  Iron pyrite (FeS$_{2}$) is the most abundant metal sulfide on Earth. Owing to
its reactivity and catalytic activity, pyrite has been studied in various
research fields such as surface science, geochemistry, and prebiotic chemistry.
Importantly, native iron-sulfur clusters are typically coordinated by cysteinyl
ligands of iron-sulfur proteins. In the present paper, we study the adsorption
of L-cysteine and its oxidized dimer, L-cystine, on the FeS$_{2}$ surface,
using electronic structure calculations based density functional theory and
Raman spectroscopy measurements. Our calculations suggest that sulfur-deficient
surfaces play an important role in the adsorption of cysteine and cystine. In
the thiol headgroup adsorption on the sulfur-vacancy site, dissociative
adsorption is found to be energetically favorable compared with molecular
adsorption. In addition, the calculations indicate that, in the cystine
adsorption on the defective surface under vacuum conditions, the formation of
the S-Fe bond is energetically favorable compared with molecular adsorption.
Raman spectroscopic measurements suggest the formation of cystine molecules
through the S-S bond on the pyrite surface in aqueous solution. Our results
might have implications for chemical evolution at mineral surfaces on the early
Earth and the origin of iron-sulfur proteins, which are believed to be one of
the most ancient families of proteins.
