ACTIVE-SITE OF BACTERIORHODOPSIN - FTIR AND H-1-NMR STUDIES USING A MODEL MOLECULE

We obtained the 2-N-methylaminoethylguanidine amide, 4, of Kemp's tria cid(all-cis-1,3,5-trimethylcyclohexane 1,3,5-tricarboxylic acid) as a model substance for the active site of bacteriorhodopsin. Compound 4 w as synthesised from Kemp's triacid triethyl ester, 1, in three reactio ns. Compound 4 and its complex with tetrabutylammonium 4-methylphenola te were studied by FTIR and H-1 NMR spectroscopy in acetonitrile solut ions. In the case of compound 4, two types of hydrogen bond are formed : one is the CO2H ... N half arrow left over half arrow right CO2- ... H+N bond. In this case, the donor is one of the two carboxylic acid g roups, and the acceptor, the guanidine group. A double-minimum proton potential is present in this bond and therefore it exhibits large prot on 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 sol ution of compound 4, the situation changes dramatically. A PhOH ... N half arrow left over half arrow left PhO- ... H+N bond with large prot on polarizability is formed between the phenolate and guanidine groups . The polarizable carboxylic acid-guanidine hydrogen bond is broken an d the asymmetrical NH ... O bond between guanidine and the O atoms of carboxylic acid becomes much stronger. The results obtained with the m odel are compared with those obtained earlier with bacteriorhodopsin.