Abstract
This study describes the interaction of low-molecular-weight DNICs with short peptides designed to explore the stability and structure of peptide-DNIC/peptide-RRE constructs. Although characterization of DNICs is possible via EPR, XAS and NRVS, combination of aqueous IR νNO and UV-vis spectra can serve as an efficient tool to characterize and discriminate the peptide-bound DNICs/RREs. The de novo chelate-cysteine-containing peptides KC(A)nCK-bound DNICs CnA-DNIC and monodentate-cysteine-containing peptides KCAAK-/KCAAHK-bound Roussin’s red esters KCAAK-RRE/ KCAAHK-RRE were synthesized and characterized by aqueous IR, UV-vis, EPR, CD, XAS and ESI-MS. In contrast to the inertness of chelate-cysteine-containing peptide-bound DNICs toward KCAAK/KCAAHK, transformation of KCAAK-RRE/KCAAHK-RRE into CnA-DNIC triggered by CnA and the reversible transformation between CnA-DNIC and CnA-RRE via the {Fe(NO)2}9-{Fe(NO)2}10 reduced-form peptide-bound RREs demonstrate that {Fe(NO)2}9 motif displays a preference for chelate-cysteine-containing peptides over monodentate-cysteine-containing peptides. Also, this study may signify that nitrosylation of [Fe-S] proteins generating protein-bound RREs, reduced protein-bound RREs or protein-bound DNICs are modulated by both the oxidation state of Fe and chelating effect of the bound proteins of [Fe-S] clusters. Nitrosylation of the chelate-thiolate-containing DNIC triggers NO activation to generate the homoleptic nitrosyl DNIC [Fe(NO)4]− made up of two nitroxyls attached to a delocalized {Fe(NO)2}9 motif. The significantly longer N(3)−O(3)/N(4)−O(4) and Fe(1)−N(3)/Fe(1)−N(4) bond distances (bent Fe(1)−N(3)−O(3)/Fe(1)−N(4)−O(4) bond angel) reflect that N(3)−O(3) and N(4)−O(4) of [Fe(NO)4]− may act as nitroxyl- coordinated ligands. That is, the electronic structure of the [Fe(NO)4]− is best described as a {Fe(NO)2}9 motif coordinated by two nitroxyl (NO−) ligands.