Preferred conformers in 3-aminopropanol have been determined by using the quantum-mechanical PCILO method. The results indicate two conformers for the molecule and both of them are stabilized due to intramolecular hydrogen bonding between O−H and N of the amino group. One of the conformers has been observed experimentally by microwave spectroscopy in excellent agreement with the theoretical prediction. The important implication of this result is that the preferred conformations of a molecule can be theoretically predetermined and transitions corresponding to the calculated conformers should be looked for in microwave spectroscopy. This will result in reducing the difficult and time-consuming efforts of scanning the microwave spectrum in wide frequency ranges.

The conformational preferences of all-trans retinal (referred to as retinal) have been investigated by the PCILO (Perturbative Configuration Interaction using Localized Orbitals) method. The results indicate that the all-trans isomer (the conformer in which all the double bonds of the polyene chain intrans conformation) is relatively higher in energy than the one in which the terminalC=O bond assumes acis-conformation. The β-ionone ring and the polyene chain are nonplanar and the angle between the two planes is 90°. The all-trans isomers withθ₅ = 180° or 0° are energetically more preferred than the other isomers of retinal, the 11-cis isomer being the least stable. A rotation, however, around C12–C13 single bond (θ₄) in 11-cis retinal predicts two distorted 11-cis, 12-s-conformers withθ₄ = 90° and 270° which are as stable as 9-cis or 13-cis isomers. The theoretically predicted distorted 11-cis conformers are in agreement with experimentally observed conformers of 11-cis retinal in solution by NMR studies.

Bacteriorhodopsin, a chromoprotein having retinal as chromophore is linked to protein opsin through a protonated Schiff base, and undergoes a cyclic photoreaction on light absorption involving a number of intermediate species. The conformational preferences of the protonated and deprotonated Schiff bases of all-trans and 13-cis retinal withn-pentylamine as model for lysine have been investigated using molecular orbital PCILO (Perturbative Configuration Interaction of Localised Orbitals) method. The theoretical calculations reveal that there is an intrinsic differential flexibility between the protonated and deprotonated Schiff bases irrespective of the retinal isomers. The deprotonation of the Schiff base enhances the flexibility of the chromophore in the molecule. Based upon this information and that there are no major changes in the conformation of the protein opsin, two models for the photoreaction cycle involving intermediates of different structures have been proposed. Of these, experimental evidences seem to favour model 1 in which deprotonation and notrans-cis isomerisation accompanies the primary photochemical event. In other words, the intermediate species of the photoreaction cycle of bacteriorhodopsin have all-trans retinal as the chromophore with varying degree of non-covalent interaction with the protein opsin.