EPR, ENDOR, TRIPLE resonance spectroscopy and MO studies on radical anion, neutral radical and radical cation of vitamin K3 in a selection of solvents

Abstract

All three radical species (radical anion, neutral radical and radical cation) from 2-methyl-1,4-naphthoquinone (vitamin were prepared in liquid solutions and measured by EPR and ENDOR/TRIPLE resonance K 3 ) spectroscopy. The methyl group protons showed the largest variation of isotropic hyperÐne coupling, from 8.3 MHz (anion) to 19.1 MHz (neutral) to 9.3 MHz (cation). Isotropic hyperÐne couplings of a proton in the quinoid ring and the methyl group protons in di †erent solutions were shown to be dependent on the relative permittivity of the solvent. A prominent alternating linewidth e †ect was detected in the EPR spectra of the radical anion of 2-methyl-1,4-naphthoquinone at di †erent concentrations and at di †erent temperatures in 1,2dimethoxyethane. This was explained as being due to the counter-cation Ñuctuation between the two quinodic oxygens. A solid dipotassium radical salt of vitamin was prepared and its EPR spectra measured. Isotropic K 3 hyperÐne coupling constants were also calculated for all three radical species using the UB3LYP density functional method.

The Hydrogen-Bonded Dianion of Vitamin K1 Produced in Aqueous–Organic Solutions Exists in Equilibrium with Its Hydrogen-Bonded Semiquinone Anion Radical

“…The spectrum shown in Figure a that was obtained at low modulation amplitude and low water content is similar to what has previously been observed and assigned to the VK 1 radical anion in organic solvents. − The total widths of the EPR spectra obtained in low- and high-water-content solutions were the same, although the shapes of the spectra were different, indicating slightly different hyperfine coupling constants. It is known that semiquinone anion radicals undergo hydrogen bonding in pure water (or in alcohols), with the interactions having been detected by Q-band pulse 2 H electron–nuclear double resonance experiments .…”

The Electronic Structure of the Flavin Cofactor in DNA Photolyase

“…An analogous effect has been observed for nitroxides and quinones: in nonpolar environments the hfc's were reduced with respect to those measured in polar and/or protic solvents. [55][56][57] Aqueous and relatively nonpolar environments have very different dielectric constants, which strongly affect electrontransfer rates, due to their effect on the reorganization energy. In model photolyase systems, Heelis and co-workers showed that the quantum yield of splitting cis,syn-1,3-dimethyluracil dimers is solvent dependent, due to changes in the relative rates for forward and backward electron transfer.…”

Substrate Binding to DNA Photolyase Studied by Electron Paramagnetic Resonance Spectroscopy

“…Altered hfcs are frequently observed in spin label EPR when one and the same spin probe is exposed to protein regions or solvents of different polarity (Kawamura et al, 1967;Joela and Lehtovuori, 1999;Steinhoff et al, 2000). Whereas such effects are well documented for nitroxide spin labels, the effect of a modified dielectric environment on flavin proton hfcs has not yet been quantified.…”