The crystal and molecular structure of the five-membered-ring analogue of vitamin-A acid

On the ordering of the first two excited electronic states in all-trans linear polyenes

Reported experimental evidence of the relative position of the first two excited electronic states in linear polyenes was carefully examined and compared with that derived from time dependent density functional theory (TDDFT) theoretical calculations performed at the B3LYP level on optimized geometries. The energy values for the first two triplet states 3Bu and 3Ag, obtained from TDDFT calculations, were found to be highly strongly correlated with the experimental values. Also, the theoretical calculations for the electronic transition 1 1Ag --> 1 1Bu were also extremely well correlated with their experimental counterparts; even more important, the three reported experimental data for 1 1Ag --> 2 1Ag transitions in these systems conformed to the correlation for the TDDFT 1 1Ag --> 1 1Bu transition. The first excited electronic state in the linear polyenes studied (from ethene to the compound consisting of 40 ethene units, P40) was found to be 1Bu. The energy gap between the excited states 2 1Ag and 1 1Bu decreased with increasing length of the polyene chain, but not to the extent required to cause inversion, at least up to P40. In the all-trans linear polyenes studied, the widely analyzed energy gap from the ground electronic state to the first excited singlet state for infinitely long chains may be meaningless as, even in P40, it is uncertain whether the ground electronic state continues to be a singlet.

Design of a helix-bundle cross-link: NMR and UV-visible spectroscopic analyses and molecular modeling of ring-oxidized retinals

In order to generate potential chemical cross-links for studying the chromophore binding site of bacteriorhodopsin and related helix-bundle proteins, MnO2 was used to oxidize all-trans-retinal's ring moiety. The structures and solution conformations of three ring-oxidized retinal analogues have been determined by using UV-visible absorption and 1H and 13C NMR spectroscopies, primarily with regard to (i) the introduction of a functional group at the ring end of the chromophore, (ii) the retention of the all-trans geometry of the polyenal side chain, and (iii) the torsional angle of the ring-polyenal bond. Analyses of their UV-visible absorption spectral parameters (lambda max, epsilon max, and vibrational fine structure) and NMR spectral parameters (1H-1H coupling constants, 1H and 13C NMR chemical shifts, and 1H homonuclear Overhauser effects) indicated the 4-oxo and the 2,3-dehydro-4-oxo derivatives both possess the twisted 6-s-cis conformation adopted by most six-membered ring analogues of retinal in solution or crystal. However, the alpha-dioxocyclopentenyl analogue exists in solution predominantly (70-80%) as the planar 6-s-trans conformer, similar to violerythrine chromophore analogues. In order to identify the minor solution forms, molecular modeling and geometry optimizations using the semiempirical molecular orbital method AM1 defined two additional symmetry-related minima at +/- 30-40 degrees in its C6-C7 torsional energy profile. Because the chromophores of bacterio- and halorhodopsins and sensory rhodopsins are bound as the 6-s-trans conformer [Harbison, G.S., Smith, S.O., Pardoen, J.A., Courtin, J.M.L., Lugtenburg, J., Herzfeld, J., Mathies, R.A., & Griffin, R.G. (1985) Biochemistry 24, 6955-6962; Baselt, D.R., Fodor, S.P.A., van der Steen, R., Lugtenburg, J., Bogomolni, R.A., & Mathies, R.A. (1989) Biophys. J. 55, 193-196], we suggest that the cyclopentenyl analogue's alpha-diketo function may be favorably positioned within the binding pocket and sufficiently reactive toward nucleophilic attack to cross-link an arginine located in or near the ring end of the chromophore cavity: Arg134 according to the current model of bacteriorhodopsin's tertiary structure [Henderson, R., Baldwin, J.M., Ceska, T.A., Zemlin, F., Beckmann, E., & Downing, K.H. (1990) J. Mol. Biol. 213, 899-929] or Arg82 as postulated from an alternate model constructed primarily to accommodate the external point charge contribution to bacteriorhodopsin's opsin shift.

Analysis of N-(4-hydroxyphenyl)retinamide polymorphic forms by X-ray powder diffraction

A quantitative X-ray diffraction method, with zinc oxide used as the internal standard, was developed for the analysis of polymorphic forms I and II of N-(4-hydroxyphenyl)retinamide. The standard curve relating peak height ratio to the percentage of form I was linear. The method was precise and accurate to within +/- 6%.