Detection of two novel lowest lying singlet-triplet transitions of the CSC chromophore in L-methionine

Abstract

Methionine sulfur constitutes a part of the coordination environments of metal ions in a number of electron transfer metalloproteins, e.g. blue copper proteins, and c-type cytochromes. Copper coordination by the methionine sulfur ligand has also been found in the recent X-ray structures of ascorbic oxidase and cytochrome c oxidase. However, the role of strongly conserved methionine in these proteins is not well defined. In this work the electronic transitions of the CSC chromophore have been studied by circular dichroism (CD) and magnetic circular dichroism (MCD) spectroscopy between 210 and 350 nm. Using L- and D-methionine, L-methioninol, and dialkyl sulfides (t-butyl methyl sulfide, diisopropyl sulfide) two novel low lying CD and/or MCD bands at about 262 and 285 nm have been detected in the tailing of the corresponding electronic absorption spectra. These bands precede all singlet-singlet transitions of the CSC chromophore and show the molar extinction coefficients, ϵ, of < 1 M 1cm −1. Moreover, based on ab initio SCF-Cl calculation the lowest triplet state energy of 4.87 eV (254 nm) has been reported for the CSC chromophore in dimethyl sulfide (C. Dézenaud-Dandine and A. Sevin. J. Mol. Struct. (Theochem). 339 (1995) 133). Based on these features both bands have been assigned to singlet-triplet transitions from the highest filled orbital of a sulfur lone pair. The effect of singlet-triplet transitions of methionine sulfur on the electronic structure of electron transfer metalloproteins is discussed.