TY - JOUR
T1 - The retinal chromophore/chloride ion pair
T2 - Structure of the photoisomerization path and interplay of charge transfer and covalent states
AU - Cembran, Alessandro
AU - Bernardi, Fernando
AU - Olivucci, Massimo
AU - Garavelli, Marco
PY - 2005/5/3
Y1 - 2005/5/3
N2 - Ab initio multireference second-order perturbation theory computations are used to explore the photochemical behavior of two ion pairs constituted by a chloride counterion interacting with either a rhodopsin or bacteriorhodopsin chromophore model (i.e., the 4-cis-γ-methylnona-2,4,6,8-tetraeniminium and all-trans-nona-2,4,6,8-tetraeniminium cations, respectively). Significant counterion effects on the structure of the photoisomerization paths are unveiled by comparison with the paths of the same chromophores in vacuo. Indeed, we demonstrate that the counterion (i) modulates the relative stability of the S0, S1, and S2 energy surfaces leading to an S1 isomerization energy profile where the S1 and S 2 states are substantially degenerate; (ii) leads to the emergence of significant S1 energy barriers along all of the isomerization paths except the one mimicking the 11-cis → all-trans isomerization of the rhodopsin chromophore model; and (iii) changes the nature of the S1 → S0 decay funnel that becomes a stable excited state minimum when the isomerizing double bond is located at the center of the chromophore moiety. We show that these (apparently very different) counterion effects can be rationalized on the basis of a simple qualitative electrostatic model, which also provides a crude basis for understanding the behavior of retinal protonated Schiff bases in solution.
AB - Ab initio multireference second-order perturbation theory computations are used to explore the photochemical behavior of two ion pairs constituted by a chloride counterion interacting with either a rhodopsin or bacteriorhodopsin chromophore model (i.e., the 4-cis-γ-methylnona-2,4,6,8-tetraeniminium and all-trans-nona-2,4,6,8-tetraeniminium cations, respectively). Significant counterion effects on the structure of the photoisomerization paths are unveiled by comparison with the paths of the same chromophores in vacuo. Indeed, we demonstrate that the counterion (i) modulates the relative stability of the S0, S1, and S2 energy surfaces leading to an S1 isomerization energy profile where the S1 and S 2 states are substantially degenerate; (ii) leads to the emergence of significant S1 energy barriers along all of the isomerization paths except the one mimicking the 11-cis → all-trans isomerization of the rhodopsin chromophore model; and (iii) changes the nature of the S1 → S0 decay funnel that becomes a stable excited state minimum when the isomerizing double bond is located at the center of the chromophore moiety. We show that these (apparently very different) counterion effects can be rationalized on the basis of a simple qualitative electrostatic model, which also provides a crude basis for understanding the behavior of retinal protonated Schiff bases in solution.
KW - Ab initio
KW - Conical intersection
KW - Counterion
KW - Protonated Schiff base
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U2 - 10.1073/pnas.0408723102
DO - 10.1073/pnas.0408723102
M3 - Article
C2 - 15855270
AN - SCOPUS:18244373415
SN - 0027-8424
VL - 102
SP - 6255
EP - 6260
JO - Proceedings of the National Academy of Sciences of the United States of America
JF - Proceedings of the National Academy of Sciences of the United States of America
IS - 18
ER -