.024 31 34 30 35 60 0.031 45 is similar to Procedure B except for 25 i o 0.028 26 30 50 31 0.034 the difference in solvent for trypto0.032 21 21 60 80 25 0.041 24 phan. It is apparent that a very slight 17 0.036 17 70 90 19 0.048 19 14 100 0.040 14 15 15 80 0.054 loss of tryptophan occurs when trypto0,044 12 12 110 12 90 11+ 0.061 phan is added in the alkaline solu10 10 0,048 100 9.3 120 0.068 9.3 tion. This loss is probably caused by 0 Per cent concentration of sodium nitrite added, in 0.1 ml. t o each test. b Transmittancy, 600 mp. the presence of a trace of oxidant in the sodium hydroxide and not by
8;
ANALYTICAL CHEMISTRY
36
phan were in the blank, 1 0 0 ~ o recovery was attained when reaction I proceeded 1 hour, Appearance of Test Solution Immediately 1 hour 30 min. and 97 and 967, when reTransmittancy after after after Wave Length Sample addition t o addition action I proceeded for 4 to addition At max. A t 600 Tryptoof Llaxinium test of sodium t o test color, mp Colorb, Quantity, phan, 30 hours, respectively. solutionc nitrited solutionc Substance Y % % mil. Y The color of blank so93.4 93.9 Canary yel- Intense ca- Pale yel570 Indole 50 0 low nary yellow low lutions containing fructose 36.1 3 6 . 1 Canary yel- Intense ca- Dull blue 590-600 Indole 50 50 progressively deepened from nary yellow green low 60.8 63.0 Canary yel- Orange yel- Pale pink 550 Skatole 50 0 light yellow in 1 hour t o low low nravish Fast deep brown in 30 hours. 580 20.4 2 0 . 8 Canary yel- Orange yel- BlueSkatole 50 50 Hom-ever, test solutions conlow low violet 590 26 . O 2 6 . 1 Faint orange Fainter orange Blue with Tryptaminee 50 0 taining fructose were coloryellow yellow violet cast less up to 4 hours but Tryptaminee 50 50 580-610 9.0 9 . 0 Faint orange Fainter orange Deep then became greenish-orange yellow yellow purple blue in 30 hours. As shown in Colorless Pale blue 50 590-600 36.0 3 6 . 0 Pink Colorless Deep blue 100 590-610 14.0 1 4 . 0 Pink Table X, tests containing a Procedure B used. Blank solution contained 23.8 N acid! 8.0 ml.; 2 N arid containing 30 m,g. of DAB, 1.0 mi. : fructose required progreswater 1.0 ml.: 0.04% sodium nitrite solution 0.1 ml. Transmittancies may be converted t o weight of tryptophan sively more sodium nitrite from curve B, Figure 4. b Determined over wave-length range of 550 t o 630 mp, to produce maximum color C Ordinary artificial light. with increased time of standd Fluorescent light. 62 y of tryptamine hydrochloride. ing. Test solutions XThich had stood 1, 4, and 30 Table X. Effect of Glucose and Fructose on Tryptophan Determination hours required 1.2, 3.0, and Molar Ratio >6.0 equivalents of sodium Time of Sodium TransmitAppearance nitrite (referred to tryptoof CarbohyKitrite t o tancy a t Reacdrate, Tryptophan &isximum Tryptophan Test before Test after phan) when read against tion I, 5.0 Mg. Required for Blank Colora, Found, Loss, color color Hours per Test Max. Color Used % y % Blank B1 developed developed blank solutions containing 14.2 101 0 Colorless Blue Colorless fructose and 1.2, 2.0, and 1 Glucose } :;; Bl; Blue Colorless B2 14.1 102 0 Colorless 4.0 equivalents, respectively, Blue Colorless B1 1 4 . 0 97 3 Colorless 4 Glucose \ when read against blanks 13.2 100 0 Blue B2 Colorless Colorless containing no fructose. The 14.2 96 4 B1 Blue Colorless Colorless 30 Glucose Blue B2 13.7 98 2 Colorless Colorless color of blank tests conBlue B1 16.2 97 3 Light yellcIW Colorless 1 Fructosed } taining fructose was proB2 Colorless 15.0 98 2 Colorless Blue gressively lightened by in3.0 B1 16.0 90 10 Slight greenOrange Colorless ish-blue creased amounts of sodium Fructose 14.3 96 4 2.0 B2 Colorless Colorless Slight greennitrite, and doubtless this ish-blue reductive competition of the >6.0 B1 24.8 69 31 Brown Greenish- Greenishblue orange 30 Fructose brown color for sodium niB2 16.8 88 12 Colorless 4.0 Greenish- Greenishblue orange trite caused the increased a Transmittancies read after reaction I had proceeded for 1 hour were converted t o weight of tryptophan from requirement of sodium nicurve B, Figure 4; those read after 4 and 30 hours were converted using curve C Figure 4. trite in tests containing b B 1 test read against a n individual blank containing carbohydrate, t r y i t o p h a n , and the same quantity of sodium nitrite used in corresponding test. fructose. Recoveries of 98, B2 test read against blank containing only acid and D.4B. d Test with fructose set up similarly t o t h a t with glucose. 96, and 88cl, of the tryptophan were obtained in 1, 4, and 30 hours, respectively, when tests were read against slightly enhanced xhen tryptamine and tryptophan are in the blanks containing no fructose and recoveries of 97, 90, and 69% same test. were likeFise obtained when the blanks contained fructose. The wave lengths of maximum color of indole, skatole. tryptaThe procedure used was similar to Procedure E. To 1.820 mg. mine, and tryptophan, when determined separately, were 570, of tryptophan, 546 mg. of p-dimethylaminobenaaldehyde, and 550 (or lower), 590, and 590 to 610 mp, respectively. When de91 mg. of glucose were added 182.0 ml. of 19 N acid a t 25" C. termined in the presence of equal weights of tryptophan, the The reaction mixture was kept a t 25 O C. in the dark and a t desired time intervals 10-ml. aliquots were removed for testing. Indiwave lengths of maximum color were 590 to 600 mp, 580 mp, and vidual blanks: To 1.629 mg. of tryptophan and 81.5 mg. of glu580 to 610 mp for indole, skatole, and tryptamine, respectively. cose were added 162.9 ml. of 19 N acid a t 25' C. This solution was Glucose and Fructose. The effect of carbohydrate on the estiset up a t the same time as the test solution, At desired intermation of tryptophan was determined using both glucose and vals 10-ml. aliquots were removed to be used with corresponding test solutions. Sodium nitrite of the desired concentration was fructose, which behave differently. Table X shows the recoveradded to each blank in 0.1 ml. of solution. Blank with no gluies of tryptophan in test solutions containing 100 micrograms of cose: 19 N acid, 10.0 ml.; p-dimethylaminobenzaldehyde, tryptophan and 5000 micrograms of glucose or fructose. The 30 mg. ; water 0.1 ml. tests were carried out by Procedure E in which reaction I.was allowed to proceed 1, 4, and 30 hours. Also shown in Table X Fructose is regarded as representative of types of carbohyis the effect of the presence of carbohydrate on the quantity of drate which give adventitious color under the conditions of the sodium nitrite required to produce maximum color. tryptophan test. But in the tests described in Table X the ratio Glucose did not produce interfering color in the test nor in the of carbohydrate to tryptophan is probably higher than will ordiblank solutions and no additional sodium nitrite was required t o narily be encountered in analytical work. Even under these develop maximum color. Using blank tests containing no glusevere conditions 98 to 100% recoveries of tryptophan were obcose, 98 to 100% of the tryptophan was recovered when 1, 4, or tained in the presence of fiftyfold quantities of carbohydrate 30 hours were allowed for reaction I. When glucose and tryptowhen reaction I was allowed t o proceed 1 hour and when the Table IX.
Effects of Indole, Skatole, and Tryptamine on Tryptophan Determinationa
j
i::
i:;
::;
1
1
~
V O L U M E 20, N O . 1, J A N U A R Y 1 9 4 8
37
quantity of sodium nitrite required to give maximum color was In comparative tests by Procedure A, final transmittancies of 9.2, 9.2, 9.2, and 9.5% were obtained in tests with 100 determined. Furthermore, the brown coloration produced by micrograms of tryptophan with an 0.068% dilution of a fructose would undoubtedly be much less if tryptophan was determined in 13.2 N acid by Procedure A. This procedure can freshly prepared 1% solution of sodium nitrite, an 0.068% he used, however, only if the tryptophan is not bound in protein dilution of a 42-day-old 1% solution, a 42-day-old 0.068% linkages. dilution, and an 0.06870 dilution of an 89-day-old lYO solution, Tryptophan-Free Proteins. Tryptophan is usually deterrespectively. .4s a further test of the stability of dilute solutions of sodium mined in the presence of other amino acids, such as are encountered in proteins. I t was of interest, therefore, to determine the nitrite the rate of Oxidative capacity of 0.1 ml. of 0.008570'dilutions of sodium nitrite of various ages was determined. This degree of recovery that could be attained when tryptophan was added to gelstin plus tl-rosine (added to make up for the tyrosine test provides a delicate measure of oxidative capacity because the quantity of sodium nitrite used is insufficient for maximum color deficiency of gelatiti) and also to a tryptophan-free allergenic developnient (21). Therefore the rate of oxidation as well as the protein fraction iCB-lC76 1 obtained from castor beans. The effect of the composition of the blank solution on the quantity end point may be compared. Results in Table XI11 show that of tryptophan recovered under the conditions of the test was also an 0.0085% dilution of an 89-day-old 1% solution of sodium determined. nitrite caused a slightly faster rate of oxidation and gave a transTables S I and S I 1 show the recovery of added tryptophan mittancy of 18% as compared to a transmittancy of 2070 for an (50 micrograms per test) when determined in the presence of 0.008570 dilution of a freshly prepared 1% solution. A transgelatin (10 mg. per test) plus tyrosine (500 micrograms per test) niittancy of l8Y0 was obtained with a freshly prepared 0.0085% and also CB-1C76 (10 mg. per test). Blank solutions of various dilution of a 42-day-old 1% solution of sodium nitrite as comconipositions were used. I n Tables S I and XI1 tryptophan pared to 21Y0 for a 42-day-old 0.008570dilution of the same soluvalues of 48.4 and 51.3 micrograms, respectively (with blank tion. These results are probably all within the experimental solution B2, test T 4 ) , are regarded as the ones to he used for comerror of the method. It is apparent, therefore, that aging up t o parison with the other values in respective tables because they three months does not cause detectable deterioration in colorwere obtained with the same blank solution xhich would normally producing capacity of sodium nitrite solutions stored in glasshe used in a tryptophan determination. stoppered Pyrex flasks exposed to indirect artificial light. -4 tryptophan value of 51.2 micrograms was obtained in the test made in the presence Table XI. Effect of Gelatin Plus Tyrosine and Effect of Composition of Blank of gelatin and tyrosine (B3, Solution on Tryptophan Determination T5, Table XI) using the blank Blank Solution" Test Solutionb Tryptophan solution that would normally Blank Trypto- T,yro- GelaTest Trypto- Tyro- GelaCorDeviaNo. phan sine tinC DAB Xo. phan sine tine Found rectedd tion be used in such a deterniinaY y .vg. Jig. Y Y avo. Y Y % tion. If this value is corrected T1 50 500 10 51.2 49.9 +3.0 by subtracting 1.3micrograms ni 0 500 10 30 T2 50 0 0 45.6 ... -5.6 (the tryptophan content of this sample of gelatin was deterI T3 50 500 10 52.7 51.3 +5.8 n2 0 0 0 30 mined as 0.0l3yG in a later T4 50 0 0 48.4 ... 0.0 study), then the quantity of I T5 50 500 10 51.2 49.9 +3.0 added tryptophan becomes B3 50 500 10 '0 iI T 6 50 0 47.2 ... -2.4 49.9 micrograms B hich devi50 500 10 5 2 . 7 51.3 +5.8 ates by +3.09', from the B4 0 500 10 , 0 T7 0 0 48.4 0.0 1 T8 50 amount actually present. Likewise a tryptophan value T o each blank was added 0.1 ml. of 0.06% a Composition of blank solution dissolved in 10 ml. of 19 N acid. sodium nitrite solution. of 51.2 micrograms mas obb Tests conducted by Procedure C. Tryptophan added in 0.5 ml. of 0.1 N sodium hydroxide solution and gelatin-tyrosine added in 0.5 ml. of same solvent. Color was developed with 0.1 ml. of 0.06% sodium nitrite solutained in the presence of CBtion. Transrnittancies read 30 minutes after adding sodium nitrite. 1C76 using the normal blank Ash- and water-free basis. Kitrogen content 18.52 (ash- and water-free basis). d Corrected on basis of tryptophan content of 0.013% a s determined b y method t o be described. (B3, T5, Table XIII. This value deviates by only -0.2'3 from the amount actually deTable XII. Effect of a Tryptophan-Free Protein (CB-lC76)o from Castor Beans termined (B2, T4, Table XII). and Effect of Composition of Blank Solution on Tryptophan Determination Deviations of from -5.6 Blank Solutionb Test SolutionC Tryptophan Test Blank t o +5.8yo from the added So. T r y p t o p h a n CB-1C76 DAB So. Tryptophan CB-1C76 Found Deviation tryptophan were obtained with Y Mg. Mg. Y Mg. Y % i T1 50 10 51.5 $0.4 blank solutions of various comni 0 10 30 i positions, as shown in the 0 51.5 -0.2 50 1 T2 t ahles. ( T3 50 10 52.5 +2.4 n2 0 0 30 i 50 0 . 51.3 0.0
1 1
I
EFFECT OF AGING OX SODIUM NITRITE A S D p-DIMETHYLAMINOBEiYZALDEHYDE SOLUTIONS
Desired dilutions of sodium nitrite were prepared from a 1% aqueous stock solution. The effect of aging on the color-developing capacity of dilutions of the stock solution was determined.
...
L T4
B3 n4
50
0
10
10
0
o
I
T5
50
10
51.2
1,
T6
50
0
49.7
-3.2
f T7 i i T8
50
10
52.7
+2.8
50
0
51.6
+0.6
-0.2
CB-1C76 was a subfraction of castor bean allergenic polysaccharidic-protein fraction CB-1C. CB-1C76 contained 19.1% nitrogen and 0.37% carbohydrate. CB-1C is similar to CB-1.4. Properties and amino acid content of CB-1A have been previously recorded ( 1 7 ) . b Composition of blank solution dissolved in 10 ml. of 19 N acid. T o each blank solution was added 0.1 ml. Of 0.04570 sodium nitrite solution. Tryptophan added t o tpst, in 0.5 "1. of water a n d CB-1C76 added in 0.5 c Tests conducted b y Procedure C. ml. of water. Color developed with 0.1 ml. of 0.048% sodium nltritesolutlon. a
38
ANALYTICAL CHEMISTRY
Table XIII. Effect of Aging o n Rate a n d Total Oxidizing Capacity of 0.0085% Solutions of S o d i u m Nitritea Age of 1% Stock Solution a n d Dilution Preoared from It ________ Stock 0 0085% Time, Minutes solution, dilution, % Transmittancy (600 mb) days days 0 1 2 3 5 7 1 0 1 5 3 0 6 0 89 0 93 58 48 41 34 29 26 22 19 18 42 42 93 62 53 47 40 35 31 27 23 21 93 60 50 43 3 5 30 27 23 19 18 42 ' 0 0 0 92 61 52 46 38 33 29 25 22 20 Tests conducted by Procedure A , like those described in Table 111, using 100 y of tryptophan. Transmittancies convertible to weight of tryptophan from curve A , Figure 4. Q
The effect of aging on solutions of p-dimethylaminobenzaldehyde in 2 N acid, stored in glass-stoppered Pyrex flasks in the dark, was also determined. Comparative tests with solutions of p-dimethylaminobenzaldehyde from 0 to 60 days old are shown in Table XIV. p-Dimethylaminobenzaldehydesolutions 1 day old gave maximum color with tryptophan, but losses of 2 to 10% were observed on 5 to 60 days' standing. Even freshly prepared solutions of p-dimethylaminobenzaldehyde should never be exposed, even briefly, to sunlight or bright artificial light, as n-ill be shown in a later paper. APPARATUS AND M A T E R I A L S
A Coleman Universal spectrophotometer, Model 11 (wave band width 35 mp), was used for most of this work. The wavelength scale was checked with a didymium filter. Round 19mm. cuvettes were used. The open ends of the cuvettes were squared so that they fitted into the holder with enough tension to prevent even slight movement. The cuvettes were always placed in the holder in the same position and the same cuvette was always used for the blank and another for the test solution. Per cent transmittancy was read directly from the galvanometer scale. A Beckman quartz spectrophotometer was used to obtain the absorption curves shon-n in Figures 1 and 2. Square 10-mm. cuvettes were used. A calibrated 1-ml. hypodermic syringe was used for measuring tryptophan solutions. Tests showed this instrument to be both more accurate and more precise than glass pipets. . A 0.25-m1. syringe vas used for measuring sodium nitrite solutions. Tryptophan. T R O samples were used. Sample I was Eastman crystalline l-tryptophan, melting point 286-288" C. decomposed (a standardized Anschutz thermometer was used). Analyl sis: calcd. for CnHl,0&2, X, 13.72; C, 64.67; H, 5.93; found, N, 13.77, 13.73; C, 64.97, 64.61; H, 6.10, 6.11. For sample 11, 1 gram of Eastman crystalline l-tryptophan was recrystallized three times from 70YGethanol used in the proportion of 40 ml. per gram of tryptophan. Yields of 0.47, 0.23, and 0.14 gram were obtained from respective recrystallizations. The final product was ground in an agate mortar and dried in a platinum boat in a vacuum Abderhalden dryer a t 110" C. for 2 hours. Analysis: found, S , 13.71, 13.76; C, 64.55, 64.65; H, 5.79, 5.86. Kitrogen was determined by the Kjeldahl micromethod. Carbon and hydrogen were determined by a micromethod. Standard solutions of tryptophan in water were freshly prepared on the same day used. p-Dimethylaminobenzaldehyde. Reagent grade p-dimethylaminobenzaldehyde was further purified by the procedure described by Adams and Coleman ( 3 ) . The purified product was washed three times with water until free from chlorides and then dried to constant weight in a vacuum desiccator over calcium chloride. In a typical experiment 74.1 grams of p-dimethylaminobenzaldehyde yielded 43.5 grams of an almost colorless crystalline product, melting point 73-74 O C. (literature value 73" C.). Analysis: calcd., N, 9.37; found, 9.46, 9.33. sulfuric acid Solutions of p-dimethylaminobenzaldehyde in 2 were freshly prepared each day unless otherwise stated. Sulfuric Acid. Reagent grade 36 N sulfuric acid was used to prepare desired dilutions, 13 hich were standardized against pure sodium carbonate with methyl red as indicator. The sulfuric acid used in this study was so free from oxidizing agents that purification was not necessary. However, it is possible that some lots of sulfuric acid might require distillation to remove traces of oxidizing agents. This point will be discussed in a subsequent paper. Sodium Nitrite. Reagent grade sodium nitrite was used. ~~
which assayed 97y0 according to the manufacturer. Dilutions were usually repared from a ly0 aqueous solution on the day used, althougf: tests shoTTed that sodium nitrite in solution retained its full color-producing strength indefinitely. Other Substances. Eastman reagent grade indole (nitrogen calculated, 11.97; found, 12.04, 12.07), skatole, and tryptamine hydrochloride r e r e used. The nitrogen content of the I-tyrosine used was 7.64%; theoretically i t is 7.74%. The glucose used was a National Bureau of Standards' standard sample. The fructose was a purified sample. The gelatin contained 18.52% nitrogen (ash- and water-free basis), The tryptophan-free protein CB-1C76 was obtained in a fractionation of an allergenic fraction CB-1C obtained from castor beans. CB-1C is similar to CB-lA, the amino acid composition of which has been previously reported ( 1 7 ) . CB-1C76 contained 19.1'% nitrogen and 0.49% carbohydrate (ash- and water-free basis). General Considerations. A total volume of 10.1 ml. (including sodium nitrite solution) was used in all tests. Tests Tere carried out in 25- or 50-ml. Pyrex, standard-tapered, glassstoppered Erlenmeyer flasks. Blank solutions were always set up as nearly as possible similar in composition to the test solution. S n equal volume of the same solvent used for the tiyptophan in the test was added to blank solutions 71 hen pure tryptophan was analyzed. I t made no difference in the value of the transmittancy observed whether or not the blank solution contained p-dimethylnminobenzaldehyde or sodium nitrite. .In equal volume of 2 S acid could be substituted for the 2 S acid solution of p-dimethylaminobenzaldehyde. Likewise 0.1 ml. of water could be added to the blank instead of thc 0.1 ml. of the appropriate concentration of sodium nitrite. When the solution to be analyzed contained constituents other than tryptophan, p-dimethylaminobenzaldehj-de was omitted from the blank and a volume of 2 -Yacid solution equal to that used in the test was added to the blank. In these casqs sodium nitrite was added to the blank solution. Table XIV. Effect of -ige on Solutions of p - D i m e t h y l aminobenzaldehyde in 2 -icida Age of DAB Solution, Days 0 1
Transmittanoy, 600 mu,
%
9 8
Tryptophan Recoi.ered, Y
100 IO0
Loss,
70 0 0
Tests made by Procedure A x i t h 30 mg. of DAB and 100 y of tryptophan per test. Transmittancies convertible t o weight of tryptophan from curve A. Figure 4.
Reaction I was carried out a t 25" =t0.1" C. in the dark in all tests except some preliminary ones which v-ere conducted a t room temperature. Reaction I1 was carried out a t room temperature in the dark. Protection from light, especially sunlight and bright artificial light, is important for reaction I because destruction of tryptophan in acid solution is accelerated by light. Protection fiom artificial light during reaction I1 \\as practiced but it is not essential. Acid solutions of p-dimethylaminobenzaldehyde should not be exposed to light. The photochemistry of these reactions d l be the subject of a later paper. The wave length of maximum color varied sli4htly over the range of 590, 600, or 610 mp; consequently, in the analytical procedures the lon-est transniittancy obtained at one of these wave lengths was used. The variation in transmittancy value over this aave-length range is very slight and for filter photometers either 590 or 600 mp would be satisfactory. The colorimetric reactions involved in the following procedures are very sensitive and may be influenced by contaminants. Therefore, care should be taken to use purified reagents, distilled water, and carefully cleaned glassware, and even t o avoid impure air. Tillnians and -Ut (20) also recognized and emphasized
V O L U M E 20, NO. 1, J A N U A R Y 1 9 4 8
39
these precautions in connection with the fornialdehyde-tryptophan colorimetric reactions. RECOMMENDED PROCEDURES FOR DETERlMINATION OF TRYPTOPHAN
Procedure A (5 hours, 13.2 N acid, tryptophan added in water solution). Seven milliliters of 18.7 N acid, 1.0 ml. of 2 N acid containing 30 mg. of p-dimethylaminobeuzaldehyde, and enough to equal 10 ml. (including the tryptophan solution) are mixed and cooled to 25" C. The tryptophan solution is added and mixed, and the resulting solution is cooled to 25" and kept in the dark a t this temperature for 4 hours. To this solution is then added 0.1 ml. of 0.0687, sodium nitrite solution in water. The solution is shaken and kept a t room temperature in the dark for 60 minutes; then the transmittancy is read. Transmittancy may be converted to weight of tryptophan from curve A, Figure 4. Procedure B (1.5 hours, 19 N acid, tryptophan added in water solution). Eight milliliters of 23.8 N acid and 1.0 ml. of 2 N acid containing 30 mg. of p-dimethylaminobenzaldehyde are mixed and cooled to 25' C. To this solution is added 1.0 ml. of a water solution of the tryptophan. The solution after shaking and cooling to 25" is kept in t,he dark a t 25" for 1 hour. To this solution is added 0.1 ml. of 0.047, sodium nitrite solution. The solution is shaken, and the color is allowed to develop for 30 minutes a t room temperature in the dark. Transmittancy is then read and may be converted to w i g h t of tryptophan from curve B, Figure 4. Procedure C (24 to 48 hours, 19 h'acid, tryptophan added in water solution). Procedure C is the same as Procedure B except that reaction I is allowed to proceed for 24 to 48 hours before color is developed. I n the presence of substances other than tryptophan it 7%-ouldbe necessary to determine whether or not standing caused destruction of the tryptophan-p-dimethylaminobenzaldehyde compound. Transmittancies are convertible to weight of tryptophan from curve C, Figure 4. Procedure D (1.5 hours, 19 N acid, tryptophan added in 1 iV sodium hydroxide solution). Procedure D is the same as Procedure B except that the tryptophan is added in 1 N sodium hydroxide solution. Other concentrations of alkali could be used if the standard curve v a s determined lvith the corresponding. concentrations. Transmittancies may be converted to weight of tryptophan from curve D, Figure 4. Procedure E (4 to 72 hours, 19 N acid, tryptophan and p dimethylaminobenzaldehyde added in solid form). This procedure was used to obtain curve C. To 1.392 mg. of tryptophan (sample 11) and 348 mg. of p-dimethylaminobenzaldehyde (30 mg. per 10 ml. of test solution) in a 125-ml. glass-stoppered Erlenmeyer flask w r e added 116.0 ml. of 19 2%- acid a t 25" C. (solut,ion -1).To 300 mg. of p-dimethylaminobenzaldehyde were added 100 nil. of 10 A- acid at 25' (solution B). Appropriate volumes of solutions -1and B were mixed a t once to give quantities of tryptophan ranging from 5 to 120 micrograms per 10 ml. of test solution. After mixing, the test solutions were reserved in the dark a t 25' for 7 hours (4 to 72 hours would give the same results according to Table VI). To each test solution was then added 0.1 ml. of 0.0470 sodium nitrite solution. The solutions Tvere shaken and kept in the dark a t room t'emperature for 30 minutes, then transmittancies were read. Results are plotted in curve C, Figure 4. Procedure F (1.5 hours, 19 'V acid, tryptophan added in water solution, p-dimethylaminobenzaldehyde in solid form). T o 30 mg. of p-dimethylaminobenzaldehyde are added 9.0 ml. of 21.4 N acid. To this solution a t 25. C. is added 1.0 ml. of tryptophan in water solution. Xft'er mixing, Procedure B is followed, using curve B, Figure 4, to convert transmittancies to n-eight of tryptophan. This procedure, like G and H, is useful for conducting a few tests where it is not desired to make up a solution of p-dimethylaminobenzaldehyde. Procedure G (24 to 48 hours, 19 N acid, tryptophan added in water solution). Procedure G is the same as Procedure F except that reaction I is alloved to proceed from 24 to 48 hours. Curve C, Figure 4, is used to convert transmittancies to weight of tryptophan. Procedure H (1.5 hours, 19 N acid, tryptophan added in 1.0 N sodium hydroxide solution). Procedure H is the same as Procedure F except that the tryptophan is added in 1 N sodium hydroxide solution. Curve D. Figure 4. is used to convert transmTttancies to n-eight of tryptophan:
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DISCUSSION AND SUMMARY
The color-forming reaction between tryptophan, p-dimethylaminobenzaldehyde, and sodium nitrite can be carried out satis-
factorily in sulfuric acid solution, thus eliminating the objectionable features of using strong hydrochloric acid. Maximum color is attained in 12 t o 13.2 N acid but the reaction is more rapid in 19 -V acid and this latter concentration is suitable for the determination of tryptophan in proteins starting with the intact protein. Optimum conditions for conducting the reaction in both 13.2 and 19 A- acid have been determined. I n 13.2 S acid transmittancies ranging from 88 to lOyo were obtained with 5 to 98 micrograms of tryptophan per 10 ml. of test solution and in 19 .V acid transmittancies of 88 to 10% were obtained with 5 to 114 micrograms of tryptophan with 19-mm. cuvettes in the Coleman, Model 11, spectrophotometer. Conformity to Beer's law over the entire range of concentration was observed. Under the conditions of the test, free tryptophan is stable in 19 .V sulfuric acid for 2 hours. This period is long enough for completion of the reaction between tryptophan and p-dimethylaminobenzaldehyde to form the much more stable condensation product which suffers no loss in color-forming capacity on standing for 2 days in the dark a t 25' C. Tryptophan can be satisfactorily determined in the presence of an equal quantity of indole, a fiftyfold quantity of glucose or fructose, and a mO-fold quantity of tryptophan-free protein. Equal quantities of skatole and tryptophan gave color equivalent to 1.6 times the quantity of tryptophan present. K h e n equal quantities of tryptamine and tryptophan xvere determined together, color equivalent to 2.6 times the quantity of tryptophan n-as obtained. The determination of free tryptophan usually involves preliminary procedures to get the tryptophan into a solution suitable for analysis. Therefore several alternative procedures have been presented in this paper ivhich can be adapted t o a variety of needs by the analyst. Tryptophan in solid form or dissolved in water, in sodium hydroxide, or in acid solution can be determined. Procedure -1, n-hich gives maximum sensitivity, requires 5 hours. Procedure B can be completed in 1.5 hours, while Procedures C and E can be set up one day and finished the next day. Even more variations can be devised from the comprehensive data concerning the reaction presented. Thus, procedures consuming only a fraction of an hour could be devised to meet special requirements. LITERATURE CITED
(1) Xdanikiewicz, d..Arch. PhysioZ. (Pj'liigers).9, 156 (1874). (2) Adams, R., and Coleman, G. H., "Organic Syntheses," Collective 1-01. I, 2nd ed., p. 214, New York, John Wiley & Sons, 1941. (3) Bates, R. W., Proc. Am. SOC.Biol. Chem., J . B i d . Chem., 119, vii (1937). (4) Cole, S. IT., J . Physiol., 30, 311 (1903-04). (5) Fearon, 11'. R., Biochem. J . , 14, 548 (1920). (6) Ghigi, E., Gam. chim. itaZ., 63,411 (1933). (7) Herzfeld, E., Biochem. Z . , 56, 258 (1913). (8) Hopkins, F. G., and Cole, S. TT., Proc. Roy. SOC.( L o n d o n ) , 68, 21 (1901). (9) Horn, >I. J., and Jones. D. B., J . Bid. Chem., 157, 153 (1945). (10) Kraus, Ida, Ibid., 63, 157 (1925). (11) Milone, H. S., and Ereritt, E. L., Proc. SOC.Exptl. B i d . Med., 51, 82 (1942). (12) lIitchell, H. H . , and Hamilton, T. S., "Biochemistry of the -Imino .Icids," Yew York, Chemical Catalog Co., 1929. (13) Reichl. C . , Monat., 10, 317 (1S89). (14) Ibid., 11, 155 (1890). (15) Rohde. E., 2. physiol. Chem.,44, 161 (1905). (16) Rosenheim, O., Biochem. J., 1,233 (1906). (17) Spies. J. R., and Coulson, E. J., J. Am. Chem. SOC.,65, 1720 (1943). (18) Sullivan, M.X., and Hess, IT. C., J . Bid. Chem., 155, 441 (1944) (19) Sullivan, 11.X., Milone, H. S..and Everitt, E. L., Ibid., 125,471 (1938). (20) Tillmans, J., and Alt, A., Biochem. Z., 164, 135 (1925). (21) Voisenet, >I. E., Bull. S O C . chim., 33 [ 3 ] ,1198 (1905). RECEIVED February 2 4 , 1 9 4 7 .
