Structure of 4'-Butyl-2,3,5,6-Tetrafluorobiphenyl-4-Carbonitrile

C17H13F4N, M(r) = 307.3, orthorhombic, P2(1)2(1)2(1), a = 7.280 (4), b = 11.253 (3), c = 17.255 (5) angstrom, V = 1413.6 angstrom3, Z = 4, D(x) = 1.444 g cm-3, Mo K-alpha, lambda = 0.71069 angstrom, mu = 0.79 cm-1, F(000) = 632, T = 147 K. Final R = 0.036 for 1206 observed reflections. The molecular conformation has three planar groups, the cyanotetrafluorophenyl, the phenyl and the butyl groups. The butyl group lies at 13.4 (4)degrees to the phenyl group and there is an angle of 40.8 (4)degrees between the tetrafluorophenyl and phenyl groups. In the unit cell, molecules are stacked in pairs (head to tail) with the phenyl ring and the tetrafluorophenyl rings almost exactly overlaying each other. Closest C...C contacts are 3.388 (5) angstrom within the pairs and 3.380 (5) angstrom between the pairs

New lamellarin alkaloids from the Australian ascidian, Didemnum chartaceum

Five novel lamellarin-class alkaloids have been isolated from a Great Barrier Reef ascidian, Didemnum chartaceum. The structures of the 20-sulfated derivatives of lamellarins B, C, and L (1-3); the 8-sulfated derivative of lamellarin G (4), plus a nonsulfated compound, lamellarin Z (5), were identified by interpretation of spectroscopic data. Lamellarin G 8-sulfate (4) is the first example of this class of compounds sulfated at the C-8 position, while lamellarin Z (5) is the first example of a dimethoxylated lamellarin. The known lamellarins A, B, C, E, G, and L (6-11), plus the triacetate derivatives of lamellarins D (12) and N (13), were also isolated. An aberration in the integration of signals in the H-1 NMR spectra of the 20-sulfated derivatives of lamellarins B, C, and L (1-3) led to NMR relaxation studies. T-1 values were calculated for all protons in the sulfated lamellarins (1-4) and their corresponding nonsulfated derivatives (7, 8, 10, 11). Interestingly, the protons ortho to the sulfate group in compounds (1-4) had T-1 values up to five times larger than the corresponding protons in their nonsulfated derivatives (7, 8, 10, 11)

Conformations of human apolipoprotein E(263-286) and E(267-289) in aqueous solutions of sodium dodecyl sulfate by CD and H-1 NMR

Structures of apoE(263-286) and apoE(267-289) have been determined in aqueous solution containing 90-old molar excess of perdeuterated sodium dodecyl sulfate by CD and H-1 NMR. Conformations were calculated by distance geometry based on 370 and 276 NOE distance restraints, respectively. RMSD for superimposing the region 265-284 from an ensemble of 41 structures for apoE(263-286) (263-286) is 0.64 +/- 0.17 Angstrom for backbone atoms (N, C-alpha, C=O) and 1.51 +/- 0.13 Angstrom for all atoms. The backbone RMSD for an ensemble of 37 structures for apoE(267-289) is 0.74 +/- 0.21 Angstrom for the region 268-275 and 0.34 +/- 0.10 Angstrom for the region 276-286. A two-domain structure was found far apoE(267-289) with the C-terminal half adopting a very well defined helix and the N-terminal segment 268-275 a less well defined helix, suggesting that the N-terminus may weakly bind to SDS. For apoE(263-286), an amphipathic helix-bend-helix structural motif was found with all hydrophobic side chains on the concave face, The existence of a bend around residues Q273 To G278 is consistent with their temperature coefficients of amide protons as well as secondary shifts of alpha-protons. Comparison of the structures of the two peptides revealed that the enhanced binding of apoE(263-286) to lipid could be attributed to the formation of a hydrophobic cluster consisting of residues W264, F265, L268, and V269. Aromatic side chains are proposed to be especially important in anchoring apolipoprotein fragments to micelles

The modified stretched-exponential model for characterization of NMR relaxation in porous media

A compact model, the modified stretched exponential (MSE), for characterization of transverse and longitudinal NMR relaxation of fluid in saturated porous media is presented. The MSE model exhibits the theoretically correct long- and short-time behavior, and its general applicability for representing transverse-relaxation data is demonstrated for a wide range of rock lithologies and saturating fluids at various magnetic field strengths and echo times. The model also provides a reliable and sample-independent method for signal back-extrapolation, which is necessary for quantitative petrophysical NMR measurements, and overcomes the limitations of the well-known stretched-exponential model. (C) 1996 Academic Press, Inc