Direct single-molecule measurements of phycocyanobilin photophysics in monomeric C-phycocyanin

Significance The efficiency and adaptability of photosynthetic antenna systems in harvesting solar energy can be ultimately traced to their nanoscale organization. By monitoring the fluorescence emission from single photosynthetic antenna proteins in free solution over long timescales, we directly observed transitions among many distinct photophysical states exhibiting different flows of energy transfer among the embedded photosensitive pigments. We show that these states are well-described by a simple physical model, which allows us to identify and characterize the properties of individual chromophores and reveals surprising heterogeneity and quenching behavior that is not predicted by bulk studies.