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Four Hg-based IR nonlinear-optical materials, AHgSnQ 4 (A = Sr, Ba; Q = S, Se), were discovered and investigated systematically. Their structures are built of two-dimensional [HgSnQ 4 ] 2- layers, which are assembled alternately by distorted (HgQ 4 and SnQ 4 ) tetrahedra and separated by eight-coordinated A 2+ cations. The two sulfides AHgSnS 4 (A = Ba, Sr) exhibit large second-harmonic-generation (SHG) responses (2.8 and 1.9 × AgGaS 2 at 2.09 μm), as well as large band gaps (2.77 and 2.72 eV). The two selenides AHgSnSe 4 (A = Ba, Sr) show even stronger SHG responses, about 5 times that of AgGaS 2 . Furthermore, all four compounds show phase-matching behavior, and the results of first-principles calculation elucidate the key role of the HgQ 4 group in the enhanced SHG effect in β-BaHgSnS 4 and BaHgSnSe 4 .

Nitrosylation of the chelate-thiolate-containing dinitrosyliron complex (DNIC) [(S(CH(2))(3)S)Fe(NO)(2)](-) triggers nitric oxide (NO) activation to generate the homoleptic nitrosyl {Fe(NO)(2)}(9) DNIC [Fe(NO)(4)](-) (1) made up of two nitroxyls (or two NO anions) attached to a delocalized {Fe(NO)(2)}(9) motif. The significantly longer N3-O3/N4-O4 [1.380(12) and 1.280(12) Å] and Fe1-N3/Fe1-N4 [2.008(11) and 2.045(10) Ǻ] bond distances reflect that N3-O3 and N4-O4 of complex 1 may act as the nitroxyl-coordinated ligands. That is, the electronic structure of the DNIC 1 is best described as a {Fe(NO)(2)}(9) motif coordinated by two nitroxyl (NO(-)) ligands.

This manuscript describes the interaction of low-molecular-weight DNICs with short peptides designed to explore the stability and structure of DNIC-peptide/RRE-peptide constructs. Although characterization of protein-bound and low-molecular-weight DNICs is possible via EPR, XAS, and NRVS, this study demonstrates that the combination of aqueous IR ν(NO) and UV-vis spectra can serve as an efficient tool to characterize and discriminate peptide-bound DNICs and RREs. The de novo chelate-cysteine-containing peptides KC(A)(n)CK-bound (n = 1-4) dinitrosyliron complexes KC(A)(n)CK-DNIC (CnA-DNIC) and monodentate-cysteine-containing peptides KCAAK-/KCAAHK-bound Roussin's red esters (RREs) 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 reversible transformation between CnA-DNIC and CnA-RRE via {Fe(NO)(2)}(9)-{Fe(NO)(2)}(10) reduced-form peptide-bound RREs demonstrate that the {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 iron and the chelating effect of the bound proteins of [Fe-S] clusters.