4 08 (4)
A.
Rainis and M. Szwarc
(8) K. D. Gundermann, H. Fiege, and &. Klockenbring, Justus Llebigs Ann. Chem., 738, 140 (1970). (9) W. R. Seitz, W. W. Suydam. and D. M. Hercules, Anal. Chem., 44, 957 (1972). (10) W. Nonidez, private communication. (1 1) H. H. Seliger In “Light and Life.” W. D. McElroy and B. Glass, Ed., Johns Hopkins Press, Baltimore, Md., 1961, pp 200-205.
E. H. White in “Light and Life,” W. D.McElroy and B. Glass, Ed., Johns
Hopkins Press, Baltimore, Md., 1961, pp 183-195. (5)E. Epstein and P. Kuwana, Photochem.Photobiol., 4, 1157 (1965). (6) P. E. Shevlin and H. A. Newfeld, J. Org. Chem., 35, 2178 (1970). (7) I 5 X lo5 M-l sec-l and the electron affinity of pyridine, Py, in DME is lower by at least 0.15 Y than that of triphenylene, Tr. The dimer, Na+(-Py-Py-)Na+, formed by this method is stablle in THF or DME. +
-
-
Pyridine, Py, may be reduced to its radical anion, Pym-, and under appropriate experimental conditions the latter may be identified by its esr spectrum.l-3 However, in contrast to radical anions of aromatic hydrocarbons, Py- salts rapidly dimerize and this reaction often prevents their detection in the investigated system. The formation of the diamagnetic dimer was first postulated by Smith4 and later by Ward5 who proposed for it a covalently bonded structure Na
PJa
Subsequently, Szwarc and his coworkers2 and simultaneously Schmulbach, Hinckley, and Wasmund6 proposed for it the dimeric dianion structure (I).The dimer is readily
(1) dehydrogenated, particularly in the presence of alkali metal, the overall reaction being I
bipyridyl
-4
.t
2NaM
and, indeed, many investigator^^-^ found bipyridyl and its radical anion as the products of pyridine reduction. These consecutive reactions have caused some confusion in the interpretation of experimental data. For example, the 335-nm band observed after brief exposure of a solution of pyridine in tetrahydrofuran (THF) to a sodium mirThe Journal of Physical Chemistry, Voi. 79, No. 2, 7975
-
-
ror was erroneously attributedg to the Py- radical anion. We.shal1 show, however, that it results from the presence of dimer (I),in agreement with previous suggestions.226 The rates of the various reactions consuming Py- are greatly influenced by the experimental conditions. For example, Talcott and Meyers found the half-life of Py- to be about 1 min when the radical was formed electrolytically in liquid ammonia, while Kemp, et aL,3 were unable to detect Py- in static experiments over shorter periods of observation when the base was mixed with a solution of sodium in liquid ammonia. The difference in the nature of the cation, NMe4+ in the electrolytic study and Na+ in Kemp’s investigation, was claimed to be responsible for these divergent observations. Substitution of a methyl group for the 4 hydrogen of pyridine seems to slow the dimerization of the respective radical anion^,^ and the dimerization of 2,6-dimethylpyridine radical anions seems to be even s10wer.~ According to Atherton, et al.,1° the radical anions of 3,5dimethylpyridine, prepared by sodium reduction in dimethoxyethane (DME), do not dimerize at all, presumably due to steric strain induced by the substituents. However, Kemp, et a1.,3as well as Talcott and Meyers,l found them to dimerize in liquid ammonia in less than 15 min. The capricious behavior of Py- is illustrated by the fact that only once was its esr spectrum recorded in HMPA2 (hexamethylphosphoric triamide), subsequent attempts at reproducing this result being unsuccessful. Nevertheless, the sharp and clear spectrum recorded in that experiment leaves no doubt about the identity of the paramagnetic species. We started, therefore, our investigation with the inten-
Dimerization of Pyridine Radical Anions in THF and DME tion of gaining more quantitative information aboot- the modes and rates of disappearance of pyridine radical anions. Our study covered reactions performed at 25' in THF and DME with sodium counterions. Experimental Section
THF and DME were dried with LiAlH4 and then distilled on a vacuum line into storage flasks containing perylenide dianion and an excess of Na-K alloy. Whenever needed, the solvent was distilled into the desired container attached to the line. Pyridine was dried with CaHz and vacuum distilled into a storage flask. Measured volumes were transferred under vacuum into ampoules and diluted with known volumes of the purified THF or DME. Triphenylene and biphenyl were purified by vacuum sublimation and stored under vacuum in ampoules fitted with breakseals. Standard high-vacuum techniques were used in the preparation of the triphenylenide radical anion solutions, the concentrations being determined spectrophotometrically. The kinetics of the reaction was investigated in a Durrum stop-flow spectrophotometer, using the technique described elsewhere.ll A solution of pyridine was mixed with that of the sodium salt of triphenylene radical anion (Tr.-,Na+) containing its parent hydrocarbon (Tr). Both pyridine (Py) and triphenylene were in excess of Tr.-,Na+. Sodium tetraphenylboride (-4 X 10-3 M) was added to suppress the dissociation of ion pairs of the reagents, ensuring that free ions do not participate in the investigated process. The progress of the reaction in THF was monitored by the absorbence at 672 nm, and in DME at 688 nm, both being the A,, of Tr.-,Na+ in the corresponding solvents. Their decimal extinction coefficients are 0.57 X lo4 and 0.60 X lo4M-l cm-l, respectively.lZ The spectrum of the yellow solution prepared with B.-,Na+ as the donor was recorded with a Cary-14 spectrophotometer. B.-,Na+ was chosen for these studies because biphenyl has a negligible absorption at 335 nm, the A,, of the yellow species. Its extinction coefficient was determined on the assumption that one molecule of the absorbing product is formed on the consumption of two molecules of B.-,Na+. Thus, the decimal e was found to be 3.6 X lo3 M-l cm-l in THF as well as in DME. Results Mixing of THF or DME solutions of pyridine and a suitable electron donor (sodium biphenylide, B--,Na+, or triphenylenide, Tr.-,Na+) ultimately leads to the formation of a pale yellow solution. No paramagnetism could be detected by esr after completion of the reaction, and hence neither Py- nor the radical anion of bipyridyl were among the products. The color of the resulting solution does not vary with time, in contrast to that of other solutions prepared by alkali metal reduction,2-6 implying that the insta,bility of the dimeric dianion is enhanced by the presence of alkali metals. In all kinetic experiments the reciprocal of the absorbence a t A,, of Tr.-,Na+ was linear with time, the slope providing the apparent bimolecular rate constant h ~The . results and the conditions maintained in each run are summarized in Tables I and 11. The value of k~ was found to be proportional to the ratio ([PyI/[Tr]lZ,as shown graphically in Figures 1and 2, implying that the reaction is due to dimerization of Py.-,I\fa+, its
107
TABLE I: Pseudo-Second Order Rate Constant, kI, for the Reaction of Tr.-, Na+ with Py in THF. 102[Py], M
102[Tr],M
[PyI/[TrI
0.675 0.99 1.43 4.1 6.0 8 .'4 16.7 24.0 0.98 0.98 0.98 0.98 0.98 0.98
1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 0.20 0.305 0.55 1.06 1.76 3.73
0.61 0.90 1.29 3.71 5.4 7.5 15.2 21.6 5 .O 3.2 1.79 0.93 0.56 0.26
k ~ M, - l 8ec-I 3 . 7 x 103 8 . 8 X 108 1 . 9 x 104 1 . 6 X lo6 3 . 4 x 106 6 . 7 X lo6 2.3 X lo6 3 . 6 X 108 (?) 3.4 x 106 1 . 3 X lo6 4.05 xi04 1.17 x 104 3.60 x 103 7.2 X l o 2
a T = 25'; [Na+, BPh4] = 4 X 10-3 M. The initial to M. concentration of T r . -, Na + was in the range
TABLE 11: Pseudo-Second-Order Rate Constant, kI, for the Reaction of Na+, T r . - with Py in DME.
lO2[Py], M 102[Tr], M 2.0 4.0 7 .O 10.0 20 .o 30 . O 10 .o 10 .o 10.0 10.0 10.0
[Py]/[Tr]
KI, M-I sec-'
2.33 4.5 7.9 11.1 22 33 2.4 3.8 9.3 22 52.2
4 . 1 X 10 16.2 X 10 48.7 X 10 10.5 X lo2 34.2 X lo2 75.6 X lo2 5 . 2 X 10 1 2 . 1 x 10 51.3 X 10 26 X lo2 20 x 102
0.86 0.89 0.89 0 .go 0.91 0.915 4.13 2.60 1.os 0.46 0.19
M. The initial a T = 25'; [Na+, BPh4-] = 4 x to M. concentration of Tr . -, Na + was in the range L " " I ' " ' I Na-, Tr'
-
"J
' " ' I " ' ' I Py -DME at 25OC
4 5 1
-1
;35[
25
I
os
'
~
'
I O
"
"
"
"
"
15
'
'
20
Log ["I/[T,]
Figure 1. Plot of log kl against log DME system.
[Py]/[Tr] for the Na+,Tr*--Py-
minute concentration being maintained by the electrontransfer equilibrium P y + Tro-, Na' -=--f Py ', Na' + Tr K,, Henoe, the pseudo-second-order rate constant, kI, is given by 2kdKtr2{[p~]/[Tr]]~, where kd denotes the bimolecular rate constant of 2Py.-, N a +
kd -+
Na+(-Py- Py')Na'
A comprehensive kinetic treatment which takes into acThe Journalof Physical Chemistry, Voi. 79, No. 2, 1975
A. Rainis and M. Szwarc
108 1 77-
No*, Tr:
-
" " I " * ' " P y - T H F at 25'C
'
~
'
"
I
"
-
No*, Try
I
-
6ot
4c*
"
' - 0I5
"
'
' 0I0
"
"
0I6
"
"
IIO
'
"
1
" I 5
Loa [PY1/[Tr]
Figure 2. Plot of log klagainst log [Py]/[Tr] for the Na+,Tr.--Py-
i
I
I
P y - D M E at 25OC
1
[Trl/[PYl Figure 3. Plot of [Py]/[Tr] kl against [Tr]/[Py] for the Na+,Tr.--
Py-DME system.
THF system.
0
I
count the finite concentration of Py.-,Na+ in the reacting solution leads to the relation d[Tr*', Na']-'/dt
=
2k&Ttr2{[ P ~ l / [ T r ] ) ~ / ( + l K,,[Pyl/[Tr]) and therefore the plots of [Py]/[Tr-]k~ us. [Tr-]/[Py] should be linear, its slope giving 1/(2kdKtr2)and the intercept 1/(2kdKtr). Such plots are shown in Figures 3 and 4 for the dimerization proceeding in THF and DME, respectivley. The intercepts are indistinguishable from 0 indicating that 1/(2kdKtr) is smaller than M sec for the reaction in THF and less than 10-3 M sec for the DME system.
Discussion The kinetic data and the negative esr results demonstrate that the species ultimately formed in the electrontransfer reduction of pyridine is the dimeric dianion Na+(-Py Py-)Na+ Its spectrum agrees with that reported by Hush, et ~ l . and , ~ the extinction coefficient a t A,, 335 nm (e 3.6 X lo3 M-l cm-l), is virtually the same as that determined by the other group,13namely, 4 X lo3. The kinetics of the reaction confirms the proposed mechanism-a rapid formation of Py.-,Na+ by the electron transfer which maintains its equilibrium concentration, followed by dimerization. The results yield a value of 5.6 X lo3 M-l sec-l for kdKtr2 in THF and 3.8 M-l sec-l in DME; the substitution of DME for THF retards this reaction by a factor of 1400. Unfortunately, the individual values of kd and Ktr cannot be determined from our data. However, the inequality, 1/(2kdKtr) < or in the THF or DME systems implies Ktr < 0.1 in the former and 6 X 105 M-l sec-l in THF and > lo5 M-l sec-l in DME. Our mechanism predicts that the dimerization initiated by electron transfer from sodium biphenylide would be too fast to be followed by our technique, while the reaction involving sodium anthracenide would be immeasurably slow. Both predictions were confirmed by experiments. The large increase in the value of k&t, resulting from the change of solvent from DME to THF calls for comment. The increase by a factor of 1400 is too large to be attributed to the enhancement of dimerization of Py.-,Na+ in THF. It is probable that Py.-,Na+ forms tight ion pairs in THF but loose pairs in DME, whereas Tr-,Na+ forms The Journal of Physical Chemistry, Vol. 79,N o . 2, 1975
No*, Tr:
-
I
Py - THF at 25OC
I
I
I
3001
G Z C C
i 566
[Trl/CPYl
Figure 4. Plot
of [Py]/[Tr]
kl
Py-THF system.
I
I O 3 M"S"
40
agdinst [Tr]/[Py] for the Na+,Tr.--
loose ion pairs in both s o l v e n t ~ . ~ Hence, ~ J ~ the equilibrium established in THF Tr.',Na+(loose) + P y e Tr + Py', Na+(tight) should be substantially shifted to the right when compared with the equilibrium established in DME Tr*',Na*(loose) + Py Tr + Py',Na+(loose) Indeed, such an effect was reported12 for the equilibria Tr-', Na'(1oose) + naphthalene e Tr
+ sodium naphthalenide (tight)
established in THF and Ti+*',Na'(1oose)
+
naphthalene Tr + sodium naphthalenide (loose) in DME. A t ambient temperature the ratio of the respective equilibrium constants is about 10. Because Py.- is smaller than the naphthalenide ion, the Py.-,Na+ ion pair in THF may be tighter than the naphthalenide ion pair. Hence, Kt, (in THF)/Kt, (in DME) should be greater than 10, whereas a value of 37 is obtained from our kdKtr2data, provided kd is assumed to be the same in THF and DME. It is likely, therefore, that k d is somewhat larger for the dimerization of tight Py.-,Na+ pairs in THF than for the loose Py.-,Na+ in DME.
Reaction of Hydroxyl Radicals with Oligopeptides
Acknowledgment. The support for this study by the National Science Foundation is gratefully acknowledged. Appendix We wish to mention here two observations pertinent to our studies. (1) It has been reported that many labile radical anions are stable in HMPA.2 Therefore, we investigated the possibility of generating Py- in this solvent by electron transfer from sodium biphenylide (Na+,B.-). Surprisingly, there was no reaction on adding dry, purified Py to a M solution of Na+,B.- in HMPA, the concentration of P y ranging from to 10-1M.It could be argued that this may be due to an unfavorable equilibrium, the electron affinity of Py being too low in this solvent or, alternatively, that the dimerization of the free Py- anions is too slow. It is more likely, however, that a specific interaction of HMPA with Py prevents or reduces the electron transfer. This anomalous behavior is currently being investigated. (2) The behavior of Li+ salts is often different from those of other alkali metals. We investigated, therefore, the dimerization of Py- induced in DME or in THF by Tr.-,Li+. The reaction was too fast to be followed and a
100
pale yellow precipitate appeared. Apparently Li+(-PyPy-)Li+ is insoluble in these solvents since addition of a slight excess of lithium tetraphenylboride to yellow solution of the sodium salt in THF and DME produced similar precipitation.
References and Notes (1) C. L. Talcott and R. J. Meyers, Mol, Phys., 12, 549 (1967). (2) J. Chaudhuri, S. Kume, J. Jagur-Grodzinski, and M. Szwarc, J. Amer. Chem. SOC.,90, 6421 (1968). (3) A. R. Buick, T. J. Kemp, G. T. Neal, and T. J. Stone, J. Chem. SOC.A, 1609 (1969). (4) C. R. Smlth, J. Amer. Chem. Soc., 46,414 (1924). (5)R. L. Ward, J. Amer. Chem. SOC.,83, 3623 (1961). (6) C. D. Schmulbach, C. C. Hlnckley, and D. Wasmund, J. Amer. Ghem. SOC.,90,6600 (1968). (7) A. Carrington and J. Santos-Velga, Mol. Phys., 5, 21 (1962). (8) R. Setton, C.R. Acad. Sci., Ser. AB, 244, 1205 (1957). (9) J. W. Dodd, F. J. Horton, and N. S. Hush, Proc. Chem. Soc., 61 (1962). (10) N. M. Atherton, F. Gerson, and J. N. Murrell. Mol. Phys., 2, 509 (1962). (11) A. Rainis, R. Tung, and M. Szwarc. J. Amer. Chem. SOC., 95, 659 (1973). (12) Y. Karasawa, G. Levln, and M. Szwarc, Proc. Roy. SOC.,Ser. A, 326, 53 f1971). (13) Since Hush, et a/., attributed the absorption to the radical anion, Py-, and not to its dimer, the value quoted in their paper has to be multiplied by a factor of 2. (14) M. Szwarc, "Carbanions, Living Polymers and Electron-Transfer Processes," Wlley, New York, N.Y., 1968, pp 314-316.
Reaction of Hydroxyl Radicals with Oligopeptides in Aqueous Solutions. A Pulse Radiolysis Study P. S. Rao and E. Hayon* Pioneering Research Laboratoty, U.S.Army Natick Laboratories, Natick, Massachusetts 0 1760 (ReceivedAugust 22, 1974) Publication costs assisted by Natick Laboratories
The reaction rate constants of OH radicals and the spectral characteristics of the free-radical intermediates produced from various oligopeptides in water have been studied in detail using the technique of pulse radiolysis. These include the amides of glycine, glycylglycine, tetraglycine, N- acetylglycylglycine, and glycyl/3-alanine, as well as glycylsarcosine and the N- acetyl derivatives of triglycine, trialanine, and trisarcosine. The k OH values are dependent, in particular, on the state of protonation of the terminal amino group: the rate constants increase on deprotonation of the -NH3+ group. The site(s) of attack by OH radicals are also dependent upon the state of protonation of the terminal amino group: on deprotonation of the -NH3+ group, radicals in an a position to the -NH2 group are preferentially formed. The transient absorption spectra of the free-radical intermediates are strongly dependent on the pK, of the parent molecules, as well as on the pK, of the free radicals produced. Based on these results, it is concluded that ionization of the peptide hydrogen occurs for various peptide radicals -CONHC(R)-CON-c(R)H+.
+
Introduction The fast-reaction technique of pulse radiolysis and kinetic absorption spectrophotometry is an important tool in the study and understanding of the free-radical chemistry of amino acids and peptides in aqueous solutions. It can provide information on the nature of the free radicals produced on reaction with eaq- and OH radicals, the acid-base properties of the peptide radicals, the effect of oxygen and
pH on the formation and reactions of the peroxy peptide radicals, and the redox properties of peptide radicals. The above information is necessary to provide reaction mechanisms for the formation of the products observed on exposure.of peptides to ionizing radiations. Using this technique, the interaction of eaq- with amino acids,l simple peptides,2 ~ligopeptides,~ and the peptide linkage,4?5as well as amides,6 have been studied in this laboratory. Reductive deamination and electron addition to The Journal of Physical Chemistry, Voi. 79, No. 2, 1975
