Vol. 60 [CONTRIBUTION FROM THE
DBPARTMENT OF BIOLOGICAL CHEMISTRY,
COLUMBIA UNIV3ZiSITY ]
Racemization of Tripeptides bprad Hydmtoins' BY M. BOVARNICK AND H. T.CLARICE Ii 0 In 1910 Dakin2 first described the racemization H NH~-C--C-NH-~-COOH 1 II of optically active hydantoins under the influence of mild alkali. At the same time he demonK K' strated that the presence of a hydrogen atom on A H OH H the asymmetric casbon atom was a necessary I / 1 condition for the occurrence of the racemization NH,-C-C=N-C-COOH and further observed that neither the optically I I R' R active amino acids themselves, their uramino acid B derivatives, nor optically active dipeptides, were racamiaed under the conditions.2a He therefore need be considered ; the keto-enol tautomerizaconcluded that the mechanism of racemization tion obviously does not take place to any appreinvolved a keto-enol tautomerism and that it ciable extent as the dipeptides do not racemize. In a tripeptide a wide variety of tautomeric was essential that both the a-amino group and forms is possible, of which oidy those represented the carboxyl group attached to the asymmetric below need be cotlsidered here. It is a well-reccarbon atom be hound up in order for the amino ognized €act that the presence of a double bond a d d to undergo racemization by this m e t h ~ d . ~ in a molecule tends to favor the production (when Extending this work, Dakin and others* later this i s a t a l l possible) of other double bonds in ideatifid the terminal amino acids of protein conjugatioa with it. It was therefore thought aking use of the assumption that an explanation of the racemizability of the o and carboxyl groups, they tripeptides might reside in the possibility that in would not rac n alkaline hydrolysis. Lethese peptide chains a previous or simultaneous vene,S working with smaller peptide chains, suboccurrence of an enolized peptide linkage (D), stantiated the findings of Dakin and reached the analogous to that outtined above (B) for a diconclusion that in a peptide composed of three necessary to induce the fonnapeptide, might be optically active amino acids only the middle tion d a second enolic double bond (E) whereby amino acid was racemized by alkali. the asymmetry of the central constitilent amino On comparing the formula&of dipeptides and acid is destroyed. tripeptide a theoretical reason presents itself for the o M e d differace in racdeabbility of H '1 H " U , the two. In both there are two type$ of strucNH-C- I &--NH-C--NH--C--COOH c of undergoing tautomerization . One I I R R' R r keto-ennl system, and the other i s H OH \I 0 t1 the amide-imide System. In the dipeptide only ii I . i I 1 the non-racemizing tautomeric equilibrium I) NHp--C--C=N --C - C - - N H -C COOH
c.,
8
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(1) Tkus report IS frbm a dissertation submitted by Max Bovatnick in palWilment of tke requiremeats for the degree of Doetor of Philosophy in the Faculty of Pure Science, Columbia University. (2) Dakin, Am. C k m . J . , 44,4$ (1810).
f;T I
R' '
I
R
~
If
- - C z C --SH-C
I
173 (1901)l. (3) Dakin, J . Bid. Chum.,18, 367 (1913). (4) Dakin and Dudley, {bid X I , 2fiB (1!?€3). 17wkin and Dale, Hiochcm.J . , 19, 248 (1919) (6) Levene, Steiacr arid Marker ./ Tirol Chnr.. 99. % t M (1871)
K" C) H
C)H
NHr-C--Cr=N
it wai, hydmlyztxi tb an extent of o+er 15% [Lemne, Ba$a and Stdgcr, J . Bid. C h . . 8% Id7 (I$S@,1. Racernizatioa of the r e sulting a d n o acids can be effected by alkali only under very vigorous co&tddons, such sa hesting d t h 15% barium hydrooxide a t 155160" for forty-eight h o e s [E. Pibeher, Z. phgsiol. Chum..88, lSl,
I
1
R
-C(JOH
15
I K"
R'
in order for this a~&&sm to be effective it1 the case of a dipeptidethe foilowing reaction would have to take place
JyP I I1
H
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