ULTRAVIOLET RESONANCE RAMAN STUDIES OF TRANS-PEPTIDES AND CIS-PEPTIDES - PHOTOCHEMICAL CONSEQUENCES OF THE TWISTED PI-STAR EXCITED-STATE

Excitation into the amide pi-pi* transitions of N-methylacetamide (NMA) and small peptides such as di- and triglycine results in the photochemical conversion of the trans-amides into cis-amides with a high quantum yield. The cis form of NMA is easily monitored since its amide II UV Raman cross section is ca. 10-fold larger than that of the trans peptide with 220-nm excitation. We can detect the ca. 1.5% Boltzmann population of the cis form of NMR present at room temperature. The amide II mode of cis peptides differs dramatically from that of trans peptides since it contains much less N-H in-plane bending. This result is inconsistent with previous normal mode calculations which assumed identical geometry and force constants for the cis and trans forms. The pi* excited state of peptides is twisted relative to the ground state in a manner reminiscent of ethylene. We also reinterpret previous studies of N-methylthioacetamide to indicate that its pi* excited state is twisted. Our data clearly indicate the correctness of the assignment of the peptide conformation sensitive band at ca. 1400 cm-1 to the overtone of the amide V vibration for dipeptides and polypeptides. The fact that the overtone of the amide V vibration is not enhanced for NMA and related derivatives indicates a significant difference between the NMA excited-state potential surface and that of dipeptides and polypeptides. This result may signal that the peptide pi* excited state is delocalized over the peptide backbone.