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

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 CS-C chromophore and show the molar extinction coefficients, epsilon, of <1 M-1 cm(-1). Moreover, based on ab initio SCF-CI 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. Dezernaud-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. (C) 1998 Elsevier Science S.A. All rights reserved.