A theoretical investigation of sulphur K-shell X-ray absorption of cysteine

The near-edge X-ray absorption fine structure (NEXAFS) spectrum at the sulphur K-edge of the cysteine amino acid is simulated by the ab initio Hartree–Fock static-exchange method. Several forms of cysteine are investigated, including neutral, zwitterionic and water-solvated forms. The simulations indicate that the presence of an environment induces a small lowering of the edge position in the spectrum of the neutral form.