Membrane-induced secondary structures of neuropeptides: a comparison of the solution conformations adopted by agonists and antagonists of the mammalian tachykinin NK1 receptor.

We present what we believe to be the first documented example of an inducement of distinctly different secondary structure types onto agonists and antagonists selective for the same G-coupled protein receptor using the same membrane-model matrix wherein the induced structures are consistent with those suggested to be biologically active by extensive analogue studies and conventional binding assays. 1H NMR chemical shift assignments for the mammalian NK1 receptor-selective agonists alpha-neurokinin (NKA) and beta-neurokinin (NKB) as well as the mammalian NK1 receptor-selective antagonists [d-Pro2,d-Phe7,d-Trp9]SP and [d-Arg1, d-Pro2,d-Phe7,d-His9]SP have been determined at 600 MHz in sodium dodecyl sulfate (SDS) micelles. The SDS micelle system simulates the membrane-interface environment the peptide experiences when in the proximity of the membrane-embedded receptor, allowing for conformational studies that are a rough approximation of in vivo conditions. Two-dimensional NMR techniques were used to assign proton resonances, and interproton distances were estimated from the observed nuclear Overhauser effects (NOEs). The experimental distances were used as constraints in a molecular dynamics and simulated annealing protocol using the modeling package DISCOVER to generate three-dimensional structures of the two agonists and two antagonists when present in a membrane-model environment to determine possible prebinding ligand conformations. It was determined that (1) NKA is helical from residues 6 to 9, with an extended N-terminus; (2) NKB is helical from residues 4 to 10, with an extended N-terminus; (3) [d-Pro2,d-Phe7,d-Trp9]SP has poorly defined helical properties in the midregion and a beta-turn structure in the C-terminus (residues 6-9); and (4) [d-Arg1,d-Pro2, d-Phe7,d-His9]SP has a helical structure in the midregion (residues 4-6) and a well-defined beta-turn structure in the C-terminus (residues 6-10). Attempts have been made to correlate the observed conformational differences between the agonists and antagonists to their binding potencies and biological activity.

Improved performance accordion heteronuclear multiple-bond correlation spectroscopy-IMPEACH-MBC.

A modification of the recently reported ACCORD-HMBC long-range heteronuclear shift correlation experiment is described. The new experiment, IMPEACH-MBC (improved performance accordion heteronuclear multiple-bond correlation), introduces a new pulse sequence element, a constant time variable delay. The incorporation of the constant time variable delay into the IMPEACH-MBC sequence suppresses (1)H-(1)H coupling modulation inherent to the utilization of the accordion principle to sample a broad range of potential long-range heteronuclear couplings. (1)H-(1)H coupling modulation, which introduces an F(1) modulation or a "skew" of responses in the second frequency domain of the ACCORD-HMBC experiment, is suppressed in the IMPEACH-MBC experiment. Results of identically optimized IMPEACH-MBC and ACCORD-HMBC experiments performed on a sample of strychnine are compared.

Long-range (1)H-(15)N heteronuclear shift correlation at natural abundance.

Despite the inherently low sensitivity of (15)N NMR because of its low gyromagnetic ratio (gamma(N)) and its relatively low natural abundance (0.37%), this important nuclide still has useful potential as a structural probe even at natural abundance. Inverse-detected NMR methods coupled with major advances in NMR probe designs have made it possible to acquire long-range (1)H-(15)N heteronuclear shift correlation data on samples as small as a micromole overnight. Chemical shift referencing schemes for (15)N and the range of (15)N shifts are discussed, followed by a discussion of the currently available pulse sequences, pulse calibration, parametrization and processing of long-range (1)H-(15)N data, and the implications of probe selection. These topics are followed by a review of the applications contained in the literature that have utilized (1)H-(15)N heteronuclear shift correlation experiments at natural abundance, with emphasis placed on the observed long-range coupling pathways.

A comparison of inverse-detected heteronuclear NMR performance: conventional vs cryogenic microprobe performance.

We report a comparison of the results obtained at 500 MHz for heteronuclear shift correlation (HSQC) experiments with very small natural product samples using conventional and cryogenically cooled 3 mm NMR probes. The cryo probe affords a 12- to 16-fold reduction in data acquisition time for a comparable signal-to-noise ratio.

Identification of an N-acetyl keto derivative of fumonisin B1 in corn cultures of Fusarium proliferatum.

A method is presented for the separation and identification of a new N-acetyl keto derivative of fumonisin B1 (FB1) produced in solid corn culture. Cultures of Fusarium proliferatum (M-1597) were purified using preparative hplc, and the new fumonisin was detected by negative-ion esms. Structures were confirmed by 1H- and 13C-nmr spectroscopy. The new fumonisin differs from FB1 in that the tricarballylic acid functionality at the C-15 position of the eicosane backbone is replaced by a ketone and the amino group is acetylated. Direct analysis of the culture material by negative-ion electrospray lc/ms confirmed that the new fumonisin is produced naturally by the fungus.

Thalibealine, a novel tetrahydroprotoberberine--aporphine dimeric alkaloid from Thalictrum wangii.

A novel tetrahydroprotoberberine-aporphine dimeric alkaloid, (-)-thalibealine (1), was isolated from the roots of Thalictrum wangiii, and its structure established via spectroscopic analysis. Three other alkaloids were isolated, including the benzyltetrahydroisoquinoline-aporphine dimer (+)-thalmelatidine, the aporphine (+)-magnoflorine, and the protoberberine berberine. This is the first reported isolation of a tetrahydroprotoberberine-aporphine dimer from nature, as well as the first reported isolation of constituents from Thalictrum wangii.

Synthesis of thalprzewalskiinone, a revision of structure.

A direct comparison of the spectral data for synthetic 2-methyl-6,7-dimethoxy-3'-methoxy-4'-hydroxyoxobenzylisoquinoline iodide (1) and its positional isomer 2-methyl-6,7-dimethoxy-3'-hydroxy-4'-methoxyoxobenzylisoquinoline iodide (2) with the data obtained for the oxobenzylisoquinoline alkaloid thalprzewalskiinone revealed that the original structural assignment of the alkaloid as 1 was in error. These results mandate the revision of structure of thalprzewalskiinone to 2-methyl-6,7-dimethoxy-3'-hydroxy-4'-methoxyoxobenzylisoquinoline iodide (2).