6959
Proton Transfer and Base Hydrolysis Reactions of Some trans-Bis( dimethylglyoximato)cobalt ( 111) Complexes' James P. Birk, Pweh Boon Chock, and Jack Halpern
Contribution from the Department of Chemistry, University of Chicago, Chicago, Illinois 60637. Received July 8, 1968 Abstract: The equilibria and kinetics of the reversible deprotonation of truns-Co(DH)z(NOz)Br- (where DH- = HON=C(CH3)C(CH,)=NO-) and of several other trans-bis(dimethylglyoximato)cobalt(III) complexes, exemplified by the reaction, Co(DH)z(N02)Br- OH- F! Co(DzH)(N02)Br2- HzO,have been examined. Equilibrium constants, determined spectrophotometrically, ranged from 31 M-' for Co(DH)Z(N02)Br- to 1.8 x 102 M-1 for C O ( D H ) ~ ( N O ~ ) ~The - . rate of approach to equilibrium, determined by the temperature-jump or stopped-flow
+
+
+
method, obeyed first-order kinetics with the apparent first-order rate constant given by kabsd = k-l kl[OH-1, where kl and k-l are the rate constants of the forward and reverse reactions, respectively. Values of kl ranged The . hydrolysis from 1.2 x 106 M-1 sec-1 for C O ( D H ) ~ ( N O ~ t) o~ -1.3 X lo6 M-' sec-l for C O ( D H ) Z ( N H ~ ) ~ + H ~ found to be accelerated by OH-, according to a rate law which of Co(DH)z(N02)Br- to C O ( D H ) ~ ( N O ~ ) Owas can be expressed as -~ ( [ C O ( D H ) ~ ( N O ~ ) B ~ [Co(D2H)(No~)Br~-l)/dr -I = ka[Co(DH)~(N02)Br-l kt,[Co(DzH)sec-l. Neither the kinetic behavior nor the results (N02)Br*-], where k, = 3.5 x 10-5 sec-1 and kb = 1.1 X , S N ~ C mechB of hydrolysis experiments in the presence of added NOz- serve to distinguish between sN2, S N ~ C Band anisms for the base hydrolysis path.
+
+
of the reaction of sodium carbonate and perchloric acid. The solutions were analyzed by ion-exchange titration. Sodium hydroxide solutions were Fisher Analyzed 1.00 or 0.10 M NaOH. All other materials were of reagent grade. Distilled water was used in the preparation of all solutions. Kinetic Measurements. The deprotonation reactions were studied in aqueous solution at 15.0" and 0.10 M ionic strength using k1 a temperature-jump apparatus manufactured by Messanlagen Co"'(DH)zXY OHCo"'(D2H)XY HzO (1) Studiengesellschaft, Gottingen, Germany. In this apparatus, a 0.05k- I pF capacitor was charged to 41 kV and then discharged through a solution previously equilibrated at 10.0", causing the temperature where DH is the dimethylglyoximate anion, HON= to rise by 5.0" in 2 psec or less. The relaxation to the new equiand Co"'(DHhXY = CoC(CH,)C(CH,)=NO-, librium position was then observed spectrophotometrically, at (DH)z(NO&-, CO(DH)~(CN)Z-, C O ( D H ) ~ ( N O , ) B ~ , wavelengths in the range 400-500 mp, on a Tektronix-type 564 or CO(DH)~(NH&+. Spectrophotometric equilibrium storage oscilloscope. A typical temperature-jump oscillogram is reproduced in Figure 1. measurements on these reactions, along with kinetic Some deprotonation reactions were also studied in 80 vol. % m e a s u r e m e n t s made by the temperature-jump method, methanol at 25.0", in an attempt to extend observations to comare described. These studies have relevance to the plexes which are only slightly soluble in water. Because of comeffect of internal hydrogen bonding on the rates of plications associated with the nonaqueous solvent, measurements on proton-transfer reactions. these solutions were made with a Durrum stopped-flow spectrophotometer rather than with the temperature-jump apparatus. The second, much slower, absorbance change corThe kinetics of the base hydrolysis of Co(DH)2(N02)Br- (reacr e s p o n d s to the base hydrolysis of the cobalt(II1) comtion 2) were determined spectrophotometrically (at 360-400 mp) plex, exemplified by the reaction at 15.0" and 0.10 M ionic strength using a Beckman DB spectrophotometer equipped with a recorder. Co(DH),(NO?)Br- OH- +CO(DH)~(NO~)OH- Br- (2a) Equilibrium Measurements. Equilibrium quotients for the proton-transfer reactions (eq 1) were determined spectrophotometrior cally, by measuring the absorbancies of solutions containing a given Co(D2H)(NO2)Br2- H 2 0 +CO(DH)~(NO~)OH- Br- (2b) total concentration of the CoI1I(DH)zXYcomplexes and various concentrations of added OH-. To minimize effects due to hydrolyA kinetic study of this reaction is described. sis of the complex, the sodium hydroxide was added immediately before the spectra were recorded with a Cary 14 spectrophotometer. Equilibrium quotients, K , defined by Experimental Section
he addition of OH- to solutions of trans-bis(dimethylglyoximato)cobalt(III) complexes results in two distinct spectral changes. The first, which is reversible and very rapid, is attributable to the deprotonation reaction
T
+
+
+
+
+
+
Materials. The following cobalt(II1) complexes were prepared2 )~],~ by published procedures : N ~ [ C O ( D H ~ ) ( N O ~H[Co(DH)2(NOZ)Br].HzO, [CO(DH)~(NOZ)(HZO)],~ K[Co(DH)z(CN)z] 1.5Hz0,5[CO(DH)Z(NH&]C~. ~ H z O[Co(DH)z(py)Br],' ,~ [Co(DH)z( p ~ ) C l land , ~ [CO(DH)Z(P(C,H~)~)BTI.~ Stock solutions of sodium perchlorate, used to maintain constant ionic strength, were prepared from the triply recrystallized product (1) This research was supported by grants from the National Science Foundation and the Advanced Research Projects Agency. (2) We are grateful to Drs. P. W. Schneider and D. N . Hague for preparation of some of these complexes. (3) L . Tschugaeff, Chem. Ber., 41, 2230 (1908). (4) A . V. Ablov, Dokl. Akad. Nauk S S S R , 91, 1019 (1954). ( 5 ) N. Maki, Bull. Chem. SOC.Japan, 38, 2013 (1965). (6) L. Tschugaeff, 2.Anorg. Allgem. Chem., 46, 160 (1905). (7) L. Tschugaeff, Chem. Ber., 40, 3498 (1907).
K
=
[COII'(D~H)X~][COI"(DH)~XY]-~[OH~~ (3)
were determined from the linear plots of (A/[CoItot [OH-I-I, shown in Figure 2, fitted to the relation
(A/[ColtOt -
€DH)-~ = (CD
-
c D ~ ) - ~us.
+ K-'[OH-],,')
EDH)-~(~
(4)
where A is the absorbance of the solution, ED and EDH are the measured extinction coefficients of ColI1(DzH)XYand CO"I(DH)~XY, respectively, [Co]t,,t = ([CoIII(DH)2XY] [CoIII(DzH)XYI), and [OH-],, is the equilibrium OH- concentration (Le,, the concentration of added OH-, less that consumed by reaction with CO"I(DH)~XY),calculated by an iterative procedure.
Birk, Chock, Halpern
+
trans-Bis(dimethylglyoximato)cobalt(IIZ)Complexes
6960
Figurc 1. Saniple tcmprraturc-jump oscillogram roar the reaction C N D H ) , ( C N ) ? ~ ~ OH- t+; CO(D?H)(CN)~~- H?O: 7.2 x 10-4 M CO(DH)ACN)~-; 0.05 M OH-; I S " ; 0.10 M ionic strength. Time scale: 1 0 0 , secldivision
+
+
Results and Discussion Deprotonation Reactions. Addition of sodium hydroxide to aqueous (or aqueous methanolic) solutions of various bis(dimethylglyoximato)cobalt(lIII) complexes, i e . , Co(DHk(NO,)z-, Co(DHk(NO,)Br-, Co(DHMCN)z-, Co(DHk(NH,k+, Co(DHX(py)Br (PY = pyridine), Co(DH)?(py)CI, and Co(DH)*(PPhn)Br, resulted in rapid reversible changes in the visible and near-ultraviolet spectra. This effect had been previously observed for a number of such complexes, e.g., Co(DHk(NO&-, Co(DH)z(NOkNHa, and Co(DHk(NH&+, by Ablov and Fillip0v,8~and attributed by them t o the reversible loss of a bridging proton by the dimethylglyoxime ligands. This process is exemplified for C o ( D H h ( N 0 k B r by eq 5 or, more generally, by eq 1.
Figure 2. Plots of (A/[Col,, us. [OH-].,-' according toeq 4: V, Co(DH)dCN)1; 0, Co(DH).(NO2)Br-; 0, CNDH)? (NO&; A, Co(DH)dNO& in 80 vol. Zmethanol.
the presence of only two absorbing species; and (iii) plots of (A/[Co],,, - eDH)-' us. [OH-]-' (Figure 2) are linear in accord with eq 4 derived for an equilibrium involving the loss of a single proton from the cobalt complex. Kinetic data, obtained by the temperature-jump technique, for Co(DH)%(NO&, Co(DHX(NO1)Br, Co(DH)*(CN),-, and CO(DH)~(NH&+,are summarized in Table I, which also includes some data for Co(DH)?(NOz)?- in 80 vol. % methanol, obtained by the stoppedflow method. In each case, the approach to equilibrium obeyed the pseudo-first-order rate law, derived for the reversible reaction described by eq 1, ;.e. -d In ( A , - AJdt
=
kobsd= k-I
+ kl[OH-]
(6)
where A is the absorbance at the indicated time. Values of kl and k , , determined from the slopes and intercepts of linear plots of kobJdus. [OH-], are listed in Table 11, along with values of K, determined both from the kinetic data (K = k,/k-,) and spectrophotomer 1*trically, as described earlier. The discrepancy between the two values of K for CO(DH)~(NO& in 80% methanol is probably not unreasonable, since the reaction was so rapid (tl,* = 1-7 msec) as to be barely within the range accessible to the stopped-flow technique used, resulting in a rather low signal-to-noise ratio. For L J Co(DH)2(N0z)Br-, the best value of K yielded by the [CdD2HXNOJ2BrlZ~ kinetic data was 61 M - ' ; however, the data could also be accommodated reasonably well by the spectroThis conclusion is supported by our observations that (i) the spectral changes for complexes such as CO(DH)~- photometrically determined value of 31 M-', which is considered more accurate. Attempts to determine K (NO& and Co(DH),(CN),- are quantitatively respectrophotometrically for CO(DH)*(NH,)~+ were versible (reacidification of a solution to which OH- has thwarted by irreversible absorbance changes of unbeen added restores the spectrum of the original soludetermined source. However, this equilibrium quotient tion); (ii) spectra of solutions of Co(DH),(N02)%-, has been determined at 25" by Ablov and Fillipov to be containing different concentrations of OH-, exhibit 1.6 X l o a M - ' ; the discrepancy from our kinetic value two isosbestic points at 345 and 363 mp, in agreement of 9.0 X IOz M-I is in the direction expected for the with Ablov and Fillipov8 (342 and 365 mp), indicating difference in temperature. (8) A. V. Ablov and M. P. Fillipov, Rum. 3. h o r g . Chem., 5, 1311 A possible complication in the study of the deprotona(1960). tion of Co(DH)z(NOz)Br- arises from the irreversible (9) A. V. Ablov and M. P. Fillipov, ibid., 7,525 (1962). Journal of zhe American Chemical Socieiy 1 90:25
/ December 4, I968
6961 Table I. Kinetic Datan for the Reactions of Various Bis(dimethylglyoximato)cobalt(III) Complexes with OH- (eq 1) Co"'(DH)zXY complex Co(DHMCNb-
Co(DH)*(NOz)Br-
104[Co"'(DH)~XY]tot, 1OZ[OH-]added, M M 9.5 9.0 5.6 7.2 5.1 8.7 9.0 5.1 7.2 4.6 4.8 4.3 3.9 3.7 4.4 5.3 8.4 5.2 6.5 3.5
Co(DH)dNH&+
l@akobw&b
3.92 1.94 0.97 0.474 0.191 0.092 5.85 4.92 4.89 3.93 3.93 2.94 1.95 0.96 0.45 3.84 2.93 1.96 1.95 0.98 0.97 0.49 0.19 0.097
8.56 f 0.10 5.35 f 0.57 3.28 f 0.14 2.42 f 0.12 1.56 f 0.05 1.52 f 0.02 9.9 9.26 f 0.36 6.50 f 0.36 7.11 i 0.18 7.30 i 0.08 5.18 f 0.06 4.14 f 0.06 2.82 f 0.05 2.87 f 0.04 5.56 f 0.31 4.78 f 0.16 2.84 f 0.17 2.66 1.76 f 0.06 1.54 f 0.06 1.56 f 0.05 1.02 f 0.03 0.62 f 0.05 0.64 f 0.03d 0.35 f 0.03d 0.23 f 0.01d 0.18 f 0.01d 0.15 f 0.01d 0.14 f 0.01d 0.11 f 0.01d 8.11 f 1.03 4.99 3.50 f 0.37 2.33 f 0.10 2.12 f 0.10
4.00 2.00 1.00 0.500 0.200 0.100 5.91 4.95 4.93 3.95 3.95 2.96 1.96 0.963 0.456 3.89 3.00 2.00 2.00 1.OO 1 .oo 0.50 0.20 0.10 2.00 1 .oo 0.50 0.50 0.20 0.20 0.10 0.50 0.35 0.25 0.10 0.05
5.5
Co(DH)z(NOz)z-
1O2[OH-]e,, M
3.5 5.2 2.5 2.0 0.7 2.0 0.7 2.0 0.7 0.7 5.6 8.9 10.7 4.9 4.7
...
...
... ... ...
...
0.453 0.283 0.179 0.0774 0.0366
SeC-1
( 10-akkobad)oalod)c seC-1
8.8 5.0 3.2 2.3 1.7 1.5 9.2 8.1 8.0 6.8 6.8 5.6 4.4 3.2 2.6 5.3 4.3 3.1 3.1 2.0 2.0 1.4 1.0 0.91 0.63 0.36 0.23 0.23 0.14 0.14 0.12 7.5 5.3 3.9 2.5 2.0
* Uncertainties cited are the average deviations of duplicate determinations on 0 Ionic strength maintained at 0.10 M with NaCIOd; 15". methanol, the same solution, c Calculated from eq 6 using the values of kl and k-1 in Table 11. d 25"; in 80 vol. Table 11. Rate Constants and Equilibrium Quotients for the Reactions of Various Bis(dimethylglyoximato)cobalt(III) Complexes with OHComplex
10-6kl, M-l sec-'
Co(DH)dCN)zCo(DHb(N0z)BrCo(DH)z(NOz)zCo(DH)z(NO&Co(DH)z(NHdz+
1.90 f 0.07 1.23 f 0.04 1.18 f 0.07 0.27 =k 0.09d 13.3 f 1.5 a Ionic strength maintained at 0.10 M with NaClOn; 15". K = Figure 2. d At 25" in 80 vol. methanol. e At 25", from ref 9.
hydrolysis reaction of this complex (eq 2), the kinetics of which are described below. Although much faster than the hydrolysis of the other complexes studied, this reaction was nevertheless slow enough to permit the temperature-jump measurements on the protontransfer reaction to be performed. Successive temperature-jump experiments on the same solution at various stages of hydrolysis were characterized by a decreasing total absorbance change but by an essentially constant relaxation time (kobsd-l), yielding constant values of kl and k-1. This is consistent with our observations that the absorbance change accompanying the deprotonation of C O ( D H ) ~ ( N O ~ ) OinH ~the wavelength range of these measurements (400-500 mp) is negligible. Co(DH)2(py)Br, Co(DH),(py)Cl, and Co(DH)*(PPh3)Br also exhibited reversible absorbance changes
10-ak-l, sec-'
10-zK,b M-1
1.35 f 0.13 2.0 f 0 . 4 0.80 f 0.13 0.09 f 0.01d 1 . 5 f 0.4
1.4 f 0.2 0.61 f 0.15 1.5 f 0.3 2.9 i 0.4d 9.0 i 3.0
(kI/k-l).
lO-W,c M-1 1.18 0.31 1.29 1.80d (15.8)e
Determined spectrophotometrically using eq 4 from data in
on addition of OH-. However, because of limited solubility in water, studies on these complexes could not be made by the temperature-jump method, and the reactions in 80 methanol proved too fast for stoppedflow measurement. Influence of Internal Hydrogen Bonding on the Rates of Proton Transfer. Eigen'O has discussed in some detail the influence of internal hydrogen bonding on the rates of proton-transfer reactions. The formation of a chelating internal hydrogen bond (such as that present in bis(dimethylglyoximato)cobalt(III) complexes) apparently interferes with hydrogen bonding between the donor proton and the acceptor (OH- or solvent), thereby decreasing the rate of proton transfer. Thus, whereas thermodynamically favorable proton-transfer (10) M. Eigen, W. Kruse, G. Maass, and L. D. Maeyer, Progr. Reaction Kinetics, 2 , 285 (1964).
Birk, Chock, Halpern
1 trans-Bis(dimethylglyoximato)cobalt(IIZ)Complexes
6962
reactions between -NH or -OH donors and OH- normally proceed with rates close to diffusion-controlled (k IO9-lO1' M-' sec-l), the transfer of a proton protected by internal H-bonding is generally appreciably slower. Rate constants reported for such protontransfer reactions to OH- are typically in the range 4 X IO6 to 4 X IO6 M-l sec-' for 0 - H . . . N donors and 2 X IO7 to 7 X lo8 M-I sec-' for 0 - H . . S O donors. Secondary influences, causing variations within these ranges, include charge and steric effects. Our results are generally consistent with this pattern and interpretation. The values of the rate constants ( k l )for proton transfer to OH- found by us for various CO"'(DH)~XY complexes, which lie in the range 1.2 X IO5 to 1.3 X IO6 M-' sec-I (corresponding to 6 X lo4to 6 X IO5 M-' sec-lperproton in CO~II(DH)~XY), are among the lowest yet reported for such proton-transfer reactions and are well below the previously observed values for proton transfer from 0-H. .O donors to OH-. This is consistent with the generally accepted presence of very strong H bonds in bis(dimethylg1yoximato)cobalt(III) complexes, and with the view that the energy required to "liberate" the proton from the internal H bond (i.e., to break the internal H bond) makes an important contribution to the barrier for proton transfer. The principal factor responsible for the variation of kl, within the range of observed values, appears to be the charge of the cobalt(II1) complex. Thus, in aqueous solution, the value of kl for Co(DH)z(NH&+ (1.3 x lo6 M-l sec-I) is appreciably higher than the values for the three anionic complexes Co(DH)z(N02)2-, Co(DH)Z(NOz)Br-, and Co(DH),(CN),-, all of which lie within the relatively narrow range, 1.2 X IO6 to 1.9 X IO5 M-' sec-l. The decrease in k1 for Co(DH)z(NO&- from 1.2 X IO5 to 2.7 X lo4 M-' sec-' in going from water to 80% methanol as solvent may reflect the reduced effectiveness of methanol (possibly because of its lower acidity), as compared with water, as a bridging medium for proton transfer. Base Hydrolysis of Co(DH),(NO2)Br-. The kinetics of the hydrolysis reaction of Co(DH)z(NOz)Br-,
-
i.e.
+ H20
-
where K is the equilibrium quotient of reaction 5, defined by eq 3. In accord with eq 9, the hydrolysis reactions, examined at 15" and 0.10 M ionic strength, exhibited pseudo-first-order kinetic behavior, yielding the values of khyd listed in Table 111. Using K = 31 M-l, these Table 111. Kinetic Data for the Base Hydrolysis of Co(DH)2(N02)Br-
1.22 1.06 1.16 1.37 1.37 1.30 ~
9.88 4.90 2.38 1.11 0.488 0.183
8.4 6.8 4.9 3.1 1.8 0.92
~~
Ionic strength maintained at 0.10 Mwith NaC104; 15". Computed from eq 9 using the values of k,, kb, and K given in the text. a
+
data yielded linear plots of kbyd(1 K[OH-I) us. [OH-] which were fitted to eq 9 to yield the values ka = 3.5 X sec-l and kb = 1.1 X sec-'. The above sec-') detervalue of ka agrees with that (4.0 X mined by Hague and Halpern" at 1.0 M ionic strength. The kinetic behavior of the OH--dependent path for the hydrolysis of Co(DH)z(NOz)Br- is consistent with, and fails to distinguish between, three alternative mechanisms, analogous to those that have been proposed for the corresponding base hydrolysis reactions of acidopentaamminecobalt(II1) complexes, which exhibit similar rate laws.'* These are (i) an s N 2 mechanism, corresponding to eq 2a, (ii) an S N ~ C mechanism, B corresponding to the rapid preequilibrium 5, followed by the sN2 hydrolysis of the conjugate base according to eq 2b, and (iii) an SNICB mechanism, corresponding to the rapid preequilibrium 5, followed by the hydrolysis of the conjugate base according to the SNI mechanism
+
Co(DzH)(NOp)Br2-+Co(D2H)(N02)Br- (rate determining) (10) Co(DzH)(N02)-
+
8.64 6.90 4.07 2.86 1.95 0.96
+ HzO
----f
Co(DH)z(NOz)OH-
(11)
In the case of the base hydrolysis of pentaamminecobalt(II1) complexes, evidence for an SNICB mechahave previously been examined in acid solutions by nism has been provided by the observation12of comHague and Halpern." We have extended this investipetition between water and a variety of anions (NOz-, gation to the region of higher pH (up to 0.1 M OH-) NCS-, etc.) for the proposed five-coordinate interand found the rate of hydrolysis to increase with the mediate, Co(NH3)?(NH2)+. Thus, the base hydrolysis OH- concentration according to the rate law of C O ( N H ~ ) ~ X ~ + = C1-, Br-, I-) in the presence (X-d[ Co( DH)z(NOz)Br-]tot/dt = of 1 M NOz- yields about 445% C O ( N H ~ ) ~ ( N O ~ ) ~ + , this yield being nearly independent of the identity of -~ ( [ C O ( D H ) ~ ~ N O ~ ) B ~ - I X, and proportional to the NOz- concentration. A [Co(DzH)(N02)Br2-])= k,[Co(DH),(NOz)Br-] similar experiment, attempted by us for CO(DH)~(N02)Br-, yielded negative results. Thus. no Cokb[Co(D2H)(N02)Br'-1 = (DH)2(N0z)z- product could be detected spectropho(ka ~~K[OH-I)[C~(DH)Z(NOZ)B~-I~,~/~ 1 tometrically when the hydrolysis of Co(DH)z(NOz)BrNOH-]) (8) was effected in a solution containing 0.2 M OH- and 1.0 to 4.0 M NOz-. In connection with the interpretaHence tion of this result, it should be noted that NOz- does -d In [ C O ( D H ) ~ ( N O , ) B ~ - ] ~=, ~khy, /~~ = not react appreciably with C O ( D H ) ~ ( N O ~ ) O Hnor ~, does C O ( D H ) ~ ( N O ~ undergo ) ~ appreciable hydrolysis, (ka kbK[OH-l)/(l K(OH-l) (9) under the conditions and during the duration of these Co(DH)2(N02)Br-
CO(DH)~(NO~)OH~ Br-
+
+
(7)
+
+
+
+
(11) D. N. Hague and J. Halpern, Inorg. Chem., 6 , 2059 (1967). The basis for the assignment of trans configurations to Co(DH)z(NOz)Br- and related complexes is discussed in this reference.
(12) D. A . Buckingham, I. I. Olsen, and A. M. Sargeson, J . A m . Chem. SOC.,88, 5443 (1966).
Journal of the American Chemical Society / 90:25 / December 4, 1968
6963
experiments. It should further be noted that, from the standpoint of the charge of the proposed S N ~ C B intermediate, the competition of NOz- with water might be expected to be less favorable for Co(D2H)(N02)-
than for C O ( N H ~ ) ~ ( N H ~ )In ~ +any . case, the mechanistic ambiguity relating to possible contributions from S N ~S, N ~ C Band , SN~CB paths to the base hydrolysis of CO(DH)~(NO~)B~remains to be resolved.
Mechanism for the Proton-Transfer Reactions of a Peptide Hydrogen in Copper ( 11) Triglycine Gordon K. Pagenkopf' and Dale W. Margerumz Contribution f r o m the Department of Chemistry, Purdue University, Lafayette, Indiana 47907. Received June 26, 1968
Abstract: The rate of interconversion of CuH-zL- and CuH-lL (where L is triglycine and the protons are ionized from the peptide linkages) is much slower than normal acid-base reactions. The rate constant for the reaction of H 3 0+ with CuH+L- is 4.9 X lo6M-l sec-', and the value for the reaction of OH- with CuH-lL is 2.5 x 104 M-1 sec-1. The reason proposed for the slower rates is the necessity to break and rearrange coordinate bonds to copper. The proton addition is general acid catalyzed and proton removal is subject to base catalysis. Two reaction paths are proposed. The copper-imide bond in CuH-?L- is not easily broken and tends to dissociate after the addition of a proton to the peptide nitrogen.
C
opper(I1) complexes of short-chain polypeptides promote the ionization of the peptide hydrogens. 3-g In the present work the speed and mechanism of this type of proton-transfer reaction is examined for copper(I1) triglycine which forms CuH+L (or CuA) and CUH-~L- (or CUB-) with the loss of one and two protons, respectively, from triglycine (L-). The pK, values for these ionizations are 5.4 and 6.6, and from infrared studies in D20 it was concluded that the structure of CUH-~L-can be represented by I.9 The crystal structures of disodium tetraglycinocuprate(I1) and of sodium triglycinocuprate(I1) (NaCuH-L.H20) have
0, I \ (-1
attached to two copper atoms and each copper atom is bonded to two different peptide molecules. The copper is coordinated to the carbonyl oxygen of the peptide link.6 Structures have been suggested for CuL+ and for CUH-~Lin solution where the -NHgroup of the peptide link is c ~ o r d i n a t e d . ~Our kinetic evidence suggests that the proton-transfer reactions between CuH-Land CuH-lL involve coppernitrogen dissociation and that CuH-1L has carbonyl oxygen coordination. The reaction of H30+with CuH-L- to form CUH-~L is much less than the diffusion-controlled rate. l 1 This proton-transfer reaction is general acid catalyzed and can be studied in the presende of ethylenediaminetetraacetate ion (EDTA) which does not react as a nucleophile with CuH-L- but does react with CuH-lL. The reactions of CuH-'L with bases were studied directly using a pH-jump method. Experimental Section Kinetic runs were followed using stopped-flow spectrophotometers. One instrument was described earlierI2 and the other was a
0
Durrum-Gibson stopped-flow, Durrum Instrument Corp., Palo Alto, Calif. Both instruments were thermostated at 25.0 =t0.1 The reaction between copper triglycine and EDTA was followed the peptide nitrogens coordinated to ~ o p p e r . lo ~ , ~ ,by the disappearance of CuH-2L- which has a much higher molar When copper triglycine is crystallized from acid absorptivity than CuH-lL or CuEDTA2- at 555 mp. This wavelength was used for reactions at pH 6.0-7.5 where a significant persolutions to give CuLCl. 1.5Hz0, the peptide chain is centage of the copper triglycine is present as CuH-,L. Above pH (1) Abstracted from the Ph.D. Thesis of G . K. P., Purdue University, 7.5 the reactions were followed at 235 mp. June 1968. Triglycine was obtained (chromatographically homogeneous) (2) Address correspondence to this author. from Mann Research Laboratories (New York, N. Y.)and was used (3) H. Dobbie and W. D. Kermack, Biochem. J . , 59, 246, 257 (1955). without further purification. A 9.85 X M stock solution of (4) A. R. Manyak, C. B. Murphy, and A. E. Martell, Arch. Biochem. Cu(ClO& was prepared from the twice-recrystallized salt and stanBiophys., 59, 373 (1955). dardized against EDTA. The copper(IIfitrig1ycine complex was (5) T. Cooper, H. C. Freeman, G. Robinson, and J. Schoone, Nature, prepared for each series of reactions by mixing copper(I1) and tri194, 1237 (1962). (6) H. C. Freeman and M. R. Taylor, Proc. Chem. Soc., 88 (1964). glycine using a 2Z molar excess of triglycine. The concentration (7) W. L. Koltun, R. H. Roth, and F. R. N. Gurd, J . Biol. Chem., of copper(I1) triglycine used for a kinetic run ranged from 2 x
I (CuH-zL-)
O,
238, 124 (1963). (8) M. K. Kim and A. E. Martell, Biochemisfry, 3, 1169 (1964). (9) M. K. Kim and A. E. Martell, J . A m . Chem. Soc., 88, 914 (1966). (10) H. C. Freeman, J. C. Schoone, and J. G. S h e , Acta C r y s f . , 18, 381 (1965).
Pagenkopf, Margerum
(11) G. K. Pagenkopf and D. W. Margerum, J . A m . Chem. Soc., 90, 501 (1968). (12) D. W. Margerum and J. D. Carr, ibid., 88, 1639 (1966).
Proton Transfer of Peptide Hydrogen in Copper(Il) Triglycine
