Photochemistry and Photophysics

• Model Subunits of Natural Bilins

Bilins are a class of pigments found in some of the oldest photosynthetic cyanobacteria, as well as some forms of algae. These linear tetrapyrrole molecules distinguish themselves from chlorophyll in their variability in conjugation length as well as their implicit structural flexibility. This enables the organism enhanced tunability in it's solar light harvesting strategy by: 1) incorporating a sequence of bilins with diverse conjugation lengths leading broader absorptions in the visible region simultaneously with the development of an energy funnel, and 2) tuning the absorption region of the light harvesting protein around the light available within its environment. This leads to pigment colors ranging from pink to blue, and it allows absorption in regions where more common chlorophyll light-harvesting proteins miss. However, the flexibility of the bilin skeleton introduces new non-radiative decay pathways, such as rapid internal conversion, where captured light energy is lost to heat. This phenomenon often occurs when light absorption induces a fast twisting motion about the bridge connecting pyrrole rings. Our research investigates the timescales, yields, and energy migration accompanying these photophysics on the simplest, yet distinctive, subunits of bilin pigments. Specifically, two unique classes of dipyrroles that incorporate the methine bridge connecting the pi-systems are of interest in our group. Characterizing the decay pathways of the excited dipyrroles in concert with an analysis of the natural biological environment enables us to pinpoint chemical strategies to steer such behavior to either favor or inhibit that motion. That chemical control inevitably facilitates a specific molecular function.