An electronic state of the chromophore, phycocyanobilin, and its interaction with the protein moiety in C-phycocyanin: protonation of the chromophore

An electronic state of phycocyanobilin (PCB, a derivative of an open tetrapyrrole) in C-phycocyanin, an antenna pigment-protein complex for photosynthesis, was estimated by including the effect of an electrostatic field of protein moieties and water molecules. We adapted our unique method for calculation of the resonance integral. The observed absorption wavelength and an oscillator strength were reproduced only when the PCB was assigned to the protonated form with a net charge of 1. This was rationalized by the negative charge of a specific aspartate residue which was equidistant from the two nitrogen atoms of two central pyrrole rings.