VIBRATIONAL SPECTROSCOPY OF LDLL-MIXTURES OF AMINO-ACIDS

In equimolar mixtures of L- and DL-proline, the vibrational spectra of the LDLL-mixture is not an average between that of the L- and the DL- isomers. Instead the DL-mixture is clearly intermediate between the L- and the LDLL-spectra. Adding L-isomer to the DL-isomer, the solid sta te spectra becomes less similar to that of the L-isomer. Replacing res pectively the LL-half and the DL-half with a second amino acid, each s et can significantly alter the conformation of proline and/or the seco nd amino acid, even though both SetS (LDLL- and LLDL-mixtures) have id entical structural composition. Large changes in spectral properties b etween LDLL- and LLDL-amino acids are consistent with diastereoismeric interactions between isomers. Infrared frequencies for chiral structu res in an achiral localized environment are not predictive of the freq uencies for the same chemical structures in a chiral environment.