ジペプチド結晶場による分子認識

抄録

A distinct feature of a dipeptide is a straight structure bearing an amino group at one terminal and a carboxyl group at another terminal because a hardly rotatable peptide linkage exits at its center. As a result, dipeptide molecules are arranged in atwo-dimensional layer.
On stirring crystalline (R) -phenylglycyl- (R) -phenylglycine (1) and isopropyl phenyl sulfoxide (2 a) in the presence of water, (S) -2 a molecules widened the channel between the peptide layers to be accomodated under asymmetric recognition. The driving forces of this accomodation is hydrogen bonding between H₃N⁺ group and sulfinyl group, phenyl-phenyl edge-to-face interaction, and CH-π-interaction of the isopropyl of 2 a with the phenyl of 1. (R) -Methyl phenyl sulfoxide was stereo-selectively intercalated into crystalline 1. These phenomena were also observed by the peptide (1) -deposited quartz-crystal microbalance.
In contrast with 1, (R) - (1-naphthyl) glycyl- (R) -phenylglycine (5) always requires an appropriate guest molecule to maintain a crystalline structure because relatively large naphthyl group disturbs contraction of the peptide layers. Indeed, the dipeptide (5) forms the crystals including a guest molecules (an alcohol, an ether, or sulfoxide) on crystallization from methanol containing the guest. It is noteworthy that the guest exchange is possible by simply stirring the inclusion crystals with other guest molecules in carbon tetrachloride. The layer arrangement in these inclusion crystals was revealed by single-crystal X-ray analysis.