ICPBCZin: a red emitting ratiometric fluorescent indicator with nanomolar affinity for Zn2+ ions

A new fluorescent Zn2+ indicator, namely, ICPBCZin was synthesized and the spectral profile of its free and Zn2+ bound forms was studied. The newly synthesized zinc indicator incorporates as chromophore the chromeno [3′,2′:3,4]pyrido[1,2a] [1,3]benzimidazole moiety and belongs to the dicarboxylate-type of zinc probes. The compound is excited with visible light, exhibits high selectivity for zinc in the presence of calcium and other common biological ions, and its Zn2+ dissociation constant is 4.0 nM. Fluorescence spectra studies of ICPBCZin indicated a clear shift in its emission wavelength maxima upon Zn2+ binding, as it belongs to the class of Photoinduced Charge Transfer (PCT) indicators, along with changes in fluorescence intensity that enable the compound to be used as a ratiometric, visible-excitable Zn2+ probe

A functional nitric oxide reductase model

A functional heme/nonheme nitric oxide reductase (NOR) model is presented. The fully reduced diiron compound reacts with two equivalents of NO leading to the formation of one equivalent of N2O and the bis-ferric product. NO binds to both heme Fe and nonheme Fe complexes forming individual ferrous nitrosyl species. The mixed-valence species with an oxidized heme and a reduced nonheme FeB does not show NO reduction activity. These results are consistent with a so-called “trans” mechanism for the reduction of NO by bacterial NOR

Aldoxime Dehydratase: Probing the Heme Environment Involved in the Synthesis of the Carbon–Nitrogen Triple Bond

Fourier transform infrared (FTIR) spectra, "light" minus "dark" difference FTIR spectra, and time-resolved step-scan (TRS 2) FTIR spectra are reported for carbonmonoxy aldoxime dehydratase. Two C-O modes of heme at 1945 and 1964 cm -1 have been identified and remained unchanged in H 2O/D 2O exchange and in the pH 5.6-8.5 range, suggesting the presence of two conformations at the active site. The observed C-O frequencies are 5 and 16 cm -1 lower and higher, respectively, than that obtained previously (Oinuma, K.-I.; et al. FEBS Lett.2004, 568, 44-48). We suggest that the strength of the Fe-His bond and the neutralization of the negatively charged propionate groups modulate the ν(Fe-CO)/ν(CO) back-bonding correlation. The "light" minus "dark" difference FTIR spectra indicate that the heme propionates are in both the protonated and deprotonated forms, and the photolyzed CO becomes trapped within a ligand docking site (ν(CO) = 2138 cm -1). The TRS 2-FTIR spectra show that the rate of recombination of CO to the heme is k 1945 cm -1 = 126 ± 20 s -1 and k 1964 cm -1 = 122 ± 20 s -1 at pH 5.6, and k 1945 cm -1 = 148 ± 30 s -1 and k 1964 cm -1 = 158 ± 32 s -1 at pH 8.5. The rate of decay of the heme propionate vibrations is on a time scale coincident with the rate of rebinding, suggesting that there is a coupling between ligation dynamics in the distal heme environment and the environment sensed by the heme propionates. The implications of these results with respect to the proximal His-Fe heme environment including the propionates and the positively charged or proton-donating residues in the distal pocket which are crucial for the synthesis of nitriles are discussed