Very happy to share our new article entitled "Electronic coupling in the reduced state lies at the origin of color changes of ommochromes", which is now availabe in the journal Dyes and Pigments! You can access the published version for free here (link available for 50 days) and the preprint here.
This interdisciplinary study tackles the molecular mechanisms that allow ommochrome pigments to change their color from yellow to red (i.e. bathochromy) upon reduction. Such bathochromic color changes are unusual for animal pigments since chemical reduction is usually associated to color losses (i.e. hypsochromy) due to double bond saturation. Yet, bathochromy is a biologically important phenomenon since some male dragonflies signal their sexual maturity through such color changes. To rationalize and to predict color changes of ommochromes, we used quantum computational methods, namely Density Functional Theory (DFT) and its Time-Dependent version (TDDFT), on ommochrome-like pigments showing either bathochromic or hypsochromic behaviors upon reduction. We found that color changes are solely driven by reduced states, whose capacity to let excited electrons flow is dependent on specific chemical groups. Specifically, because reduced ommochromes possess Electron-Withdrawing Auxochromes (EWA) attached to their phenoxazine core, they absorb light more easily than their oxidized form; hence their batochromic color changes. Those results also point toward a possible better antiradical/antioxidant function for ommochromes since EWA are associated to higher electron affinity. Therefore, besides visual display, color changes in animals might help preventing oxidative stress in some contexts.
This work was performed both at the Insect Biology Research Institute (IRBI, UMR 7261, CNRS - University of Tours) and in the Chemical Theory and Modelling group (CTM, Chimie ParisTech, PSL University, CNRS, Institute of Chemistry for Life and Health Sciences).
Figon, F., Casas, J., Ciofini, I. & Adamo, C. (2021). Electronic coupling in the reduced state lies at the origin of color changes of ommochromes. Dyes and Pigments. 185, 108661. DOI: 10.1016/j.dyepig.2020.108661