Azobenzene‐Containing polypeptides: Photoregulation of conformation in solution

Abstract

SynopsisPhotochromic polypeptides, with 16 to 56% azobenzene groups in the side chains, have been prepared by reaction of poly(L-glutamic acid) with paminoazobenzene, both in the presence of dicyclohexylcarbodiimide/N-hydroxybenzotriizole and of pivaloyl chloride. Analogous modification reactions carried out on polyk-aspartic acid) were unsuccessful owing to the formation of N-succinimide rings. In trimethylphosphate, all the azopolypeptides exhibit the a-helix CD pattern. Irradiation produces the tram-tocis isomerization of the azo side chains, but does not induce any variations of the backbone conformation. In water, the CD spectra indicate the presence of appreciable amounts of a helix in 16 and 21% mol azo-containing poly(bglutamates), while a fi structure is present in a 36% mol azopolypeptide. Light produces conformational changes of the polypeptide conformation which are completely reversed in the dark. The extent and kind of photobehavior depend on the azo content and the pH value at which irradiation is carried out. The light-induced effects are discussed on the basis of the pH-induced order-disorder conformational transitions. In fact, the pK values and the transition curves of the dark-adapted samples were found to be different from those of the irradiated ones.

Towards Photocontrol over the Helix–Coil Transition in Foldamers: Synthesis and Photoresponsive Behavior of Azobenzene‐Core Amphiphilic Oligo(meta‐phenylene ethynylene)s

“…Irradiation induces a conformational transition from the random coil to the a-helical structure. [7] Furthermore, polyA C H T U N G T R E N N U N G (l-lysine) appended with azobenzene chromophores has been shown to transform from a b-sheet structure to an a-helix structure upon photoisomerization. [8] The remarkable conformational transitions in all of these cases were found to be strongly dependent on the polymers environment and employed solvent mixtures and hence were attributed to the changes in the polarity of the photochromic side chains affecting the surrounding media.…”

Photoswitchable Polypeptides

“…The polymers were obtained by treating high molecular weight poly(l-glutamic acid) with p-aminoazobenzene in the presence of dicyclohexylcarbodiimide (DCCI) and N-hydroxy-benzotriazole (HOBt). [21,22] The photochromic reactions are illustrated in Figure 3. When the modification reaction was carried out in the presence only of 13 DCCI, the modified polymers were found to contain considerable amounts of N-acylureic groups originating from side reactions between DCCI and the c-carboxylic acid functions.…”

“…In particular, the trans-cis isomerization is characterized by a strong decrease in the intense band at about 350 nm, associated with a p-p* transition, and a concomitant intensification in the band at 450 nm, associated with the n-p* transition of the azo chromophore. [21,22] In organic solvents such as trimethylphosphate or trifluoroethanol, the azo-modified polymers show the CD pattern of the a-helix structure, with the two typical minima at 208 and 222 nm ( Figure 5). Using a 200 W lamp, irradiation for 1 or 2 minutes is enough to achieve the photostationary state.…”

“…[23,24] Upon irradiation, the polypeptides displayed a reversible change of optical rotation at 589 nm, associated with the trans-cis photoisomerization of the azo units, even though the photoisomerization of the side chains did not induce any change in the a-helical main chain conformation. [20,22] The mechanism of the photoresponse was explained as follows. al.…”

Photocontrol of Polypeptide Membrane Structure and Functions by cis-trans Isomerization in Side-Chain Azobenzenesulfonate Groups

“…As a result, it can be estimated that the azo·S-14.1-PGA membrane changes the helix content from 76% to 46% at pH 7.5 by light. Ciardelli et al 10 reported the a-helix structure of PGA containing up to 16 mol% azobenzene groups (without sulfQnate anions) in the side chains in the solution system to be independent of the ultra-violet light irradiation at any pH. H should be emphasized, therefore, that increase in the photoPolymer J., Vol.…”