TY - JOUR

T1 - Reduction of Transient Histidine Radicals by Tyrosine

T2 - Influence of the Protonation State of Reactants

AU - Morozova, Olga B.

N1 - © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

PY - 2020/1/3

Y1 - 2020/1/3

N2 - The role of tyrosine radicals as mediators of electron transfer reactions in enzymes is well established, as is the involvement of histidine as a binding partner. But how environmental factors affect these reactions remains poorly explored. In the study presented here, kinetic data on the influence of the protonation state of the reactants on the reduction of transient histidine radicals by tyrosine were obtained in neutral and basic aqueous solution (pH 6–12) using time-resolved chemically induced dynamic nuclear polarization (CIDNP). The histidine radicals were generated in the photo-induced reaction with the photosensitizer 3,3′,4,4′-tetracarboxy benzophenone. From model simulations of the detected CIDNP kinetics, pH dependent second-order rate constants of the reduction of histidine radicals were obtained for four possible combinations of the amino acids and their N-acetyl derivatives, and also for the systems histidine-phenylalanine dipeptide/N-acetyl tyrosine, and N-acetyl histidine/tyrosine-glutamine dipeptide. The pH dependences of the rate constant of the reduction reaction are explained accounting for the protonation states of reactants, and also protonation state of the equilibrium form of the product - reduced form of histidine radical, which is histidine with neutral or a positively charged imidazole.

AB - The role of tyrosine radicals as mediators of electron transfer reactions in enzymes is well established, as is the involvement of histidine as a binding partner. But how environmental factors affect these reactions remains poorly explored. In the study presented here, kinetic data on the influence of the protonation state of the reactants on the reduction of transient histidine radicals by tyrosine were obtained in neutral and basic aqueous solution (pH 6–12) using time-resolved chemically induced dynamic nuclear polarization (CIDNP). The histidine radicals were generated in the photo-induced reaction with the photosensitizer 3,3′,4,4′-tetracarboxy benzophenone. From model simulations of the detected CIDNP kinetics, pH dependent second-order rate constants of the reduction of histidine radicals were obtained for four possible combinations of the amino acids and their N-acetyl derivatives, and also for the systems histidine-phenylalanine dipeptide/N-acetyl tyrosine, and N-acetyl histidine/tyrosine-glutamine dipeptide. The pH dependences of the rate constant of the reduction reaction are explained accounting for the protonation states of reactants, and also protonation state of the equilibrium form of the product - reduced form of histidine radical, which is histidine with neutral or a positively charged imidazole.

KW - CIDNP

KW - electron transfer

KW - histidine radical

KW - reaction mechanisms

KW - tyrosine radical

UR - http://www.scopus.com/inward/record.url?scp=85076374281&partnerID=8YFLogxK

U2 - 10.1002/cphc.201901020

DO - 10.1002/cphc.201901020

M3 - Article

C2 - 31709709

AN - SCOPUS:85076374281

VL - 21

SP - 43

EP - 50

JO - ChemPhysChem

JF - ChemPhysChem

SN - 1439-4235

IS - 1

ER -