Photomodulation of conformational states of phenylazobenzyloxycarbonyl- l-proline and related peptides

Abstract

In linear flexible peptides the isomeric equilibria of aminoacyl- l-proline peptide bonds are known to be shifted in favor of the trans configuration, whereas in N-urethanyl- l-proline compounds the cis configuration is preferred with stabilization of a C 7 conformation, i.e. of an inverse γ-type turn. To analyze the effects of a photoswitchable configuration in the urethane moiety on this small ordered structure element the photochrome azobenzene was incorporated as a urethane group into proline and the praline peptides ProPhe and ProPheGly. The phenylazobenzyloxycarbonyl compounds retain the preferred cis configuration of the C-N urethane bond and undergo a reversible photomodulated trans to cis isomerization of the azobenzene moiety with more or less pronounced effects on the distribution of the four possible isomers as detected by FTIR and NMR spectroscopy. Correspondingly, detectable changes in the population of the C 7 conformation were observed upon irradiation making this system highly promising for monitoring dynamics by time-resolved spectroscopy.