Peptides in the Days of Photonics

Abstract

Introduction Light represents an ideal primary energy source for manipulating systems of either microscopic or macroscopic size since photons can be applied in well-controlled and clean manner and with extreme spatial and temporal resolution to induce reversible or irreversible photochemical reactions. While irreversible reactions have been applied towards the photocleavage of protecting groups as well as in photocaging and photoaffinity methodology, reversible photoreactions generate interconvertable distinct structures upon the external light stimuli that perfectly suit for molecular light-switches. Among the known photochromic compounds azobenzene, spiropyran, filgides, and diarylethene derivatives have found widespread application in light-switching of optical, chemical and mechanical properties of small molecules, macromolecules, and biopolymers [1-4]. In this context azobenzene proved to be particularly well suited because of the pronounced changes in geometry and polarity upon the light-triggered cis-trans isomerization, but foremost because of the high (photo)stability, the high isomerization and quantum yields [5]. The pioneering work of Murray Goodman [6,7] with azobenzene for photomodulation of conformational states in poly-α-amino acids has stimulated extensive studies in the field [8]. Because of the inherent drawbacks of using multiple photochromic units in molecular architectures, increasing attention has been paid more recently to the design of simple peptide systems with single light-switches appropriately positioned to constrain and relax the conformational space of the peptide backbone in reversible manner. By this approach the extremely fast and fully reversible isomerization process [9] was exploited to monitor folding/unfolding processes at femtoto picosecond time scales with ultrafast UV [10,11] and IR spectroscopy [12,13]. Model systems were used which were head-to-tail [14] and side chain-to-side chain [13] crossbridged with appropriate azobenzene derivatives. The most recent advances in the field obtained in our laboratory with azobenzene as a photoswitch are summarized below.