期刊
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
卷 102, 期 18, 页码 6255-6260出版社
NATL ACAD SCIENCES
DOI: 10.1073/pnas.0408723102
关键词
ab initio; counterion; conical intersection; protonated Schiff base
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-gamma-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 S-0, S-1, and S-2 energy surfaces leading to an S-1 isomerization energy profile where the S-1 and S-2 states are substantially degenerate; (h) leads to the emergence of significant S-1 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 S-1 --> S-0 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.
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