THEORETICAL STUDIES OF BILIPROTEIN CHROMOPHORES AND RELATED BILE PIGMENTS BY MOLECULAR ORBITAL AND RAMACHANDRAN TYPE CALCULATIONS
b'Ramachandran calculations have been used to gain insight into steric hindrance in bile\\npigments related to biliprotein chromophores. The high optical activity of denatured phycocyanin, as\\ncompared to phycoerythrin, has been related to the asymmetric substitution at ring A, which shifts the\\nequilibrium towards the P-helical form of the chromophore. Geometric effects on the electronic structures\\nand transitions have then been studied by molecular orbital calculations for several conjugation\\nsystems including the chromophores of phycocyanin. phytochrome P,, cations, cation radicals and\\ntautomeric forms. For these different chromophores some general trends can be deduced. For instance,\\nfor a given change in the gross shape (e.g. either unfolding of the molecule from a cyclic-helical to a fully\\nextended geometry, or upon out-of-plane twists of the pyrrole ring A) of the molecules under study, the\\npredicted absorption spectra all change in a simikar way. Nonetheless, there are characteristic distinctions\\nbetween the different n-systems, both in the transition energies and the charge distribution, which\\ncan be related to their known differences in spectroscopic properties and their reactivity.'