SYNTHESIS AND VIBRATIONAL ANALYSIS OF A LOCKED 14-S-CIS CONFORMER OF RETINAL

A 12-N-ethano retinal Schiff base is synthesized to study the vibrational properties of a C-14-C-15 s-cis conformer of retinal. The synthesis of the 12-N-ethano retinal Schiff base and seven of its isotopomers is described. Raman and FTIR spectra of this compound and its isotopic derivatives are analyzed and the normal mode assignments compared to MNDO normal coordinate calculations. The frequency of the C-14-C-15 stretch at 1096 cm-1 is approximately 100 cm-1 below typical values for s-trans conformers of retinal polyenes, supporting previous arguments that s-cis conformers will have characteristically low frequencies for the C-C stretch of the s-cis bond (Smith et al., Proc. Natl. Acad. Sci. U.S.A. 1986, 83, 967-971). Deuteriation at the 15-position produces two modes at 950 and 1044 cm-1 that contain C-15D rock character. The elevated frequency (1044 cm-1) of the high-frequency component is a marker band for the C=N syn geometry and confirms an earlier analysis of the sensitivity of the C-15D rock frequency to Schiff base configuration (Ames et al., Biochemistry 1989, 28, 3681-3687). This work on retinals with chemically defined configuration and conformation provides a basis for further analysis of the geometry of these bonds in retinal-containing pigments.