PEPTIDE PEROXYL RADICALS - BASE-INDUCED O-2(.-) ELIMINATION VERSUS BIMOLECULAR DECAY - A PULSE-RADIOLYSIS AND PRODUCT STUDY

Radiolytically generated OH radicals react with the cyclic dipeptides glycine anhydride (1) and alanine anhydride (2), forming a single type of peptide radical in each case by abstracting a carbon-bound H atom at the ring. In the case of sarcosine anhydride (3), besides the C(3) or C(6) H atoms (78%), the H atoms at the N-methyl groups are also tar gets of the OH radical attack (22%). In N2O/O2 (4:1 v/v) saturated sol utions these peptide radicals add oxygen (k almost-equal-to 2 x 10(9) dm3 mol-1 s-1) to form the corresponding peroxyl radicals 6 (from 1), 7 (from 2), and 12 and 13 (from 3). The kinetics of O2.- elimination f rom the radicals 6 and 7 has been monitored by pulse radiolysis techni ques. The pK(a) values of the peroxyl radicals 6 and 7 have been deter mined to be 10.8 and 11.2, respectively. The anions of these peroxyl r adicals (6a and 7a) rapidly eliminate O2.- with the rate constants 1.6 x 10(5) and 3.7 x 10(6) s-1, respectively. In contrast, the spontaneo us HO2. elimination reactions of the peroxyl radicals 6 and 7 are very slow, with rate constants of < 1 and approximately 0.5 s-1 as estimat ed from product analysis. The overall bimolecular decay rate constants of the cyclic dipeptide peroxyl radicals have been determined by puls e radiolysis measurements (2k(6) = 8.6 x 10(8) dm3 mol-1 s-1; 2k(7) = 1.6 x 10(8) dm3 mol-1 s-1; 2k(12/13) = 4.0 X 10(8) dm3 mol-1 s-1). The main products (G values) in the gamma radiolysis of glycine anhydride in N2O/O2-saturated solution at pH 6 are 3-hydroxy-2,5-dioxopiperazin e (14) (3.5 x 10(-7) mol J-1), 2,3,5-trioxopiperazine (17) (1.6 X 10(- 7) mol J-1), and 2,5-dioxo-2,3,4,5-tetrahydropyrazine (8) (0.4 x 10(-7 ) mol J-1). The product 8 is in equilibrium with its hydrate 14, which is again in equilibrium with its ring-opened form N-glyoxylyl glycina mide (16). In acidic solutions radical 6 decays essentially bimolecula rly, giving equal amounts of 14 (present in an equilibrium mixture of 8, 14, and 16) and 17. In basic solutions 14 becomes the single main p roduct as the OH--induced O2.- elimination reaction becomes much faste r than the bimolecular decay. This is in good agreement with the yield of O2.- formation as monitored by its reaction with tetranitromethane under various pH conditions. With alanine anhydride (2), the main rea ction under gamma radiolysis conditions even at pH 6 is still the OH-- induced O2.- elimination reaction with 3-hydroxy-2,5-dioxo-3,6-dimethy lpiperazine (15) as the single main product (G(15) = 4.7 x 10(-7) mol J-1). The major peroxyl radical in the sarcosine anhydride system (12) only decays bimolecularly either by self-termination or by cross-term ination with the minor peroxy radical 13 (products at pH 6 in gamma ra diolysis: 2,3,5-trioxo-1,4-dimethylpiperazine (20), G = 4.0 x 10(-7) m ol J-1; 3-hydroxy-2,5-dioxo-1,4-dimethylpiperazine(21), G = 0.4 x 10(- 7) mol J-1; 1-formyl-2,5-dioxo-4-methylpiperazine (22), G = 0.6 x 10(- 7) mol J-1; and 2,5-dioxo-1-methylpiperazine (24), G = 0.6 x 10(-7) mo l J-1). The O2.-elimination reaction of radical 13 is estimated to hav e a rate constant of < 5 s-1.