P04 Reactivity of inorganic sulfide species towards hemeproteins model compounds. Theoretical and experimental perspective

Abstract

There are several hemeproteins which are able to bind sulfide species with variable affinity [1–4] . Both ligand migation and the stabilization of the coordinated sulfide show unusual behaviour in the different hemeproteins studied, as the binding affinity can not be explained only by similar structural features. In order to disentangle the molecular determinants which control sulfide affinity to hemeproteins, we focused in the characterization of the binding of H 2 S and its conjugate bases to hemeprotein model compounds, from a theoretical and experimental perspective. QM calculations were performed for five and six-coordinated core-porphyrins with H 2 S, SH − or S 2− and an imidazol ring as fifth and sixth ligands, respectively. Fully optimized geometries were obtained at the DFT level using the B3LYP, PBE and OPBE functionals and the 6–31G** basis set. We considered the low spin states of such complexes at ferric and ferrous states. Frequency calculations under the normal mode approximation were obtained at the same levels of theory. Strikingly, we note that structural parameters and vibrational frequencies are only slightly dependent of the protonation state of the sulfide. In addition, the six-coordinated active site was parametrized by means of electronic structure calculations in order to perform a classical MD simulation for microperoxidase 11 (MP11) bound to SH 2 ,SH - or S 2- . MP11 is a hemepeptide derived cytochrome c that retains the proximal histidine as fifth ligand, among the other ten aminoacids. The classical MD approach allows us to analyze the environment of the SH - , and to build initial structures for a more refined QM–MM simulation. With the aid of the latter simulation, we obtained the vibrational spectra of SH − bound to the MP11, as predictive experimental results. The Fe (III) N-acetyl microperoxidase 11 (Fe (III) NAcMP11) [5] derivative was the model compound of choice for the solution experiments of binding of inorganic sulfides, as it prevents the aggregation observed with MP11. The reaction was studied in deoxygenated solutions of Fe (III) NAcMP11 (10 -5 M, buffer phosphate pH = 6.8, 25 °C, λ max = 398 nm) by addition of aliquots of freshly prepared solutions of SHNa, at pH 11. Unlike the control experiments with free Fe (III) porphyrinates, the corresponding reduced reaction product (Fe (II) NAcMP11 λ max = 417 nm) was not obtained. Instead, a new and stable complex was formed, as evidenced by UV/Vis spectroscopy ( λ max = 410 nm); regular features describing a change from a high spin to a low spin state FeIII compound were identified [6,7] .