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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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