Active site of bacteriorhodopsin. FTIR and 1H NMR studies using a model molecule

Abstract

We obtained the 2-N-methylaminoethylguanidine amide, 4, of Kemp's triacid(all-cis-1,3,5-trimethylcyclohexane 1,3,5-tricarboxylic acid) as a model substance for the active site of bacteriorhodopsin. Compound 4 was synthesised from Kemp's triacid triethyl ester, 1, in three reactions. Compound 4 and its complex with tetrabutylammonium 4-methylphenolate were studied by FTIR and ¹H NMR spectroscopy in acetonitrile solutions. In the case of compound 4, two types of hydrogen bond are formed: one is the CO₂H⋯N⇌CO^–₂⋯H⁺N bond. In this case, the donor is one of the two carboxylic acid groups, and the acceptor, the guanidine group. A double-minimum proton potential is present in this bond and therefore it exhibits large proton polarizability. The second NH⋯OC hydrogen bond formed between the protonated guanidine (proton donor group) and the carbonyl O atom of the other carboxylic group is asymmetrical. The proton is localised at the guanidine residue. If a phenolate molecule is added to the solution of compound 4, the situation changes dramatically. A PhOH⋯N⇌PhO^–⋯H⁺N bond with large proton polarizability is formed between the phenolate and guanidine groups. The polarizable carboxylic acid–guanidine hydrogen bond is broken and the asymmetrical NH⋯O bond between guanidine and the O atoms of carboxylic acid becomes much stronger. The results obtained with the model are compared with those obtained earlier with bacteriorhodopsin.