Conformational Studies of Bis(cyclic tetrapeptide), S,S'-Bis. A Proposed Castanet-type Structure in a Dimethyl Sulfoxide Solution

Article abstract of DOI:10.1246/bcsj.55.3808

A bis(cyclic tetrapeptide), S,S'-bis (BCGCSP), was prepared by coupling two homodetic cyclotetrapeptide.The conformations of BCGCSP in solutions were investigated by means of NMR, Raman, and CD spectroscopy.The 1H-NMR data in dimethyl-d₆ sulfoxide (Me2SO-d₆) demonstrated that the two cyclic skeletons of BCGCSP take the same conformations as that of an authentic cyclic tetrapeptide, cyclo (CGCSP).The 1H-NMR spectrum of CGCSP contains three sets of resonances, corresponding to three different molecular conformations.From the Raman lines of BCGCSP at 529 cm^-1 and 683 cm^-1 in Me2SO-d₆, it is deduced that the two HC_α-C_βS dihedral angles are 180 deg (trans), while one of the SS-C_βC_α dihedral angles is 20-30 deg and the other is either near 90 deg or near 180 deg.In addition, the values of two Raman lines predict that the C_βS-SC_β dihedral angle is in the range of near 90 deg.Such a conformation around the C_βS-SC_β bond is also reasonable in the light of the presence of a CD transition (CS-SC) at λ>250 nm in water.By combining the above data, the CPK molecular model of BCGCSP was constructed.An inspection of the model reveals that this bis(cyclic tetrapeptide) is likely to adopt a "castanet-type" structure in Me2SO-d₆.At the same time the individual cyclic skeleton is considered to take three different ring conformations in equilibrium.