Two-Dimensional NMR Studies of Polyene Systems. I. All-trans-Retinal

Selective J-resolved spectra: a double pulsed field gradient spin-echo approach

A simple method to obtain selective J-resolved spectra is presented, which relies on the refocusing properties of double pulsed field gradient spin-echoes and provides unambiguous assignment of the measured coupling constants. The proposed examples show how this method is of general applicability, and requires no more than a simple optimization strategy to produce artifact-free spectra. Examples of application include the determination of a small, long-range coupling constant (0.7 Hz) in trans-retinal and Halpha couplings in a tripeptide.

Highly Stereoselective Synthesis of Conjugated Polyenes via a Homocoupling Reaction of Unsaturated Silanes

A new method for the synthesis of conjugated polyenes containing up to eight double bonds with all-E configuration is reported. The procedure is based upon a homocoupling reaction of dienyl-, trienyl-, or tetraenylsilanes, promoted by PdCl(2) in methanol and in the presence of LiCl and CuCl(2). Configurational and conformational assignments were rigorously made on the basis of NMR spectra. The compounds obtained represent a novel interesting class of symmetrically substituted polyenes with potential optical and electrooptical properties. By changing the solvent, the homocoupling process can be switched to the halogenation of the silyl-substituted terminal double bond, thus leading to polyenyl halides.

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.