Comparison of the theoretical spectroscopic signatures of scytonemin with its monomer

Abstract

Certain strains of cyanobacteria produce the photoprotective metabolite scytonemin formed from a monomeric precursor. However, its specific mechanisms of interaction with light is still an active area of research. These gaps act as fetters upon the complete understanding of cyanobacteria photoprotection and scytonemin's assimilation for material science applications. Hence, we carried out in vacuo quantum chemical calculations using density functional theory to compare the infrared and Raman activity of scytonemin and its monomeric form. We found that the infrared spectra are highly similar, with the dimeric form having stronger intensities due to pairs of nearly-degenerate vibrational modes that correspond to weakly-coupled vibrations between monomeric halves. The two compounds thus have comparable infrared absorption. Most of the discrepancy is found in the non-resonant Raman spectra, where there is major differences in the induced change in polarizability from aromatic and unsaturated C-H stretching and bending modes. Resonance Raman spectra further suggest that excitation to the low-lying states mainly enhance the Raman activity of the aromatic stretching. Resonance Raman spectra of dimeric scytonemin in two low-lying excited states also suggest localized and delocalized excitations. The results of this paper therefore suggest the differences with the scytonemin monomer and dimer lie in their aromatic electronic structures.