MOLECULAR-DYNAMICS OF THE INCLUSION COMPLEXES OF CYCLOHEXAAMYLOSE WITH SOME AROMATIC AMINO-ACIDS AND DIPEPTIDES

The molecular dynamics of the inclusion complexes of cyclohexaamylose as a model of enzyme have been studied by means of carbon-13 NMR spectroscopy. As substrates we have chosen L-phenylalanine, L-tyrosine, L-tryptophan, glycyl-L-phenylalanine, and L-phenylalanyl-L-lysine. The molecular motion of both the cyclohexaamylose and the substrates in D2O-DCl solutions have been investigated by dividing the spin-lattice relaxation time into two contributions, the overall molecular reorientation and the anisotropic internal rotation. Upon complex formation, the correlation times for the internal motion of the phenyl ring of phenylalanine residue increase by a factor of up to 8, while those for the overall reorientation increase by a factor of only 2. These results indicate that the complex formation of the substrate with the cyclohexaamylose are induced by the insertion of aromatic side chain into a cavity of cyclohexaamylose even in the aqueous solution. The overall correlation times of the substrates are about three to seven times shorter than those of the host molecule. Thus the forces which bind the host cyclohexaamylose and the substrate are relatively weak. It was observed that the tightness of the inclusion varies with the types of aromatic amino acids and dipeptides.