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The Excited‐State Chemistry of Phycocyanobilin: A Semiempirical Study
Author(s) -
Göller Andreas H.,
Strehlow Dietmar,
Hermann Gudrun
Publication year - 2005
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200400667
Subject(s) - phycocyanobilin , protonation , chemistry , excited state , deprotonation , photochemistry , chromophore , photoexcitation , absorption spectroscopy , computational chemistry , atomic physics , organic chemistry , phycocyanin , ion , cyanobacteria , genetics , physics , quantum mechanics , bacteria , biology
Based on previous time‐resolved absorption studies, phycocyanobilin undergoes a photoreaction from an A ‐ into a B ‐ and C ‐form, with the latter two photoproducts showing absorption spectra red‐shifted from A . To identify the molecular mechanism involved in the excited‐state reactions, the structural origin of the red shift in the absorption spectra is investigated. Using semiempirical AM1 calculations that include configuration interaction by pair doubles excitation configuration interaction, the absorption spectra of different conformers as well as different protonation states were calculated. The results clearly indicate a pronounced red shift in the spectra of structures either protonated or deprotonated at the basic/acidic centres of the tetrapyrrole chromophore whereas, in contrast, conformational changes alone result in a blue shift. Furthermore, it is shown by quantum chemical calculations that the basicity of phycocyanobilin is much higher in the excited than in the ground state, with a decrease in the excited‐state p K B * of ≈9.5 units. The acidity is only slightly enhanced with a drop in p K A * of only ≈1.6 units. From these findings, a reaction model for the excited‐state processes in phycocyanobilin is proposed. According to this model, photoexcitation of phycocyanobilin triggers an excited‐state proton transfer giving rise to the formation of a protonated species. In parallel, the local increase in the medium pH associated with protonation then forwards a deprotonation at an acidic NH‐group so that in effect both protonated and deprotonated phycocyanobilin would arise from the initial photoreaction and account for the observed red shift in the spectra of the B ‐ and C ‐forms.