REACTIVITY OF HOMOCYSTEINE-THIOLACTONE AND ALPHA-METHYLHOMOCYSTEINE-THIOLACTONE WITH E(AQ)(-) AND OH-RADICAL - A PULSE-RADIOLYSIS STUDY

The efficient radiation protecting agents homocysteine-thiolactone . H Cl (HCTL . HCl) and its alpha-alkylated derivative (alpha-methyl-homoc ysteine, alpha-MHCTL . HCl) have been investigated in respect to the i dentification of the primarily formed species after absorption of ioni zing radiation using pulse radiolysis technique. The reaction of e(aq) (-) with the unprotonated form of HCTL (k = 2.1x10(9) dm(3) mol(-1) s( -1)) is leading to the formation of a radical anion-having two absorpt ion bands: at 275 nm (epsilon = 2500 dm(3) mol(-1) cm(-1)) and 510 nm (epsilon = 930 dm(3) mol(-1) cm(-1)), which decay with 2k = 2.3 x 10(9 ) dm(3) mol(-1) s(-1). The protonated form of HCTL reacts with e(aq)(- ) with k = 4.0x10(10) dm(3) mol(-1) s(-1). The OH-radicals react with HCTL with k = 1.95 x 10(9) dm 3 mol(-1).s(-1) resulting in a transient spectrum with lambda(max) = 265 nm (epsilon =2000 dm(3) mol(-1) cm(-1 )).The transients disappear with 2k = 2.1 x 10(9) dm(3) mol(-1) s(-1). The reactivity of e(aq)(-) with alpha-MHCTL was determined for both f orms: for the protonated, k = 1.25x10(10) dm (3) mol (-1) s (-1) and f or the unprotonated, k = 2.6x10 (9) dm (3) mol (-1) s (-1). The transi ent absorption spectrum at pH 8.4 shows two absorption bands: lambda = 275 nm (epsilon = 3500 dm(3) . mol(-1).cm.') and 490 nm (epsilon = 11 60 dm(3). mol(-1).cm(-1)). The transients disappear with 2k = 2.2x10 ( 9) dm(3).mol (-1).s (-1). The reaction of OH with alpha-MHCTL.HCl,k = 8.4x10 (9) dm(3).mol (-1).s (-1) (pH = 8.6) is resulting in an absorpt ion spectrum with lambda(max) < 260 nm and an absorption band at 350 n m (epsilon = 510 dm(3). mol (-1).cm(-1)). Upto 50 mu s after pulse the transients decay with 2k = 5.5x10(9) dm(3).mol(-1).s(-1) and thereaft er by a k = 8.4x10(9) dm(3).mol(-1).s(-1) (pH 8.6) is resulting in an absorption spectrum with lambda(max) < 260 nm and an absorption band a t 350 nm (epsilon = 510 dm(3). mol(-1).cm(-1)). Upto 50 mu s after pul se the transients decay with 2k = 5.5x109 dm(3).mol(-1).s(-1) and ther eafter by a first order reaction. In addition, the formation of some p roducts was also studied.The yield of ammonia resulting from alpha-MHC TL.HCl strongly depends on pH, e.g. at pH = 5.1 G(i) (NH3) = 0.95, whe reas at pH 9.15 it increases to G(i) = 3.1. Hydrogen sulphide is forme d in airfree solutions, G(i) (H2S) = 0.29, whereas in the presence of N2O reduced to G(i) (H2S) = 0.10. Some probable reaction mechanisms ar e presented.