Model Studies of DNA Photorepair:  Reduction Potentials of Thymine and Cytosine Cyclobutane Dimers Measured by Fluorescence Quenching

Abstract

The interactions of various pyrimidines (1,3-dimethylthymine, DMT, 1,3-bis(N 4 ,N 4 -dimethylcytosin-1yl)propane, DMC) and their corresponding cis-syn cyclobutane dimers (DMTD and DMCD) with a series of excitedstate electron donors were examined with the goal of understanding the energetics and mechanism of UV repair by DNA photolyase. For each substrate there is a good correlation between the excited state oxidation potential (E ox *) and the quenching rate constant (k q ). The value for k q increases as E ox * becomes more negative, asymptotically approaching a value that is at or below the solvent diffusion limit. These data all showed good fits to the Rehm-Weller equation. Reduction potentials for each of the substrates could be extracted from this analysis: -2.20 V (vs SCE) for DMTD; -2.14 V for DMT; -2.17 V for DMCD; and -2.16 for DMC. These values show that the initial electron transfer step in the photolyase mechanism is exergonic by ca. 10-15 kcal/mol. Thus these data support the reductive electron transfer mechanism for DNA photolyases proposed by Jorns et al.

New thiophene-based azo ligands containing azo methine group in the main chain for the determination of copper(II) ions

“…Millimolar solutions of the dyes were prepared in spectrophotometric grade acetonitrile. The oxidation potential of ferrocene which was used as an internal electrode was exhibited at about þ0.45 V. Redox potentials of anthracene and pyrene used in quenching studies were taken as À1.09 V and À1.16 V versus calomel electrode [30]. Redox potential of Co 2þ ion was obtained from the principles of instrumental analysis versus standard hydrogen electrode [31].…”

Reductive Electron Transfer and Transport of Excess Electrons in DNA

“…[30] In its reduced, deprotonated, and photoexcited state, flavine has a redox potential of À2.8 V and is capable of reducing all four DNA bases. [31] 2) Additionally, a special T-T dimer was incorporated into the oligonucleotides, which lacks the connecting phosphodiester bridge between the 3'-and the 5'-hydroxy groups of the two ribofuranoside moieties. [32] Hence, cycloreversion of the T-T dimer, which is initiated by a one-electron reduction, yields a strand break of the oligonucleotide, which can be analyzed and quantified by HPLC analysis.…”

Reduktiver Elektronentransfer und Transport von Überschusselektronen in DNA

“…[30] Dieses Flavin hat im reduzierten, deprotonierten und photoangeregten Zustand ein Redoxpotential von À2.8 V und ist dadurch in der Lage, alle vier DNA-Basen zu reduzieren. [31] 2) In die Oligonucleotide wurde zusätzlich ein spezielles T-T-Dimer ohne verbindende Phosphodiesterbrücke zwischen der 3'-und der 5'-Hydroxygruppe der beiden Ribofuranosid-Einheiten eingebaut. [32] Die Cycloreversion des T-T-Dimers, die durch eine Ein-Elektronen-Reduktion ausgelöst wird, bewirkt einen Strangbruch des Oligonucleotids, der durch HPLC-Analyse quantifiziert werden kann.…”