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Controlling the photoreactivity of the photoactive yellow protein chromophore by substituting at the p-coumaric acid group.

Boggio-Pasqua M, Groenhof G - J Phys Chem B (2011)

Bottom Line: The results of the calculations demonstrate that pCA(2-) can undergo only photoisomerization of the double bond.The substitution alters the nature of the first excited states and the associated potential energy landscape.In water, however, hydrogen bonding with water molecules reduces this barrier to 9 kJ/mol.

View Article: PubMed Central - PubMed

Affiliation: Laboratoire de Chimie et Physique Quantiques, IRSAMC, CNRS et Université de Toulouse, Toulouse, France.

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(-).

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Representation of the two PYP chromophore analogues used in this study: the deprotonated p-coumaric ketone (pCK−) and the doubly deprotonated p-coumaric acid (pCA2−). Rotations around the single bond (SB) and double bond (DB) are shown. Mulliken charge distributions of the ground state and first two (π,π*) excited states at the ground-state geometry are indicated.
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fig1: Representation of the two PYP chromophore analogues used in this study: the deprotonated p-coumaric ketone (pCK−) and the doubly deprotonated p-coumaric acid (pCA2−). Rotations around the single bond (SB) and double bond (DB) are shown. Mulliken charge distributions of the ground state and first two (π,π*) excited states at the ground-state geometry are indicated.

Mentions: To understand how different environments influence the isomerization process, we have more recently performed calculations on a chromophore analogue (p-coumaric ketone (pCK−), Figure 1) in water and in vacuo.(8) In both situations the predominant relaxation process in S1 involves a rotation of the single bond (SB), adjacent to the ring (Figure 1), rather than rotation of the double bond (DB). In vacuo, only the double-bond rotation can lead to radiationless decay, whereas in water both channels lead to decay. Both the single- and double-bond twisted structures are minima on the excited-state potential energy surface but only in water is the S1/S0 seam lying near these minima.8−10 The origin for the displacement of the seam is an electrostatic stabilization of the chromophore’s excited state by hydrogen-bond interactions with water molecules.8,9


Controlling the photoreactivity of the photoactive yellow protein chromophore by substituting at the p-coumaric acid group.

Boggio-Pasqua M, Groenhof G - J Phys Chem B (2011)

Representation of the two PYP chromophore analogues used in this study: the deprotonated p-coumaric ketone (pCK−) and the doubly deprotonated p-coumaric acid (pCA2−). Rotations around the single bond (SB) and double bond (DB) are shown. Mulliken charge distributions of the ground state and first two (π,π*) excited states at the ground-state geometry are indicated.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3102441&req=5

fig1: Representation of the two PYP chromophore analogues used in this study: the deprotonated p-coumaric ketone (pCK−) and the doubly deprotonated p-coumaric acid (pCA2−). Rotations around the single bond (SB) and double bond (DB) are shown. Mulliken charge distributions of the ground state and first two (π,π*) excited states at the ground-state geometry are indicated.
Mentions: To understand how different environments influence the isomerization process, we have more recently performed calculations on a chromophore analogue (p-coumaric ketone (pCK−), Figure 1) in water and in vacuo.(8) In both situations the predominant relaxation process in S1 involves a rotation of the single bond (SB), adjacent to the ring (Figure 1), rather than rotation of the double bond (DB). In vacuo, only the double-bond rotation can lead to radiationless decay, whereas in water both channels lead to decay. Both the single- and double-bond twisted structures are minima on the excited-state potential energy surface but only in water is the S1/S0 seam lying near these minima.8−10 The origin for the displacement of the seam is an electrostatic stabilization of the chromophore’s excited state by hydrogen-bond interactions with water molecules.8,9

Bottom Line: The results of the calculations demonstrate that pCA(2-) can undergo only photoisomerization of the double bond.The substitution alters the nature of the first excited states and the associated potential energy landscape.In water, however, hydrogen bonding with water molecules reduces this barrier to 9 kJ/mol.

View Article: PubMed Central - PubMed

Affiliation: Laboratoire de Chimie et Physique Quantiques, IRSAMC, CNRS et Université de Toulouse, Toulouse, France.

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(-).

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Related in: MedlinePlus