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A semiempirical study of the optimized ground and excited state potential energy surfaces of retinal and its protonated Schiff baseThe electronic ground and first excited states of retinal and its Schiff base are optimized for the first time using the semiempirical AM1 Hamiltonian. The barrier for rotation about the C(11)-C(12) double bond is characterized by variation of both the twist angle delta(C(10)-C(11)-C(12)-C(13)) and the bond length d(C(11)-C(12)). The potential energy surface is obtained by varying these two parameters. The calculated ground state rotational barrier is equal to 15.6 kcal/mol for retinal and 20.5 kcal/mol for its Schiff base. The all-trans conformation is more stable by 3.7 kcal/mol than the 11-cis geometry. For the first excited state, S(1,) the 90 degrees twisted geometry represents a saddle point for retinal with the rotational barrier of 14.6 kcal/mol. In contrast, this conformation is an energy minimum for the Schiff base. It can be easily reached at room temperature from the planar minima since it is separated from them by a barrier of only 0.6 kcal/mol. The 90 degrees minimum conformation is more stable than the all-trans by 8.6 kcal/mol. We are thus able to present a reaction path on the S(1) surface of the retinal Schiff base with an almost barrier-less geometrical relaxation into a twisted minimum geometry, as observed experimentally. The character of the ground and first excited singlet states underscores the need for the inclusion of double excitations in the calculations.
Document ID
20040088602
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
Authors
Parusel, A. B.
(NASA Ames Research Center Moffett Field CA United States)
Pohorille, A.
Date Acquired
August 21, 2013
Publication Date
December 1, 2001
Publication Information
Publication: Journal of photochemistry and photobiology. B, Biology
Volume: 65
Issue: 1
ISSN: 1011-1344
Subject Category
Life Sciences (General)
Distribution Limits
Public
Copyright
Other

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