We used a macroscopic dielectric model to study the effects of solvation and interaction between titratable and permanent partial charges on the protein conformational energy and the acid–base equilibria in the cyanobacterial photoreceptor phycoerythrocyanin, whose photoreversible photochromic response is attributed to a Z/E isomerization of the covalently bound tetrapyrrole chromophore. The calculations revealed the stabilization of the charged protonation state of the chromophore by a small set of strong local interactions. Although the protein is globular and water-soluble, the complex counterion structure has a striking similarity to the arrangement found for the photochemical active transmembrane protein bacteriorhodopsin. This could be attributed to the fact that the protonation site in the α-subunit of phycoerythrocyanin is buried in the interior of the protein. Due to the strong shielding from solvent, the interaction pattern is conserved upon a ground-state isomerization of the chromophore. The partial solvent exposure of the isomerization site resulted in a drastic influence of the chromophore configuration on the aqueous solvation energy of the protein. Implications for the sensitivity of the photochemistry to environmental factors and molecular binding are discussed. © 1993 John Wiley & Sons, Inc.
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