title: Controlling the photoreactivity of the photoactive yellow protein chromophore by substituting at the p-coumaric acid group.
abstract: We have performed ab initio CASSCF, CASPT2, and EOM-CCSD calculations on doubly deprotonated p-coumaric acid (pCA(2-)), the chromophore precursor of the photoactive yellow protein. The results of the calculations demonstrate that pCA(2-) can undergo only photoisomerization of the double bond. In contrast, the chromophore derivative with the acid replaced by a ketone (p-hydroxybenzylidene acetone, pCK(-)) undergoes both single- and double-bond photoisomerization, with the single-bond relaxation channel more favorable than the double-bond channel. The substitution alters the nature of the first excited states and the associated potential energy landscape. The calculations show that the electronic nature of the first two (π,π*) excited states are interchanged in vacuo due to the substitution. In pCK(-), the first excited state is a charge-transfer (CT π,π*) state, in which the negative charge has migrated from the phenolate ring onto the alkene tail of the chromophore, whereas the locally excited (LE π,π*) state, in which the excitation involves the orbitals on the phenol ring, lies higher in energy and is the fourth excited state. In pCA(2-), the CT state is higher in energy due the presence of a negative charge on the tail of the chromophore, and the first excited state is the LE state. In isolated pCA(2-), there is a 68 kJ/mol barrier for double-bond photoisomerization on the potential energy surface of this LE state. In water, however, hydrogen bonding with water molecules reduces this barrier to 9 kJ/mol. The barrier separates the local trans minimum near the Franck-Condon region from the global minimum on the excited-state potential energy surface. The lowest energy conical intersection was located near this minimum. In contrast to pCK(-), single-bond isomerization is highly unfavorable both in the LE and CT states of pCA(2-). These results demonstrate that pCA(2-) can only decay efficiently in water and exclusively by double-bond photoisomerization. These findings provide a rationale for the experimental observations that pCA(2-) has both a longer excited-state lifetime and a higher isomerization quantum yield than pCK(-).
authors.affiliation: Laboratoire de Chimie et Physique Quantiques, IRSAMC, CNRS et Université de Toulouse, Toulouse, France.
authors.forename: Martial
authors.initials: M
authors.lastname: Boggio-Pasqua
authors.forename: Gerrit
authors.initials: G
authors.lastname: Groenhof
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D003373:Coumaric Acids / Q000737:chemistry*; D020607:Halorhodospira halophila / Q000737:chemistry; D015394:Molecular Structure; D010777:Photochemistry; D020283:Photoreceptors, Microbial / Q000737:chemistry*; D011422:Propionates; D011789:Quantum Theory; D013237:Stereoisomerism
chemical_list: D001426:Bacterial Proteins; D003373:Coumaric Acids; D020283:Photoreceptors, Microbial; D011422:Propionates; C050162:photoactive yellow protein, Bacteria; C495469:p-coumaric acid