An Extended INDO-CI Study on Protonated Retinal Schiff-Base

The bond lengths, bond orders, electron densities and optical absorption of the ground state of retinal forming a protonated Schiff-base linkage with a lysine residue (PRSB-Lys) are calculated using the extended INDO-CI molecular orbital method, and the result is compared with the previous one obtained by the π-electron approximation. It is, thus, shown that the π-electron system of PRSB-Lys is appreciably polarized, and that the σ – π interaction in PRSB-Lys is too large to be neglected when a C-C bond in its polyene chain is twisted. It is also shown that the negative counter point-charge exerts influence on the π-electron distribution in a similar way to that shown by the π-electron approximation.