Steady-state and pulse radiolysis of triethylsilane in cyclohexane with product analysis

Pulse radiolysis studies of beta-carotene in oxygenated DMSO solution. Formation of beta-carotene radical cation

The spectroscopic and kinetic characteristics of beta-carotene radical cation (beta-carotene(.+)) were studied by pulse radiolysis in aerated DMSO solution. The buildup of beta-carotene(.+) with k(1) = (4.8 +/- 0.2) x 10(8) dm(3) mol(-1) s(-1) [lambda(max) = 942 nm, epsilon = (1.6 +/- 0.1) x 10(4) dm(3) mol(-1) cm(-1)] results from an electron transfer from beta-carotene to DMSO(.+). The beta-carotene(.+) species decays exclusively by first-order reaction, k = (2.1 +/- 0.1) x 10(3) s(-1), probably by two processes: (1) at low substrate concentration by hydrolysis and (2) at high concentrations also by formation of dimer radical cation (beta-carotene)(2)(.+). Under the experimental conditions, a small additional beta-carotene triplet-state absorption ((3)beta-carotene) in the range of 525 to 660 nm was observed. This triplet absorption is quenched by oxygen (k = 7 x 10(4) s(-1)), resulting in singlet oxygen ((1)O(2)), whose reactions can also lead to additional formation of beta-carotene(.+).

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 x HCl (HCTL x HCl) and its alpha-alkylated derivative (alpha-methyl-homocysteine, alpha-MHCTL x HCl) have been investigated in respect to the identification of the primarily formed species after absorption of ionizing radiation using pulse radiolysis technique.The reaction of e-(aq) with the unprotonated form of HCTL (k = 2.1 x 10(9) dm3 mol(-1) s(-1)) is leading to the formation of a radical anion having two absorption bands: at 275 nm (epsilon = 2500 dm3 mol(-1) cm(-1)) and 510 nm (epsilon = 930 dm3 mol(-1) cm(-1)), which decay with 2k = 2.3 x 10(9) dm3 mol(-1) s(-1). The protonated form of HCTL reacts with e-(aq) with k = 4.0 x 10(10) dm3 mol(-1) s(-1). The OH-radicals react with HCTL with k = 1.95 x 10(9) dm 3 mol(-1) x s(-1) resulting in a transient spectrum with lambda(max) = 265 nm (epsilon =2000 dm3 mol(-1) cm(-1)). The transients disappear with 2k = 2.1 x 10(9) dm3 mol(-1) s(-1). The reactivity of e-(aq) with alpha-MHCTL was determined for both forms: for the protonated, k = 1.25 x 10(10) dm3 mol(-1) s(-1) and for the unprotonated, k = 2.6 x 10(9) dm3 mol(-1) s(-1). The transient absorption spectrum at pH = 8.4 shows two absorption bands: lambda = 275 nm (epsilon = 3500 dm3 x mol(-1) x cm(-1)) and 490 nm (epsilon = 1160 dm3 x mol(-1) x cm(-1)). The transients disappear with 2k = 2.2 x 10(9) dm3 x mol(-1) x s(-1). The reaction of OH with alpha-MHCTL x HCl,k = 8.4 x 10(9) dm3 x mol(-1) x s(-1) (pH = 8.6) is resulting in an absorption spectrum with lambda(max) < 260 nm and an absorption band at 350 nm (epsilon = 510 dm3 x mol(-1) x cm(-1)). Up to 50 micros after pulse the transients decay with 2k = 5.5 x 10(9) dm3 x mol(-1) x s(-1) and thereafter by a k = 8.4 x 10(9) dm3 x mol(-1) x s(-1) (pH = 8.6) is resulting in an absorption spectrum with lambda(max) < 260 nm and an absorption band at 350 nm (epsilon = 510 dm3 x mol(-1) x cm(-1)). Up to 50 micros after pulse the transients decay with 2k = 5.5 x 10(9) dm3 x mol(-1) x s(-1) and thereafter by a first order reaction. In addition, the formation of some products was also studied. The yield of ammonia resulting from alpha-MHCTL x HCl strongly depends on pH, e.g. at pH = 5.1 Gi (NH3) = 0.95, whereas at pH = 9.15 it increases to Gi = 3.1. Hydrogen sulphide is formed in airfree solutions, Gi (H2S) = 0.29, whereas in the presence of N2O it is reduced to Gi (H2S) = 0.10. Some probable reaction mechanisms are presented.

Radiation-induced formation of haematoporphyrin-transients in aqueous solution. A pulse radiolysis study

The radiation-induced formation of some haematoporphyrin (HP) transients in aqueous solution was studied by pulse radiolysis. The OH radicals attack HP (rate constant k = (1.3 +/- 0.2) x 10(10) dm3 mol-1 s-1) and the resulting transients decay with 2k = (2.5 +/- 0.2) x 10(8) dm3 mol-1 s-1. The spectrum of the HP-OH species shows several maxima in the range of 450-850 nm (epsilon 590 = 3880, epsilon 640 = 3850, epsilon 800 = 3670 dm3 mol-1 cm-1). k for one electron reduction of HP (radical anion, HP.-) was determined by a direct eaq- attack on HP (at pH 9.0, k = (1.6 +/- 0.2) x 10(10) dm3 mol-1 s-1) as well as by electron transfer from hydroxyisopropyl radicals to the substrate (at pH 8.6, k = (3.2 +/- 0.1) x 10(8), at pH 3.8, k = (4.0 +/- 0.2) x 10(8), and at pH 2.0, k = (2.1 +/- 0.1) x 10(9) dm3 mol-1 s-1). In slightly alkaline media (pH 8.6 - 9.5) the HP.- transients have main absorption bands at lambda < 450 and 640 nm (epsilon 660 = 7500 dm3 mol-1 cm-1), whereas at pH 2.0 they are at lambda < 430 and 640 nm (epsilon 640 = 8170 dm3 mol-1 cm-1) and 710 nm (epsilon 710 = 8500 dm3 mol-1 cm-1). The one-electron oxidation of HP (formation of HP.+, radical cation) was studied by means of N3. or SO4.- species as electron acceptors. Using N3. at pH 9.5 k (HP + N3.) = (3.2 +/- 0.2) x 10(9) and for the decay 2k = (3.5 x 0.3) x 10(8) dm3 mol-1 s-1 were determined; at pH 3.6, k = (HP + N3.) = (7.1 +/- 0.3) x 10(8) and 2k = (2.6 +/- 0.3) x 10(7) dm3 mol-1 s-1 were found. In the case of SO4.- at pH9.8 a buildup (5.6 +/- 0.3) x 10.7 dm3 mol-1 s-1) was obtained. The spectrum of the HP.+ transients at pH 9.6 possesses several maxima with epsilon 345 = 38,800, epsilon 450 = 2400, epsilon 595 = 2880, and epsilon 660 = 3300 dm3 mol-1 cm-1. The corresponding values at pH 3.8 are epsilon 640 = 2450, epsilon 710 = 2600, and epsilon 780 = 1580 dm3 mol-1 cm-1.(ABSTRACT TRUNCATED AT 400 WORDS)