Abstract:
:Ab initio multi-reference 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 S0, S1, and S2 energy surfaces leading to an S1 isomerization energy profile where the S1 and S2 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.
journal_name
Proc Natl Acad Sci U S Aauthors
Cembran A,Bernardi F,Olivucci M,Garavelli Mdoi
10.1073/pnas.0408723102keywords:
subject
Has Abstractpub_date
2005-05-03 00:00:00pages
6255-60issue
18eissn
0027-8424issn
1091-6490pii
0408723102journal_volume
102pub_type
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