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 state spectra becomes less similar to that of the L-isomer. Replacing respectively the LL-half and the DL-half with a second amino acid, each set can significantly alter the conformation of proline and/or the second amino acid, even though both SetS (LDLL- and LLDL-mixtures) have identical structural composition. Large changes in spectral properties between LDLL- and LLDL-amino acids are consistent with diastereoismeric interactions between isomers. Infrared frequencies for chiral structures in an achiral localized environment are not predictive of the frequencies for the same chemical structures in a chiral environment.