Electronic Structure of the Cysteine Thiyl Radical: A DFT and Correlated ab Initio Study

The electronic structure and the unusual EPR parameters of sulfur-centered alkyl thiyl radical from cysteine are investigated by density functional theory (DFT) and correlated ab initio calculations. Three geometry-optimized, staggered conformations of the radical are found that lie within 630 cm-1 in energy. The EPR g-values are sensitive to the energy difference between the nearly-degenerate singly occupied orbital and one of the lone-pair orbitals (excitation energies of 1732, 1083, and 3429 cm-1 from Multireference Configuration Interaction calculations for the structures corresponding to the three minima), both of which are almost pure sulfur 3p orbitals. Because of the near degeneracy, the second order correction to the g tensor, which is widely used to analyze g-values of paramagnetic systems, is insufficient to obtain accurate g-values of the cysteine thiyl radical. Instead, an expression for the g tensor must be used in which third order corrections are taken into account. The near-degeneracy can be affected to roughly equal extents by changes in the structure of the radical and by hydrogen bonds to the sulfur. The magnitude of the hyperfine coupling constants for the β protons of the cysteine thiyl radical is found to depend on the structure of the radical. On the basis of a detailed comparison between experimental and calculated g-values and hyperfine coupling constants an attempt is made to identify the structure of thiyl radicals and the number of hydrogen bonds to the sulfur.