Koshikamide B, a cytotoxic peptide lactone from a marine sponge Theonella sp

Koshikamide B (1) has been isolated from two separate collections of the marine sponge Theonella sp. as the major cytotoxic constituent. Koshikamide B is a 17-residue peptide lactone composed of six proteinogenic amino acids, two D-isomers of proteinogenic amino acids, seven N-methylated amino acids, and two unusual amino acid residues. The unusual amino acids are N(delta)-carbamoylasparagine and 2-(3-amino-2-hydroxy-5-oxopyrrolidin-2-yl)propionic acid (AHPP); the former is first found as the constituent of peptides, whereas the latter is a new amino acid residue. The N-terminus of koshikamide B is blocked by a methoxyacetyl group. The structure of koshikamide B (1) has been determined by interpretation of spectral data and analysis of chemical degradation products. Koshikamide B (1) exhibits cytotoxicity against P388 murine leukemia cells and the human colon tumor (HCT-116) cell line with an IC50 value of 0.45 and 7.5 microg/mL, respectively.     

Methyl proton contacts obtained using heteronuclear through-bond transfers in solid-state NMR spectroscopy

A two-dimensional proton-mediated carbon-carbon correlation experiment that relies on through-bond heteronuclear magnetization transfers is demonstrated in the context of solid-state NMR of proteins. This new experiment, dubbed J-CHHC by analogy to the previously developed dipolar CHHC techniques, is shown to provide selective and sensitive correlations in the methyl region of 2D spectra of crystalline organic compounds. The method is then demonstrated on a microcrystalline sample of the dimeric protein Crh (2 x 10.4 kDa). A total of 34 new proton-proton contacts involving side-chain methyl groups were observed in the J-CHHC spectrum, which had not been observed with the conventional experiment. The contacts were then used as additional distance restraints for the 3D structure determination of this microcrystalline protein. Upon addition of these new distance restraints, which are in large part located in the hydrophobic core of the protein, the root-mean-square deviation with respect to the X-ray structure of the backbone atom coordinates of the 10 best conformers of the new ensemble of structures is reduced from 1.8 to 1.1 A.     

The accuracy of distance measurements in solid-state NMR.

The accuracy with which distances can be measured using dipolar recoupling experiments in solid-state NMR is investigated. The relative precision of experiments in a three spin system versus an isolated spin pair is found to depend very strongly on the nature of the coupling Hamiltonian. The accuracy of distances measured in even the simplified three spin system is seen to be very poor for existing homonuclear recoupling Hamiltonians. This suggests that it would be difficult to exploit broadband homonuclear recoupling to measure geometrical information reliably in complex spin systems. These conclusions apply equally to both single-crystal studies and powder samples. In contrast, the presence of additional spins has marginal impact on the accuracy when the coupling Hamiltonians commute with each other, as in the case of heteronuclear recoupling. The possibility of creating such a Hamiltonian for homonuclear recoupling using a suitable rotor-synchronized pulse sequence is discussed.     

Better characterization of surface organometallic catalysts through resolution enhancement in proton solid state NMR spectra

Delayed-acquisition methods, namely, echo and constant-time-acquisition approaches, allow a significant improvement in resolution in the proton solid state NMR spectra of surface organometallic catalysts such as [syn-(SiO)Mo(=NAr)(=CH(t)Bu)(CH2(t)Bu)] and [(SiO)Re(C(t)Bu)(=CH(t)Bu)(CH2(t)Bu)] (syn/anti ratio = 1:1). This enables the observation of all of the proton resonances, which is not possible with the simple proton single-pulse technique under magic-angle spinning. For example, the methylene protons of the neopentyl ligands, buried in the large peak associated with all of the methyls in the 1H MAS spectrum, can easily be identified by recording a delayed-acquisition spectrum (resolution enhancement of a factor of 3 is obtained). Moreover, combining constant-time acquisition with heteronuclear carbon-proton correlation spectroscopy also improves the resolution of the 2D HETCOR spectra.     

Saxitoxin, a toxic marine natural product that targets a multitude of receptors

Saxitoxin (STX) was discovered early last century and can contaminate seafood and drinking water, and over time has become an invaluable research tool and an internationally regulated chemical weapon. Among natural products, toxins obtain a unique reputation from their high affinity and selectivity for their target pharmacological receptor, which for STX has long been considered to only be the voltage gated sodium channel. In recent times however, STX has been discovered to also bind to calcium and potassium channels, neuronal nitric oxide synthase, STX metabolizing enzymes and two circulatory fluid proteins, namely a transferrin-like family of proteins and a unique protein found in the blood of pufferfish.     

An investigation of the structure and bond rotational potential of some fluorinated ethanes by NMR spectroscopy of solutions in nematic liquid crystalline solvents

NMR spectra of 1,2-dibromo-1,1-difluoroethane and 1-bromo-2-iodo-tetrafluoroethane dissolved in nematic liquid crystalline solvents have been analysed to yield the magnitudes and signs of the scalar couplings, J(ij), and total anisotropic couplings, T(ij), between all the (1)H, (19)F, and (13)C nuclei, except for those between two (13)C nuclei. The values obtained for T(ij) in principle contain a contribution from J(ij)(aniso), the component along the static applied magnetic field of the anisotropic part of the electron-mediated spin-spin coupling. Neglecting this contribution allows partially averaged dipolar couplings, D(ij), to be extracted from the T(ij), and these were used to determine the structure, orientational order, and the conformational distribution generated by rotation about the C-C bond. The values obtained are compared with the results of calculations by ab initio and density functional methods. The differences found are no greater than those obtained for similar compounds which do not contain fluorine, so that there is no definitive evidence for significant contributions from J(CF)(aniso) or J(FF)(aniso) in the two compounds studied.     

Nanomaterials for Ocular Drug Delivery

Efficient drug delivery to the eye remains a challenging task for pharmaceutical scientists. Due to the various anatomical barriers and the clearance mechanisms prevailing in the eye, conventional drug delivery systems, such as eye drop solutions, suffer from low bioavailability. More invasive methods, such as intravitreal injections and implants, cause adverse effects in the eye. Recently, an increasing number of scientists have turned to nanomaterial‐based drug delivery systems to address the challenges faced by conventional methods. This paper highlights recent applications of various nanomaterials, such as polymeric micelles, hydrogels, liposomes, niosomes, dendrimers, and cyclodextrins as ocular drug delivery systems to enhance the bioavailability of ocular therapeutic agents.