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Reversible intramolecular hydrogen transfer between cysteine thiyl radicals and glycine and alanine in model peptides: absolute rate constants derived from pulse radiolysis and laser flash photolysis.

ABSTRACT: The intramolecular reaction of cysteine thiyl radicals with peptide and protein alphaC-H bonds represents a potential mechanism for irreversible protein oxidation. Here, we have measured absolute rate constants for these reversible hydrogen transfer reactions by means of pulse radiolysis and laser flash photolysis of model peptides. For N-Ac-CysGly6 and N-Ac-CysGly2AspGly3, Cys thiyl radicals abstract hydrogen atoms from Gly with k(f) = (1.0-1.1 x 10(5) s(-1), generating carbon-centered radicals, while the reverse reaction proceeds with k(r) = (8.0-8.9) x 10(5) s(-1). The forward reaction shows a normal kinetic isotope effect of k(H)/k(D) = 6.9, while the reverse reaction shows a significantly higher normal kinetic isotope effect of 17.6, suggesting a contribution of tunneling. For N-Ac-CysAla2AspAla3, cysteine thiyl radicals abstract hydrogen atoms from Ala with k(f) = (0.9-1.0) x 10(4) s(-1), while the reverse reaction proceeds with k(r) = 1.0 x 10(5) s(-1). The order of reactivity, Gly > Ala, is in accord with previous studies on intermolecular reactions of thiyl radicals with these amino acids. The fact that k(f) < k(r) suggests some secondary structure of the model peptides, which prevents the adoption of extended conformations, for which calculations of homolytic bond dissociation energies would have predicted k(f) > k(r). Despite k(f) < k(r), model calculations show that intramolecular hydrogen abstraction by Cys thiyl radicals can lead to significant oxidation of other amino acids in the presence of physiologic oxygen concentrations.

SUBMITTER: Nauser T

PROVIDER: S-EPMC2662404 | biostudies-other | 2008 Nov

REPOSITORIES: biostudies-other

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Reversible intramolecular hydrogen transfer between cysteine thiyl radicals and glycine and alanine in model peptides: absolute rate constants derived from pulse radiolysis and laser flash photolysis.

Nauser Thomas T Casi Giulio G Koppenol Willem H WH Schöneich Christian C

The journal of physical chemistry. B 20081101 47

The intramolecular reaction of cysteine thiyl radicals with peptide and protein alphaC-H bonds represents a potential mechanism for irreversible protein oxidation. Here, we have measured absolute rate constants for these reversible hydrogen transfer reactions by means of pulse radiolysis and laser flash photolysis of model peptides. For N-Ac-CysGly6 and N-Ac-CysGly2AspGly3, Cys thiyl radicals abstract hydrogen atoms from Gly with k(f) = (1.0-1.1 x 10(5) s(-1), generating carbon-centered radicals ...[more]

PMID: 18973367

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Project description:In this study, steady-state and time-resolved radiolysis methods were used to determine the primary reaction pathways and kinetic parameters for the reactions of hydroxyl radical with microcystin-LR (MC-LR). The fundamental kinetic data is critical for the accurate evaluation of hydroxyl-radical based technologies for the destruction of this problematic class of cyanotoxins. The bimolecular rate constant for the reaction of hydroxyl radical with MC-LR is 2.3 (+/-0.1) x 10(10) M(-1)s(-1) based on time-resolved competition kinetics with SCN-at low conversions using pulsed radiolysis experiments. The reaction of hydroxyl radical with MC-LR can occur via a number of competing reaction pathways, including addition to the benzene ring and diene and abstraction of aliphatic hydrogen atoms. LC-MS analyses indicate the major products from the reaction of hydroxyl radicals with MC-LR involve addition of hydroxyl radical to the benzene ring and diene moieties of the Adda side chain. Transient absorption spectroscopy monitored between 260-500 nm, following pulsed hydroxyl radical generation, indicate the formation of a transient species with absorption maxima at 270 and 310 nm. The absorption maxima and lifetime of the transient species are characteristic of hydroxycyclohexadienyl radicals resulting from the addition of hydroxyl radical to the benzene ring. The rate constant for the formation of hydroxycyclohexadienyl radical is 1.0 (+/-0.1) x 10(10) M(-1)s(-1) accounting for approximately 40% of the primary reaction pathways. Representative rate constants and partitioning of hydroxyl radical reactions were assessed based on the reactivities of surrogate substrates and individual amino acids. Summation of the individual reactivities of hydroxyl radical at the different reactive sites (amino acids) leads to a rate constant of 2.1 x 10(10) M(-1) s(-1) in good agreementwith the rate constant determined in our studies. The relative magnitude of the rate constants for the reactions of hydroxyl radical with the individual amino acids and appropriate surrogates, suggest 60-70% reactions of hydroxyl radical occur at the benzene and diene functional groups of the Adda moiety.

Dataset Information

Reversible intramolecular hydrogen transfer between cysteine thiyl radicals and glycine and alanine in model peptides: absolute rate constants derived from pulse radiolysis and laser flash photolysis.

Publications

Reversible intramolecular hydrogen transfer between cysteine thiyl radicals and glycine and alanine in model peptides: absolute rate constants derived from pulse radiolysis and laser flash photolysis.

Similar Datasets

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