Cysteine and cystine adsorption on FeS$_{2}$(100)

TL;DR

This study investigates how cysteine and cystine molecules adsorb onto FeS₂ (pyrite) surfaces, revealing the importance of sulfur vacancies and providing insights into mineral surface chemistry relevant to early Earth and biological evolution.

Contribution

It combines density functional theory calculations and Raman spectroscopy to elucidate the adsorption mechanisms of cysteine and cystine on pyrite surfaces, highlighting the role of sulfur vacancies.

Findings

Dissociative adsorption is energetically favored at sulfur-vacancy sites.

Cystine forms S-Fe bonds on defective surfaces under vacuum.

Raman spectra indicate cystine formation via S-S bonds on pyrite.

Abstract

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.