Design and activity of cationic fullerene derivatives as inhibitors of acetylcholinesterase

Four different regioisomers of cationic bis-N,N-dimethylfulleropyrrolidinium salts have been prepared and evaluated as inhibitors of the enzymatic activity of acetylcholinesterase. These fullerene-based derivatives were found to be noncompetitive inhibitors of acetylthiocholine hydrolysis. Molecular modelling was used to describe the possible interactions between the fullerene cage and the amino acids surrounding the cavity of the enzyme. The cationic C(60) derivatives used in this study represent a new class of molecules potentially able to modulate the enzymatic activity of acetylcholinesterase.     

Spin-State-Selective TPPI: A New Method for Suppression of Heteronuclear Coupling Constants in Multidimensional NMR Experiments

A novel multidimensional NMR pulse sequence tool, spin-state-selective time-proportional phase incrementation (S(3) TPPI), is introduced. It amounts to application of different TPPIs on the two components of doublets so that their frequencies can be manipulated independently. The chief application is for suppression of large heteronuclear one-bond coupling constants in indirect dimensions of multidimensional experiments without interchanging the two transverse magnetization components of doublets as conventional decoupling does, which is advantageous when they relax at different rates such as by partial compensation of dipolar and CSA relaxation contributions. For experimental confirmation we use a sample of (15)N-labeled neural cell adhesion molecule modules 1 and 2, a protein with a molecular weight of about 20 kDa. The new tool is general and can be combined with many multidimensional NMR experiments for proteins.     

Diastereoselective hydroxylation of 6-substituted piperidin-2-ones. An efficient synthesis of (2S,5R)-5-hydroxylysine and related alpha-amino acids

The synthesis of (2S,5R)-5-hydroxy-6-oxo-1,2-piperidinedicarboxylates (5) and related (3S,6R)-3-hydroxy-6-alkyl-2-oxo-1-piperidinecarboxylates has been developed. The approach is based on the asymmetric hydroxylation of enolates generated from the corresponding N-protected-6-substituted piperidin-2-ones. The utility of 5a as a precursor in the synthesis of (2S,5R)-5-hydroxylysine (1), an amino acid unique to collagen and collagen-like proteins, has also been demonstrated. (2S)-6-oxo-1,2-piperidinedicarboxylates (6) required for hydroxylation studies were prepared in 38-74% yield, starting from conveniently protected aspartic acid as inexpensive chiral adduct. Hydroxylation of 6 to 5 proceeds in high yield and excellent diastereoselectivity by treatment of their Li-enolate with (+)-camphorsulfonyloxaziridine at -78 degrees C. Ring opening of di-tert-butyl (2S,5R)-6-oxo-1,2-piperidinedicarboxylate ((5R)-5a) under reductive conditions afforded the corresponding 1,2-diol (17) in 91%, which was further transformed to (2S,5R)-5-hydroxylysine in four steps (84%). 17 is also a versatile intermediate in the preparation of tert-butyl (2S,5R)-2-[(tert-butoxycarbonyl)amino]-5-hydroxy-6-iodohexanoate (3) and tert-butyl (2S)-2-[(tert-butoxycarbonyl)amino]-4-[(2R)-oxiranyl]butanoate (4), two amino acid derivatives used in the total synthesis of the bone collagen cross-link (+)-pyridinoline (2a).     

Comparison of UV and visible Raman spectroscopy of bulk metal molybdate and metal vanadate catalysts

The visible (532 and 442 nm) and UV (325 nm) Raman spectra of bulk mixed metal oxides (metal molybdates and metal vanadates) were compared on the same spectrometer, for the first time, to allow examination of how varying the excitation energy from visible to UV affects the resulting Raman spectra. The quality of the Raman spectra was found to be a strong function of the absorption properties of the bulk mixed oxide. For bulk mixed metal oxides that absorb weakly in the visible and UV regions, both the visible and UV Raman spectra were of high quality and exhibit identical vibrational bands, but with slightly different relative intensities. For bulk mixed metal oxides that absorb strongly in the UV and visible regions and/or strongly in the UV and weakly in the visible regions, the visible Raman spectra are much richer in structural information and of higher resolution than the corresponding UV Raman spectra. This is a consequence of the strong UV absorption that significantly reduces the sampling volume and number of scatterers giving rise to the Raman signal. The shallower escape depth of UV Raman, however, was not sufficient to detect vibrations from the surface metal oxide species that are present on the outermost surface layer of these crystalline mixed metal oxide phases as previously suggested. It was also demonstrated that there is no sample damage by the more energetic UV excitation when very low laser powers and fast detectors are employed, thus avoiding the need of complicated fluidized bed sample arrangements sometimes used for UV Raman investigations. The current comparative Raman investigation carefully documents, for the first time, the advantages and disadvantages of applying different excitation energies in collecting Raman spectra of bulk mixed metal oxide materials.     

Chirality and Spectroscopic Changes Induced by the Recognition of Ethyl 5-Amino-2-methyl-1,2-dihydro-3-phenylpyrido[3,4-b]pyrazin-7-yl Carbamate Analogs by Tubulin

The two chiral isomers of ethyl 5-amino-2-methyl-l,2-di-hydro-3-phenylpyrido[3,4-b]pyrazin-7-yl carbamate, NSC 613863 (R-isomer)-(+) and NSC 613862 (S-isomer)-(-) (CI980) and the three achiral analogs NSC 330770 (2-de-methylated analog A), NSC 337238 and C179 are potent microtubule inhibitors. These ligands interact with tubulin overlapping the colchicine binding site. This study addresses the effects of recognition by tubulin on the conformational properties of the ligands. The near-UV CD (circular dichroism) band of the R-isomer was suppressed, while that of the S-isomer displayed a more intense negative band when these compounds were bound to tubulin. Interestingly, the three other initially achiral compounds became optically active upon binding to tubulin; particularly, analog A exhibited a negative CD band on the order of magnitude of chiral compounds. The CD changes are reversible, highly specific and actually permit measuring the binding of the ligands by tubulin. These CD changes are compatible with the deformation of the bound ligands. Fluorescence emission is strongly enhanced and blue shifted upon binding to tubulin. Water among a solvent series had a specific solvent effect, except on the 1,2-dehydro analogs NSC 337238 and C179, suggesting hydrogen bonding to Nl. The emission of tubulin-bound R-isomer, S-isomer and analog A could be mimicked by solvent viscosity, supporting the notion that the intramolecular rotation between the pyridopyrazine and phenyl rings is frozen upon binding.     

Stereoselective alkylation of N-Boc-protected-5-substituted delta-lactams: synthesis of alpha, delta-disubstituted delta-amino acids

N-Boc-protected-5-substituted delta-lactams were readily prepared from the corresponding beta 3-amino acids. Alkylation reactions of their Na enolates with various electrophiles proceeded in high yields with high facial selectivity. The structure of the alkylation products was confirmed by single-crystal X-ray analysis. This method provides a fast access to optically active alpha, delta-disubstituted delta-amino acids.