Robinlin: a novel bioactive homo-monoterpene from Robinia pseudoacacia L. (Fabaceae).

A bioactivity-directed fractionation of the ethanolic extracts of Robinia pseudoacacia L. (Fabaceae) afforded robinlin (1), a novel homo-monoterpene. The structure of 1 was elucidated by spectral analyses of the parent compound as well as its derivatives; 1 showed strong bioactivity in the brine shrimp lethality test (BST).

A new mechanism for spin--lattice relaxation of heavy nuclei in the solid state: 207Pb relaxation in lead nitrate.

A detailed investigation of the spin-lattice relaxation time, T1, for 207Pb in solid lead nitrate has been undertaken in an effort to understand the mechanism of relaxation. The results show that the 207Pb T1 is independent of magnetic field strength and inversely proportional to the square of the temperature. These are signatures of relaxation by a spin-phonon Raman scattering mechanism. Nuclear spin-lattice relaxation in solid lead salts is more efficient for sites with smaller magnetic shielding anisotropy. A coupling mechanism is proposed whereby phonons create a local magnetic field by modulating the valence electron shell motion relative to the nuclear/electron core. Literature data suggest that spin-phonon scattering is a common relaxation pathway for other spin-1/2 heavy nuclei in solids.

Membrane conformations and their relation to cytotoxicity of asimicin and its analogues.

Certain plant species belonging to the family Annonaceae produce Annonaceous acetogenins, which are a unique class of long-chain fatty acid derivatives with potent cytotoxicity. Putative protein targets of the acetogenins are membrane-associated proteins, including complex I. Asimicin and its analogues constitute a class of Annonaceous acetogenins containing two tetrahydrofuran (THF) rings with hydrocarbon chains tethered to each ring; an alpha,beta-unsaturated gamma-lactone ring is terminal to one of the alkyl chains. The compounds examined in this study differ in the length of the alkyl chain between the THF rings and the lactone ring. The positions of both the THF and the lactone rings within liposomal membranes were determined by proton (1H) nuclear magnetic resonance spectroscopy. The depth of membrane penetration of acetogenins, coupled to membrane diffusion, controls the conformation of acetogenins as they diffuse to an active site. Based on 1H intermolecular nuclear Overhauser effects (NOEs), the THF rings of all acetogenins studied reside near the polar interfacial head group region of the DMPC. This was corroborated by 1H two-dimensional NOE spectroscopy and differential scanning calorimetry studies. The 1H difference NOE spectra indicated that the lactone rings of asimicin and parviflorin, the latter of which has two fewer carbons in its alkyl chain, are located below the glycerol backbone in the membrane. In contrast with asimicin and parviflorin, the lactone ring of longimicin B, an asimicin analogue with an alkyl chain four carbons shorter, resides close to the midplane in the membrane. This was corroborated by manganese-induced broadening studies. Since the THF rings are located near the center of the acetogenin molecules and the lactone ring is terminal to a long alkyl chain, these observations indicate that an asimicin-type acetogenin can be in either sickle-shaped or U-shaped conformations, depending on the length of the alkyl chain between the THF rings and the lactone ring. Interestingly, longimicin B does not exhibit significant cytotoxicity, but parviflorin is as cytotoxic as asimicin. The cytotoxicity of the asimicin-type of acetogenins would seem to be strongly related to the membrane conformation. This is the first report elucidating the conformation of Annonaceous acetogenins in membranes.

Natural-abundance 13C nuclear magnetic resonance spectra of terpenes and carotenes.

Natural-abundance (13)C nuclear magnetic resonance spectra are reported for some simple terpenes and carotenes. The techniques involved in the assignment of the resonances to specific carbons are outlined. The potential of this nondegradative procedure for structural analysis is demonstrated for the investigation of carbon atoms in chemical and biochemical systems.