Solid-state Vibrational Circular Dichroism Spectra of Isoleucine and Its Related Compounds: Effects of Interplay between Two Chiral Centers

The solid-state vibrational circular dichroism spectra were measured for isoleucine and its related compounds. These amino acids are characterized by the presence of two chiral centers (the asymmetric carbon in an alkyl chain and the one at the α-position). The couplet peaks were observed for the stretching vibrations of –COOH and the bending vibration of –NH2. The signs of the peaks due to –COOH were determined uniquely by the RS-chirality of the α-carbon, while those due to –NH2 were dependent on the combination of the chirality of the α-carbon and the carbon in an alkyl chain. The results implied that the asymmetric vibrational behavior of an –NH2 group was affected by the interplay between two chiral centers. The solid-state vibrational circular dichroism spectra were measured for isoleucine and its related compounds, revealing that the signs of the peaks due to –COOH were determined uniquely by the RS-chirality of the α-carbon, while those due to –NH2 were dependent on the combination of the chirality of the α-carbon and the carbon in an alkyl chain.