The Excited‐State Chemistry of Phycocyanobilin: A Semiempirical Study

Abstract

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 pK(B)* of approximately 9.5 units. The acidity is only slightly enhanced with a drop in pK(A)* of only approximately 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.

The Structure of Phytochrome: A Picture Is Worth a Thousand Spectra

“…Consistent with this hypothesis, a recent theoretical study of PCB provided evidence for considerable double-bond character about the C15,16 bond in the ground state but not in the excited state (Gö ller et al, 2005). The DrBphP structure provides new insight into the process of photoconversion.…”

“…The red-shifted intermediate is thought to reflect chromophore protonation and adoption of the more extended configuration characteristic of Pr. A recent theoretical study of PCB indicates that the extended conformation is primarily responsible for enhanced red absorbance, with protonation driving the red shift (Gö ller et al, 2005).…”

Femtosecond time‐resolved spectroscopy on biological photoreceptor chromophores

“…However, semiempirical AM1 studies revealed the coexistence of other most stable cyclic-helical structure and suggested a more stretched conformation for PCB B and PCB C compared to PCB A [70]. However, it remains to be seen whether the structural differences between PCB A , PCB B and PCB C are confined to conformational differences alone or whether they are also due to changes in the protonation state of the tetrapyrrolic ring system [71].…”

“…The chromophore of phytochrome, phytochromobilin, differs from PCB only by substitution of a vinyl instead of an ethyl group at pyrrole ring D. Thus, PCB is considered to be a suitable model for the phytochrome chromophore. For this reason, the photochemistry and photophysics of PCB has been extensively studied by a series of theoretical and spectroscopic investigations over the past years [63][64][65][66][67][68][69][70][71].…”

A Singular Bacteriophytochrome Acquired by Lateral Gene Transfer

“…5A), where the chromophore has been shown to be in the C15-Z,anti conformation (30). One should note, however, that besides the isomerization structure, the protonation state of the chromophore is also predicted to have a strong effect on the spectral properties (31). Furthermore, the fluorescence properties of BrBphP3.ORS278 do not support a simple identification of its light-induced state with the Pr state of classical phytochromes.…”