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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
| ABSTRACT - Internet Electron. J. Mol. Des. January 2004, Volume 3, Number 1, 37-44 |
Trans-Cis Isomerization of Protonated Schiff Base of Retinal
at the Ground and Triplet States: A Hybrid DFT Study
Tetsuji Iyama and Hiroto Tachikawa
Internet Electron. J. Mol. Des. 2004, 3, 37-44
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Abstract:
Trans-cis isomerization is one of the important phenomena in
the human retinal visual pigment. However, the mechanism
of the isomerization is not clearly understood. In the present
study, potential energy curves for the isomerization of all
trans-protonated Schiff base of retinal (PSBR) at the ground
state have been calculated by means of hybrid density
functional theory (DFT) calculations. Hybrid density
functional calculations have been carried out for the
isomerization reactions. Potential energy curves for the
ground singlet states and lowest triplet states were calculated
at the B3LYP/3-21G(d) level. The calculations of the
potential energy curves for the ground singlet and triplet
states (S0 and T1) of PSBR indicated that the transition state
for the ground state isomerization was located at θ13-14 = 90°,
where θ13-14 means twist angle around the C13=C14 double
bond of PSBR (θ13-14 = 0 and 180° mean all trans- and
cis-form, respectively). The energy barrier calculated at the
B3LYP/6-311G(d,p)//B3LYP/3-21G(d) level was 1.30 eV
relative to the trans-form (θ13-14 = 0°).
The cis-form of PSBR
was 0.17 eV higher in energy than that of trans-form. The
vibrational frequencies were calculated for cis- and
trans-forms at the B3LYP/3-21G(d) level. The N=C stretching
mode in Schiff base and C=C stretching mode for the
C13=C14 double bond of PSBR were red-shifted by the
isomerization from trans- to cis-forms (1678 vs. 1668 cm-1
for the N=C bond and 1613 vs. 1612 cm-1 for the C13=C14
bond. For triplet state, the barrier was located at θ13-14 = 0.0°,
and the barrier height was calculated to be 1.16 eV relative to
the trans-form at the triplet state. The barrier height for the
isomerization was estimated to be 1.20-1.30 eV. The C=C
stretching mode was red-shifted by the isomerization from
trans to 13-cis form. These features were in good agreement
with recent experimental results.
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