Theoretical Studies of Biliprotein Chromophores and Related Bile Pigments by Molecular Orbital and Ramachandran Type Calculations

Abstract

Abstract-Ramachandran calculations have been used to gain insight into steric hindrance in bile pigments related to biliprotein chromophores. The high optical activity of denatured phycocyanin, as compared to phycoerythrin, has been related to the asymmetric substitution at ring A, which shifts the equilibrium towards the P-helical form of the chromophore. Geometric effects on the electronic structures and transitions have then been studied by molecular orbital calculations for several conjugation systems including the chromophores of phycocyanin. phytochrome P,, cations, cation radicals and tautomeric forms. For these different chromophores some general trends can be deduced. For instance, for a given change in the gross shape (e.g. either unfolding of the molecule from a cyclic-helical to a fully extended geometry, or upon out-of-plane twists of the pyrrole ring A) of the molecules under study, the predicted absorption spectra all change in a simikar way. Nonetheless, there are characteristic distinctions between the different n-systems, both in the transition energies and the charge distribution, which can be related to their known differences in spectroscopic properties and their reactivity.

Circular Dichroism of Chromopeptides from Phycocyanin

“…Positive signs are defined for counterclockwise rotations viewing from C5. The set of param eters developed for biliverdin by W agniere et al was used with minor modifications chosen in analogy with our earlier MO calculations [26,27], N22 is considered to be of the "pyridine", the other three nitrogen atoms (N21, N23 and N24) to be of the "pyrrole" type. The former contributes only one electron to the n system; for this atom (N22) we took y(NC) = 7.35 eV (instead of 7.65 eV), y (N N )= 11.30eV (12.27 eV), 25 20 Brought to you by | Royal Melbourne Institute of Technology Authenticated Download Date | 7/26/15 9:33 PM ß (NC) = -2.63 eV ( -2.50 eV).…”

“…Falk and Thirring [15] have observed the "norm al" spectral changes (red shift and increased intensity of the visible band) in a N 21-N 24-m ethanobilindion which is restricted to cyclic conformations. MO calculations predict as well a red shift, but by contrast a decreased intensity for a cyclic-helical protonated bilindione [27], If one assumes that only helical conform ations have large cd signals, such a conform ation would be predominant in the 8 m urea solution, but not in other solvents.…”

“…The differences observed here remain well within this range. According to band position and in particular the intensity ratio o f the near uv and the red bands [21,22,27], the chrom o phores are predom inantly present in cyclic-helical conformations. It should be noted, however, that uv-vis spectroscopy is insufficient to analyze conformer mixtures in bile pigments.…”

“…In hydrogen bonding solvents, this situation is complicated by an interplay between intra-and /«rm nolecular hydrogen bonding which may even give rise to a solvent and tem per ature dependent cd sign reversal [11]. M oreover has the influence of asymmetric centers w ithin the chromophore on the preferential form ation of one type of helix been dem onstrated for the chrom o phores of biliproteins [9,13,27],…”

Chromophores in Photomorphogenesis

“…Because bile pigments are flexible molecules, the influence of chromophore conformation on spectral properties has been studied theoretically by several authors (BURKE et al 1972, BLAUER and WAGNIÈRE 1975, SHAE and SONG 1975, SUGIMOTO et al 1976, PASTERNAK and WAGNIÈRE 1979, SCHEER, FORMANEK and SCHNEIDER 1982. The main conclusion was that the oscillator strength of the long-wavelength band (fj is small compared to that of the short-wavelength band (f 2 ) in cyclic conformations.…”