Conformational search of scytonemin and its stereoisomers from first-principles: Towards green cosmetics

Abstract

Scytonemin is a UV-absorbing metabolite produced by cyanobacteria that may be used as a sustainable sunscreen ingredient. However, ab initio studies on scytonemin structure and excited states are sparse. We present herewith a theoretical study to predict its molecular structure using density functional theory. Predicted conformational energy barriers from the def2-QZVPP basis and select exchange-correlational functionals were benchmarked against a coupled-cluster calculation at the basis set limit. We show that the E and Z isomeric forms of the scytonemin monomer are planar compounds; moreover, we verify that scytonemin itself, as well as its geometric isomers, is non-planar, and can exist as two stable rotamers in the syn– and anticlinal regimes, the former being the more favorable state. The benchmarking suggests the BHHLYP method predicts torsion barriers well. Studies of scytonemin photochemistry can thus be facilitated by the provided structural parameters.