A Molecular Orbital Study of C-H…Cl- and N-H…Cl- Hydrogen Bonds. Inferences on Selected Metal Complexes and on Protein ClC Cl- Channels

Hydrogen bond type interactions X–H···Y– (X: C, N, O; Y: Cl) for systems that contain 1,3-imidazole (IM), 1,3-pyrimidine (PYM), N-methylacetamide (MAA), methylammonium (MA), methylamine (MAB), 1-hydroxy-4-methylbenzene (HMB), N-methylguanidinium (MGU), methanol (MeOH), have been investigated via the methods of density functional theory (DFT) at the B3LYP functional level and ab initio MP2, by using mostly the standard 6-31G** and 6-31+G** basis sets. The study helps in understanding structural aspects of at least Re/Ru-imidazole, Ru-pyrimidine and Ru-arene complexes and allows to evaluate the adduct formation energy (electronic), for species of the type M(IM)N–H···Cl–, M(IM/PYM)C–H···Cl–, whose upper limits are ca. –24 and –10 kcal at gas phase. Computed structural and energy parameters help also in evaluating the mechanism of extrusion of Cl– anions in certain ClC Chloride channels from membrane proteins. The hydrogen bond formation energy for selected aminoacid residues with Cl– ranges ca. –106 to –15 kcal mol–1. Owing to the predominance of CONH peptide bonds in every protein system, the formation of the C(=O)–N–H···Cl– hydrogen bond (DEel ≈ –21 kcal) is often revealed in X-ray structures of protein···chloride adducts.