Langmuir 1995,11, 1477-1481
1477
Micellar Catalytic Effects on the Kinetics of the Ionization of Basic Amino Acid and Acidic Amino Acid Studied by the Ultrasonic Absorption Method Teruyo Yamashita (Isoda),la Miyuki Yamasaki,la Takayuki Sano,lb Shoji Harada,lCand Hiroshige Yano*Ja Daiichi College of Pharmaceutical Sciences, 22-1, Tamagawa-cho, Minami-ku, Fukuoka 815, Japan, Department of Materials Science, Faculty of Science, Hiroshima University, Higashi-hiroshima 724, Japan, and Hiroshima Bunkyo Women's College, Kabehigashi, Asakita-ku, Hiroshima 731-02, Japan Received November 29, 1993. In Final Form: January 23, 1995@ Base equilibria of arginine (basic amino acid) in the absence of a micelle and then in a micellar solution of sodium dodecyl sulfate (SDS)have been studied by ultrasonic absorption measurements. Forward (y2kf) and backward (kb)rate constants, the apparent base dissociation constant Kb (=kdy2kf),and the volume change ( A m for the reaction were obtained. Micellar effects for arginine were greater than those for a neutral amino acid measured previously. This is discussed in terms of total charge of the amino acids. Similar experiments were carried out for aspartic acid (acidic amino acid)-cationic micelle of dodecylammonium chloride (DAC). Relaxation was considered due to the protolysis of aspartic acid. A slightly lower micellar effect than that for an aromatic carboxylicacid-dodecylammonium chloride (DAC) system was noted. AV was smaller than that for an aromatic carboxylic acid with the same pKa.
Introduction An important biological reaction is ionization of amino or carboxyl groups of amino acids in proteins. First, such reactions of simple amino acids in aggregates should be investigated. Ionization of amino acids in aqueous solution has been kinetically studied,2-6but reaction in aggregates has been considered only recently. The previous paper reported7 anionic micellar effects on the reaction rate of a neutral amino acid to be weaker than on that of a n amine8-14 and the rate determining step to be intramolecular proton transfer in a micellar solution. This study was conducted to determine the effects of adding a positive or negative charge to a neutral amino acid on the kinetics and AV of ionization. A basic amino acid (arginine)and acidic amino acid (aspartic acid) were used for this purpose.
Experimental Section Arginine and aspartic acid purchased from Nakarai, SDS from BDH, and DAC from Tokyo Kasei were used without purification. Ultrasonic absorption measurements were conducted in a frequency range 5.5-105 MHz with a pulse method. The Abstract published in Advance A C S Abstracts, April 15,1995. (1)(a) Daiichi College of Pharmaceutical Sciences. (b) Hiroshima University. (c) Hiroshima Bunkyo Women's College. (2)Applegate, K.; Slutsky, L. J.; Parker, R. C. J . Am. Chem. SOC. 1968,90,6909. (3)Hammes, G.G.;Pace, C. N. J . Phys. Chem. 1968,72,2227. (4) Hussey, M.;Edmonds, P. D. J . Acoust. SOC.Am. 1971,49,1309. (5)Hussey, M.;Edmonds, P. D. J . Acoust. SOC.Am. 1971,49,1907. (6)Tabuchi, D.; Inoue, N.; Okuwa, K.; Ohno, K. Acoustica 1975,32, 236. (7)Yamashita, T.; Tanaka, K.; Yano, H.; Harada, S. J . Chem. SOC. 1991,87,1857. (8)Yamashita, T.;Yano, H.; Harada, S.;Yasunaga, T. J.Phys. Chem. @
~1983.87. _ _ _- - 5482. (9) Yamashita,T.;Yano, H.;Harada, S.;Yasunaga,T.J.Phys. Chem. I
>
1984,88,2671. (10)Yamashita,T.; Sumino, M.; Yano, H.; Harada, S.;Yasunaga, T. Bull. Chem. SOC.Jpn. 1984,57,2352. (11)Yamashita, T. J . Soc. Hiroshima Uniu. Ser. A 1984,48,87. (12)Harada, S.; Okada, H.; Sano, T.; Yamashita, T.; Yano, H. J . Phys. Chem. 1990,94,7648. (13)Harada, S.;Doi, T.; Sano, T.; Yamashita (Isoda), T.; Yano, H. Bull. Chem. SOC.Jpn. 1990,63,2698. (14)Harada, S.;Sano, T.; Yamashita, T.; Yano, H.; Inoue, T.; Yasunaga, T. Bull. Chem. SOC.Jpn. 1988,61, 1045.
0743-746319512411-1477$09.00/0
apparatus was that used previ0us1y.l~Sound velocity was measured by the sing around method at 1.92 MHz and density by a pycnometer. pH and ultrasonic measurements for the arginine solution were carried out under a dry nitrogen gas atmosphere at 30.0 "C. For the reference solution in the absence of SDS, ionic strength was kept constant at 0.30 by NaC1. All relaxation absorption spectra were expressed by the following single relaxation equationle-1s
where a is the absorption coefficient, f the frequency, f r the relaxation frequency, and A and B the relaxing and nonrelaxing absorptions,respectively. The absorptionparameters, f,,A,and B , were obtained by fitting the data to eq 1 by computer simulation.
Results and Discussion 1. Base Equilibrium o f a Basic Amino Acid. (A) Ultrasonic RelaxationAbsorption of Arginine in the Absence of SDS Micelles. Ultrasonic relaxation absorption was measured while keeping arginine concentration constant with variation in pH. All ultrasonic absorption spectra were analyzed by relaxation equation 1. Representative spectra are shown in Figure 1. Experimental conditions and absorption parameters are listed in Table 1. pH dependence of the relaxation parameters indicated the relaxation absorption to be due to the base equilibrium of arginine. For the base equilibrium of arginine, the twostep reaction in eq 2 is proposed. The forward reaction is the proton-transfer reaction from arginine to OH-. The backward reaction is the proton-transfer reaction from HzO to arginine. The first step in eq 2 is the base equilibrium of the a-amino group with pKal = 9.04 (acid dissociation constant) and the second step is that of the (15)Tatsumoto,N. J. Chem. Phys. 1967,47,4561. (16)Blandamer, M.J.Introduction to Chemical Ultrasonics;Academic Press: New York, 1973. (17)Bernasconic, C. Relaxation Kinetics;Academic Press: New York, 1976. (18)Hammes, G.G.,Ed. Technique ofchemistry; Weissberger, A., Ed.; Wiley: New York, 1974;Part 11, Vol. 16.
0 1995 American Chemical Society
1478 Langmuir, Vol. 11, No. 5, 1995
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Yamashita et al.
I
I
10 50 100 f , MHz Figure 1. Representative ultrasonic absorption spectra for aqueous arginine (0.05 M) solutions at 30.0 “C, in the presence of NaCl (0.30M) (0,pH = 11.38)and in the presence of SDS (0.30M) (0,pH = 11.30). The arrows indicate the relaxation frequency.
5
PH Figure 2. Plots of concentrations of A, B, C, and OH- in eq 2 vs pH (A, - * -; B, - - -. , C , - - -. OH-, -). I
Table 1. Ultrasonic Absorption Parameters for Aqueous Solutions of Arginine (0.05 M) in the Presence of NaCl (0.30 M) at Various pH and 30.0 “C s2 cm-l
PH 10.64 10.74 10.84 11.00 10.93 11.17 11.38
B 19 19 17 19 17 18 19
A 29 58 58 66 53 27 17
f,(MHz) 11.8 8.8 8.5 8.4 9.2 10.8 14.7
L
O9.5
guanidine group withpK,z = 12.48.19 The rate ofreaction is controlled by kfandk b . Since the rate-determining step
10.5
11.5
12.5
PH Figure 3. Plots of (2Jtf,)vs pH for aqueous arginine (0.05 M) solution at 30.0 “C. The circles are the experimental data: 0 , in the presence of NaCl (0.30MI; 0, in the presence of SDS (0.30M). The dotted and solid lines are the curves calculated using eq 8 with the values of y2kf, k b , and Kb in Table 2,
respectively.
coo-
HN
*CNH(CH~)~CH< +ZH~O Ha’ NH e
(2)
C
of the base equilibrium of amino group in aqueous solution is the diffusion controlled process2-6 (as described later), kfl is essentially equal to ka. Thus, the rates of the first and second steps depend on k b l and kb2, respectively. Furthermore, from the definition ofK, (=KWkdkb)and the above values of pKal and pK’,2 for arginine, kb2 is larger than 103kbl. Accordingly, the first and second steps can be treated independently. Indeed, only one ultrasonic absorption was observed. Ultrasonic absorption is thus considered due to the first or second step of eq 2. In either case, the relaxation time (z) and maximum relaxation absorption per wavelength, (a’,Uma are given
2nf, (=UT)= y2k&[RNH3+1+ [OH-])
(a’4” =
r-1 =
([R;H,’l
ng u 2( A V)’ 2RT
1 +-[OH-]
+ kb
r-l
(3)
(4) -1
(5)
where RNHs+ is A o r B in eq 2, a’ the excess absorption coefficient, A the wavelength, Q the density, u the sound velocity, AV the volume change of the reaction, and the (19)Hardy, P. M. Chemistry and biochemistry ofthe Amino Acid; Barrett, G. C., Ed.; Chapman and Hall: London, New York, 1985.
subscript max the maximum value. The pH range in which only one excess absorption was observed was between pKal and pKa2 as shown in Table 1. Plots of the concentration of A, B, or C in eq 2 and OH- vs pH are shown in Figure 2. If ultrasonic absorption is due to the first step in eq 2 (base equilibrium of the a-amino group in arginine), [RNH3+1and [RNHzIin eqs 3 and 5 should correspond to concentrations of A and B in eq 2, respectively. Plots of eqs 3 and 4 vs pH should indicate a minimum and maximum as is apparent from Figure 2. If the second step (base equilibrium of the guanidine group in arginine) is the cause of ultrasonic absorption, [RNHs+] and [RNH21 in eqs 3-5 will correspond to [Bl and [Cl in eq 2, respectively, and the values of (2zfr)in eq 3 vs pH should increase with pH, since the extent of increase in [OH-] is greater than that of decrease in [B] with pH as shown in Figure 2. The values of (a’,Vmax in eq 4 must also increase over the experimental pH range due to the pH dependency of B, C, and OH- as shown in Figure 2. The experimental results on (27cfr)and (a’lImax in response t o pH indicate a minimum and maximum as shown in Figures 3 and 4, respectively. Absorption is thus due to the first step in eq 2, i.e., the base equilibrium of the a-amino group in arginine. When the concentrations of amino acid and its cation are expressed as the base dissociation constant Kb and total concentration of amino acid CO,eqs 3-5 may be rewritten, respectively, as follows
Micellar Catalytic Effects on Ionization
z
Langmuir, Vol. 11, No. 5, 1995 1479 Table 2. Kinetic Parameters (in Aqueous Solutions of NaC1) and the Apparent Parameters (in the Micellar Solution)for the Base Equilibrium of Arginine (0.05 M) at 30.0 "Ca y2kf (10' k b ( lo6 AV (cm3 M-l s-l) s-l) mol-l) PKb
1
'T
arginine-water arginine-SDS (0.3 MI "9.5
10.5
11.5
PH Figure 4. Plots of (a'&= vs pH for aqueous arginine (0.05M) solutions at 30.0 "C. The circles are the experimental data: e, in the presence of NaCl(O.30 M); 0, in the presence of SDS (0.30 M). The dotted and solid lines are the curves calculated using eqs 4 and 9 with the values of Kb and AV in Table 2.
(&,1-'[0H-]
16.7 15.20
4.12 3.26 (3.33)
a The values in parentheses are the kinetic values obtained by use of eq 14.
12.5
r-' = Kbl-lC~[OH-]/[Kbl-lCg + (Kbl-l[OH-]
14 1.0 3.2 (1.5) 1.7 (0.7)
f 1)'
+
+ l)[oH-12~b2-21(7)
The third term in the denominator of eqs 6 and 7 is much smaller than the other terms. Equations 6 and 7 may thus be expressed by eqs 8 and 9, respectively
where& is&. Equations 8 and 9 imply that whenKb-lCo >> 1, as in this study, the values of 2 4 r (eq 8) and r-l (eq 9) against pH show a minimum and maximum, respectively, at a definite pH (pH*), as follows
As shown in Figures 3 and 4, the experimental data of ( 2 4 , ) and (a'Alma obtained with variation in pH have a minimum and maximum, respectively, a t pH*. Thus, Kb can be obtained from the experimental pH*. Kb, y2kf,k b , and AV were determined so as to give the best fit for the experimental values of (2nfr)and (a'A),, and are given in Table 2. (B)Ultrasonic Relaxation Absorption in the Presence of SDS Micelles. Ultrasonic absorption was measured while keeping the concentrations of SDS and arginine constant with variation in pH in the range of 10 to 12. Relaxation absorption in all cases was analyzed by relaxation equation 1. Table 3 shows the ultrasonic absorption parameters together with experimental conditions, and typical spectra are given in Figure 1. Plots of (2nf,) and (a'AImaagainst pH are shown in Figures 3 and 4,respectively. Values of pH* ofthe relaxation frequency and relaxation amplitude in the presence of SDS exceeded those in aqueous solution. Since the same reasoning for a n aqueous solution of arginine is applicable to an arginine-SDS system, ultrasonic absorption of arginine in the presence of SDS micelles should be due to the base equilibrium of the a-amino group, i.e., the first step in eq 2, on a micellar surface. Equations 3-9 can be used to determine relaxation parameters. Figures 3 and 4 indicate the experimental data to be consistent with calculated results using eqs 3-9, thus confirming the assignment of absorption to the reaction. The apparent dissociation constant Kb and kinetic parameters (Table 2) obtained in the presence of SDS
Table 3. Ultrasonic Absorption Parameters for Arginine (0.05 M)in the Presence of SDS (0.30 M) at Various pH and 30.0 "C
10.63 10.96 11.30 11.55 11.81 12.12
31
101 150 140 100 32
27 28 28 28 27 27
9.8 7.1 5.8 5.9 7.7 15.2
micelles differ from those in the absence of micelle as follows: (i) forward and backward rate constants are smaller and larger in SDS micellar solution, respectively; (ii)pH* is higher, i.e., p& becomes smaller in SDS micellar solution; (iii) the AV value is smaller in SDS micellar solution. From the solubility of arginine, arginine must be partitioned between the bulk (aqueous)phase and micellar phase. In the micellar phase, arginine is not incorporated into a micellar hydrophobic core and may possibly be on the surface of SDS micelles. Most interactions between arginine and SDS may thus be considered to involve the binding of the associated form of arginine to the anionic surface of SDS micelles as the counterion.20 The coupled reaction mechanism for various other amino acids' is proposed in eq 11, where M indicates the surface of the
SDS micelles and BW the bulk water (bulk phase), (i) R-NHdBW) indicates species B partitioned to the bulk phase in eq 2, (ii) R-NH3+(M) indicates species A partitioned to the micellar phase, and (iii) RNH3+(BW) indicates species A partitioned to the bulk phase in eq 2. Taking into consideration the fact that the association and dissociation of the counterion to and from micelles are fast and assuming RNH3+ as c ~ u n t e r i o n , ~the ~-~~ (20) In the previous study on the methylamine-SDS system,14 a four-state model was proposed in which not only dissociated methylamine (CH3NH3+)but also undissociated methylamine (CH3NH2)were considered to be partially partitioned in SDS micelles. Ultrasonic absorption and pH data indicated the partitioning of undissociated methylamine in SDS micelle to be negligible. The data for the arginineSDS system were examined by the four-state model as in the methylamine-SDS system and the same result was obtained. (21) Kato, S.; Nomura, H.; Zielinski, R.; Ikeda, S. J.Phys. Chem. 1988,92,2305.
Yamashita et al.
1480 Langmuir, Vol. 11, No. 5, 1995
relaxation frequency for the slow process is represented as follows12~24
-
whereX= Kb(M)/Kb(B)and&@) and KdM) are apparent dissociation constants for the equilibria (i) (ii) and (i) (iii) in eq 11, respectively. By introducing the relationships, [ZRNH3+1= [RNH3+(M)I [RNH3+(B)Iand &(MI/ Kb(B) = [RNH,+(M)]/[RNH3+(B)]
-
+
k 2 + k - , (13)
Equation 14 becomes equivalent to eq 3 by substituting
kf and k b for (klh-~(k-1+ kz))/(klkz+ k-lk-2), K Z + k - ~ ,
respectively. Thus, a s shown above, that relaxation parameters can be analyzed by eqs 3-9 in a micellar solution is reasonable. The rate constants (y2k-z, Kz) for the reaction in the micellar solution were obtianed from eq 14 by assuming the rate constants (y2k1, k - 1 ) in the bulk to be equal to those in aqueous solution in Table 2. y2k-2 and kz become 1.5 x lo9 M-l s-l, 0.69 x lo6 s-l, respectively and pKb(M) is 3.33. The following stepwise mechanism has been proposedz5 for the base equilibrium of amines
-
-
where species I1is an ion-paired intermediate in the steady state, I I1 is a diffusion process, and I1 I11 is a n intramolecular proton-transfer process. The forward and backward rate constants for the neutral amino acids were previously shown7 to depend on Kb in micellar solution, while in aqueous solution the forward rate constant was essentially constant and only the backward rate constant depended on Kb. It was thus evident that the ratedetermining step of the stepwise mechanism, shown by eq 15, in SDS micellar solution differs from that in the absence of micelle, i.e., in the former case I1 I11 and in the latter case I I1 in eq 15. Plots of y2k-z and kz of arginine-SDS on p& fell on the lines obtained for neutral amino a ~ i d s - S D S . ~This indicates the rate-determining step of base equilibrium of arginine to be the same intramolecular proton-transfer process as with the neutral amino acid in SDS micellar solutions and the reaction mechanism of base equilibrium of a-amino group of amino acid to be determined primarily by the electrostatic effect
-
Table 4. Ultrasonic Absorption Parameters for Aspartic Acid (0.04 M)in the Presence of DAC (0.20 M)Micelles at Various pH and 30.0 “C
1.57 1.81 2.00 2.20 2.54 2.67
26 25 26 25 26 25
24.3 14.4 11.2
11.1 13.9 13.1
between a-NH3+or a-COO- and anionic surface of SDS micelles. Whether R is hydrophobic, or polar, uncharged or positively charged, is not important. Differences in pKb in micellar and aqueous solution are denoted by ApKb. ApKb is considered as an index of the interaction of solubilized molecules and micelles a t equilibrium. ApKb of arginine is about 0.8, this being larger than previouslydetermined7for neutral amino acids except tryptophane with the lowest solubility. The addition of only one cationic group to a neutral amino acid thus clearly has a n effect on pKb but not on the reaction mechanism of the base equilibrium of an a-amino group in micellar solution. AVvalues (15.2mL, 16.7 mL) ofthe arginine-SDS and the arginine-aqueous systems are smaller than those (20.8mL, 25.8 mL) of neutral amino acid-SDS and neutral amino acid-aqueous systems, respectively. The addition of one cationic charged group decreases AV a t base equilibrium in spite ofthe absence or presence of a micelle. 2. Protolysisof AcidicAmino Acid in the Presence of DAC micelles. Ultrasonic relaxation absorption was measured for the aspartic acid (acidic amino acid)-DAC (cationic surfactant) system. Relaxation absorption depended strongly on the concentration of aspartic acid and pH. The results are shown in Table 4 along with absorption parameters. Ultrasonic relaxation absorption in the aromatic carboxylic acid-DAC system was previously assigned to the protolysis of the aromatic carboxylic acid.26 The data of the present study are consistent with those for aromatic carboxylic acid-DAC systems. Aspartic acid in an experimental pH range is only slightly soluble in water, and thus all aspartic acid may probably be partitioned into cationic micelles. Relaxation absorption is thus considered to be a function of the protolysis of the carboxyl group of aspartic acid on the surface of DAC micelles. Aspartic acid has two carboxyl groups, a-carboxyl and /I-carboxyl, and the protolysis reactions in eq 16 occur. Relaxation absorption spectra for aspartic acid.OOC-CH~H’-OOCCH~CH’.~HOOC-CH~CH NHJ’ NH’
-
(22) Dickmann, S . Ber. Bunsenges. Phys. Chem. 1979,83, 528. (23) The ultrasonic relaxation absorption for the base equilibrium of amino group has been measured in the frequency range of several megahertz as shown in refs 2-6, while that for the association and dissociation of counterion to and from micelles was observed at higher frequency than 100 MHz in ref 22. (24) Czerlinski, C. H. Chemical Relaxation; Marcel Dekker: New York, 1966. (25) Eigen, M.;Maass, G.; Schwarz, G.Z. Phys. Chem. 1971,74,319.
10 22 35 34 14 12
H
A
,NH.’
‘COO
‘COO
( 16)
‘COOH H’ k a z
‘I
6’
C’
DAC can all be expressed by a single relaxation equation. The cause of excess absorption may be either protolysis of ,&carboxyl group (A’ B’) in eq 16 o r a-carboxyl group (B’ C’). In either case, (2nfr)and (a’&,ax can be given by replacing Kb, [RNH3+],[OH-], and [RNHz] in eqs 3-9 to Ka, [RCOO-I, [H+l, and [RCOOH], respectively. The kinetic data were determined so as to give the best fit for the experimental values of (2nfr)and (a’A),, as shown in Figures 5 and 6 and are listed in Table 5. In the case of aromatic carboxylic acid solubilized in cationic surfactants, the dissociation of the carboxyl group was f a ~ i l i t a t e d . ~ ~ , ~ ~
-
-
(26) Harada, S.; Yamashita, T.; Yano, H.;Higa, N.; Yasunaga, T. J . Phvs. Chem. 1984.88. 5406. 727) Harada, S.f Yano, H.; Yamashita, T.; Nishioka, S.; Yasunaga, T.J . Colloid Interface Sci. 1986, 110, 272.
Micellar Catalytic Effects on Ionization
Langmuir, Vol. 11, No, 5, 1995 1481 Table 5. Kinetic Parameters for the Protolysis of Various Acids in the Presence of DAC (0.20 M) Micelles at 30.0 "C
-02
Aspartic Acid (0.04 M) 5.7 7.8
3.9 2.9
m-Nitrobenzoic Acid (0.05 MP 7.0 12.0 m-Chlorobenzoic Acid (0.05 M)"
4.0
u 2 3 O1 PH Figure 5. Plots of (2nf,) vs pH for aqueous aspartic acid (0.04 M) solution in the presence of DAC (0.20 M) at 30.0 "C. The
circles are the experimental data. The solid line is the curve calculated by replacing Kb and [OH-] in eq 8 to K, and Hfwith the values of y2kf,kb, and K, in Table 5.
u 2 3
O1
PH
Figure 6. Plots of (a'&= vs pH for aqueous aspartic acid (0.04 M) solution in the presence of DAC (0.20 M) at 30.0 "C. The circles are the experimental data. The solid line is the curve calculated by replacing Kb and [OH-] in eqs 4 and 9 to K, and Hfwith the values of K, and AV in Table 5.
This is probably due to not only the electrostatic effect of cationic micellar surface but also some additional one, hydrogen bonding, for example. The similar micellar effects probably bring about the acceleration of dissociation of a- or ,&carboxyl group of aspartic acid in the DAC micellar solution. Indeed, by the addition of DAC (0.2 M) to aqueous aspartic acid (0.005 M) solution, pH decreased from 3.22 to 2.77. On the other hand, pH under the addition of ethylamine hydrochloride (0.2 M) to aqueous aspartic acid (0.005 M) solution is 3.24. This means that aspartic acid interact with the DAC micelles and the dissociation of aspartic acid in DAC micelles is enhanced. The value of pKa obtained by the analysis of the observed relaxation absorption is 2.8 as shown in Table 5, while
a
4.0
13.5
2.8
0.9
2.6
1.2
3.0
1.2
Reference 26.
the values of pKa of aspartic acid in the absence of a micellelg are 1.88 (pKal)for the a-carboxyl group and 3.65 (pKap)for the P-carboxyl group. The observed relaxation absorption in the DAC micellar solution is thus assigned to the protolysis of @-carboxylgroup. The values of y2kf, kb of acids have depended on pKa.26,27The values of y2kf, kb of aspartic acid are similar,to the values of those of aromatic carboxylic acids indicating similar pKa in DAC micellar solutionsp6as shown in Table 5. The ApK, value of aspartic acid is small compared with those values of aromatic carboxylic acids as shown in Table 5. AV of protolysis in the cationic micellar solution has been obtained to be dependent on pKa and the kind of m i ~ e l l e . The ~ ~ ,value ~ ~ of AV of the /3-carboxyl group of aspartic acid obtained in this paper is 7.8 mL and the value of AV is about 12-13 mL for aromatic carboxylic acids with similar pKa in the DAC micellesz5as shown in Table 5. The value of AV depends on many factors. One of them has been reported by Kauzmann et a1.28 as follows: Changes in the partial specific volume of protolysis depend on the nature of group R attached to the carboxyl group; Le., if R is positively charged, the volume change is 6 or 7 mL, and if uncharged, it is from 10 to 14 mL. Thus, as one or two of the factors in the causation of the difference in the value of AV between aspartic acid and aromatic carboxylic acid, the following will be considered: (1)The P-carboxyl group in aspartic acid can form the intramolecular interaction with a-NH3+,but the carboxyl group in the aromatic carboxylic acid can not. (2) The degree of intermolecular interaction between ,&carboxyl group in aspartic acid and the abundant NH3+head groups of DAC differ from that between the carboxylgroup in aromatic carboxylic acid and the abundant NH3+ head groups of DAC. LA9306903 (28) Kauzmann, W.; Bodanszky, A.; Rasper, J. J.Am, Chem. SOC. 1962, 84,177.
