Theoretical and experimental sulfur K-edge X-ray absorption spectroscopic study of cysteine, cystine, homocysteine, penicillamine, methionine and methionine sulfoxide

Abstract

The experimental sulfur K-edge X-ray absorption near-edge structure (XANES) spectra of the amino acids cysteine, homocysteine, penicillamine, methionine, including the oxidation products methionine sulfoxide and the disulfide cystine, have been analyzed by transition potential DFT calculations. The absolute energies and intensities of the main pre-edge sulfur 1s electron transitions have been computed to determine the character of the receiving unoccupied molecular orbitals (MO), and to investigate the influence of external interactions, especially by introducing water molecules hydrogen-bonded to the ionic species present in different pH ranges. When the thiol group deprotonates for cysteine, homocysteine and penicillamine and also for the cysteine residue in glutathione the energy of the main transition, to an MO with antibonding σ*(S–H) character, reduces by ∼1.1 eV and the receiving MO obtains σ*(S–C) character. The changes in transition energy due to hydrogen-bonding were in most cases found to be relatively small, although the transition intensities could vary significantly due to the changes induced in the molecular charge distribution, thereby affecting the shapes of the spectral features. For the cysteine and penicillamine zwitterions deconvolution of the experimental spectra allowed the microscopic acid dissociation constants to be extracted separately for the thiol and the protonated amine groups, pK_a^S = 8.5 ± 0.1 and 8.2 ± 0.1, and pK_a^N = 8.9 ± 0.1 and 8.8 ± 0.1, respectively, with the thiol group in both cases being the more acidic. Coordination of cysteine to nickel(II) or mercury(II) introduced a new low energy transition involving metal ion orbitals in the receiving LUMO. The small experimentally observed energy differences between the similar main absorption features of the cysteine and methionine zwitterions, 0.2–0.3 eV in comparable surrounding, as well as a minor difference in their intensities, are reflected in the calculated transitions. The S K-edge XANES spectrum of the disulfide cystine displays a characteristic double peak with the lower energy transition (2469.9 eV) into the antibonding σ*(S–S) MO. The second peak, at 2471.5 eV in aqueous solution, contains several transitions into MOs with σ*(S–C) character involving also charge transfer to the water molecules hydrating the protonated amine groups (NH₃⁺) of cystine. For solid cystine without hydrogen bonding the experimental energy difference between the two peaks is 0.2 eV larger, while no such increase occurs for the oxidized disulfide of glutathione, with a similar –S–S– bond between its cysteine residues as in cystine, because the amine groups are engaged in peptide bonds. This study shows that externally induced changes in the intramolecular bonding, e.g., by coordination, conformation geometry or hydrogen-bonding, can significantly influence the S K-edge spectra, and emphasizes the importance of a similar chemical surrounding when choosing the model compounds for standard spectra of sulfur functional groups, used to deconvolute composite experimental spectra.