Photomodulation of conformational states. I. Mono- and bicyclic peptides with (4-amino)phenylazobenzoic acid as backbone constituent

The thioredoxin reductase active-site fragment H-Ala-Cys-Ala-Thr-Cys-Asp-Gl y-Phe-OH [134-141], which is known for its high tendency to assume an almos t identical conformation as in the intact enzyme, was backbone cyclized wit h the photoresponsive (4-amino)phenylazobenzoic acid (APB) to produce a mon ocyclic and disulfide-bridged bicyclic APB-peptide. Light-induced reversibl e cis/trans isomerization occurs at identical extents in both the linear an d the two cyclic forms. Nuclear magnetic resonance conformational analysis clearly revealed that in the bicyclic APB-peptide both as a trans- and cis- azo-isomer the constraints imparted by the bicyclic structure do not allow the molecule to relax into a defined low energy conformation, thus making t he molecule a frustrated system that flip-flops between multiple conformati onal states. Conversely, the monocyclic APB peptide folds into a well-defin ed lowest energy structure as a trans-azo-isomer, which upon photoisomeriza tion to the cis-azo configuration relaxes into a less restricted conformati onal space. First femtosecond spectroscopic analysis of the dynamics of the photoreaction confirm a fast first phase on the femtosecond time scale rel ated to the cis/trans isomerization of the azobenzene moiety followed by a slower phase in the picosecond time scale that involves an adjustment of th e peptide backbone. Due to the well-defined photoresponsive two-state trans ition of this monocyclic peptide molecule, it represents a model system wel l suited for studying the ultrafast dynamics of conformational transitions by time-solved spectroscopy. (C) 2000 John Wiley & Sons, Inc.