Conformational properties of tripeptides having cyclic dipeptide backbone and their catalytic properties for enantiomer-selective hydrolysis |
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Authors: | Yasushi Kikuchi Masao Tanihara Yukio Imanishi |
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Institution: | Department of Polymer Chemistry, Kyoto University, Yoshida Honmachi, Sakyo, Kyoto 606, Japan |
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Abstract: | Conformation in aqueous solution at pH 6.95 of tripeptides having cyclic dipeptide backbones, cyclol-Glu(l-Leu-OBzl)-l-His] and cyclol-Glu(l-Leu-OH)-l-His], was investigated by u.v., c.d. and n.m.r. spectroscopy and by the lanthanide probe method. In the major conformation of cyclol-Glu(l-Leu-OBzl)-l-His], the cyclic dipeptide backbone takes a flagpole-boat conformation in which the sidechain of the l-His residue is nearly parallel with the backbone plane and the sidechain of the l-Glu residue protrudes outside the backbone plane. In the major conformation of cyclol-Glu(l-Leu-OH)-l-His], the cyclic dipeptide backbone takes a flagpole-boat conformation in which the sidechains of the l-His and l-Glu residues are accommodated in the same side of the backbone plane so that the imidazolyl sidechain of l-His residue is twisted slightly. Tripeptides were not found to change the conformation when metal salts or ammonium salts such as Cl?H3N?(CH2)11 COOEt, Gly-OEt-HCl, dl-Val-OEt-HCl and l-Leu-OEt-HCl were added, but a significant conformation change occurred upon adding d-Leu-OEt·HCl. If the same situation holds with the addition of α-amino acid p-nitrophenyl ester hydrochlorides, the previously reported enantiomer-selective catalysis by the tripeptides which hydrolysed d-Leu-OPh(NO2·HCl faster than l-Leu-OPh(NO2)·HCl can be explained; that is, the tripeptides change the conformation only when d-Leu-OPh(NO2)·HCl is bound and consequently the intramolecular reaction is facilitated. This phenomenon may be compared with that of ‘induced fit’ in enzyme catalysis. |
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Keywords: | Catalysis tripeptide cyclic dipeptide backbone conformation induced fit enantiomer-selective catalysis nuclear magnetic resonance circular dichroism |
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