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

Abstract The thioredoxin reductase active‐site fragment H–Ala–Cys–Ala–Thr–Cys–Asp–Gly–Phe–OH [134–141], which is known for its high tendency to assume an almost identical conformation as in the intact enzyme, was backbone cyclized with the photoresponsive (4‐amino)phenylazobenzoic acid (APB) to produce a monocyclic and disulfide‐bridged bicyclic APB‐peptide. Light‐induced reversible cis / trans isomerization occurs at identical extents in both the linear and 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 the molecule a frustrated system that flip‐flops between multiple conformational states. Conversely, the monocyclic APB peptide folds into a well‐defined lowest energy structure as a trans‐ azo‐isomer, which upon photoisomerization to the cis‐ azo configuration relaxes into a less restricted conformational space. First femtosecond spectroscopic analysis of the dynamics of the photoreaction confirm a fast first phase on the femtosecond time scale related to the cis/trans isomerization of the azobenzene moiety followed by a slower phase in the picosecond time scale that involves an adjustment of the peptide backbone. Due to the well‐ defined photoresponsive two‐state transition of this monocyclic peptide molecule, it represents a model system well suited for studying the ultrafast dynamics of conformational transitions by time‐resolved spectroscopy. © 2000 John Wiley & Sons, Inc. Biopoly 54: 489–500, 2000