Cryptophyte Phycobiliproteins Demonstrate Robustness to Solvent in their Light Harvesting Process

Abstract

Cryptophyte phycobiliproteins (PBPs) have been on the forefront of the quantum biology revolution since long-lasting quantum coherences were found to be present. A great deal is understood about the mechanism of energy transfer and light harvesting of PBPs in buffer, but far less is known about how this process may change in more biologically relevant conditions or solvent in general. In this work, I investigate the effect of solvent changes on two phycobiliproteins (PC 645 and PE 545) using both synthetic crowding agents and mixtures of DMSO and water that mimic the electronic properties of crowding agents. Steady state spectrophotometry and fluorometry, and Time-Correlated Single Photon Counting (TCSPC) are used to probe for differences in protein operation caused by solvent. I also employ ultrafast narrow-band transient absorbance spectroscopy (TA) for the first time on these proteins to provide even greater insight into ultrafast energy transfer and its possible solvent dependence. I find that there is little to no change in steady state absorbance spectra, no change in the main radiative lifetime, and a small change in the timescale for inter-chromophore transfer. I also propose an updated set of time constants for the transfer. I conclude that changing solvent from water to less polar crowded solutions mimicking the inside of the cell has little effect on the light harvesting of these PBPs, probably due to screening of chromophores by the protein scaffold. This all points to a view of PBPs as more robust than previously thought.