Active Site of Bacteriorhodopsin. FTIR and 1H NMR Studies using a Model Molecule

Article abstract of DOI:10.1039/ft9949001095

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 ->/<- CO2(-)...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...O=C 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 compoud 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.

Full text of DOI:10.1039/ft9949001095

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