Protonated 2,3-Dihydrobilindiones—Models for the Chromophores of Phycocyanin and the Red-Absorbing Form of Phytochrome

In light-harvesting phycocyanins the stable protonation states of the chromophores are fixed by the highly ordered environment of consensus sequences, whereas in phytochromes proton transfer dynamics may be controlled by geometrical changes initiated photochemically. We drew these conclusions from a study of the cysteine adduct of phycocyanobilin 1 and its corresponding zwitterion 1± intended to mimic the acid–base chemistry between protein and chromophores in biliproteins. Proton transfer was found to be entropically controlled.