Theoretical studies of a novel photo-switchable self-organized peptide system

Synthetic organic functional nanomaterials have been the subject of intense study lately due to their potential use in many fields. As a result, theoretical studies of such materials have also received extensive attention. In this paper, seven models of a novel photo-switchable self-organized peptide system were optimized using the semi-empirical molecular orbital method AM1, and single point calculations of three of them were investigated by means of the density functional B3LYP/3-21G* method. The geometries, energetics and frontier orbital interactions of these photo-switchable peptide subsystems were calculated and analyzed. Our results provide insight into the formation, self-assembly and photo-isomerization of the system. Weak interactions between two polypeptide rings, especially hydrogen-bonding interactions, are crucial for stabilizing the conformation and self-assembly. Remarkably, oligomers of the E form show almost no cooperative effect, strongly supporting the notion that it is possible to break the intermolecular hydrogen bonds to enable E → Z isomerization reactions. Moreover, the polypeptide rings strengthen the conjugation effect on azobenzene subunits and lead to a reduction in the frontier molecular orbital energy differences. The self-assembling process of the E form also reinforces frontier orbital interactions and favors E → Z isomerization.