Silvano Piani

Conformational analysis of heparin epoxide in aqueous solution. An NMR relaxation study

1H and 13C NMR relaxation measurements at various magnetic fields have been used to characterize the nature of overall and internal motions in heparin epoxide in aqueous solution. A two-dimensional homonuclear NOESY experiment showed a considerable number of cross-relaxing protons in the molecule. The inter-proton distances calculated from NOE data were compared with those obtained by molecular mechanics calculations. Several discrepancies between the experimental and the theoretical inter-proton distances as well as the variations in 13C spin-lattice relaxation times, measured at two magnetic fields, indicated that the polysaccharide tumbles anisotropically in solution. The rates of overall and internal motions as well as the order parameters have been calculated using a model-free spectral density function. The numerical values indicate that the correlation times which characterize overall molecular motion are outside the extreme narrowing limit (tau parallel = 8 x 10(-10) s and tau perpendicular = 4.2 x 10(-8) s) and that internal motion correlation time is on a picosecond timescale.

Alkali-Induced Optical Rotation Changes in Heparins and Heparan Sulfates, and Their Relation to Iduronic Acid-Containing Sequences

Abstract Under specific basic conditions, the glucosaminoglycans heparin and heparan sulfate, containing α-L-iduronic acid 2-O-sulfate, undergo selective epoxidation between C-2 and C-3 of this residue, with formation of a residue of 2,3-anhydro-α-L-guluronic acid. The epoxidation reaction was studied by means of 13C NMR and optical rotation measurements. The optical rotation values correlate well with the composition of the reaction products as determined by 13C NMR, thus indicating that the heparin- or heparan-like character of both natural and semisynthetic polysaccharides can be easily determined also through optical rotation measurements.

Conformational analysis of heparin epoxide: molecular mechanics computations.

The conformation of models of the epoxy-derivative of the glycosaminoglycan heparin has been studied by molecular mechanics calculations using a MM2-like force field extended with parameters for the oxirane ring. Two dimers, two trimers and several higher homologs modeling heparin epoxide were investigated, assuming the preferred 5H0 ring form of 2,3-anhydro-alpha-L-guluronic acid residue. Two-dimensional (phi; psi) maps of dimers showed the location of the energetically preferred conformers. Starting from the most stable dimer conformers, structures of trimers and other oligomers were derived and optimized, with an exhaustive search of the preferred sidechain conformers. The effect of solvation on conformation was analyzed using a continuum model of solvent. The present calculations indicate a significant flexibility of the heparin epoxide chain.

Semi-synthetic Heparins with 2-Deoxy-2-sulfamino-α-l-iduronic Acid Residues: Chemical Reactivity and Biological Activity1

Abstract Ammonolysis of the epoxide rings of 2,3-anhydro-α-L-guluronic acid residues, generated in alkaline medium from 2-O-sulfated α-L-iduronic acid residues of heparin, quantitatively afforded 2-amino-2-deoxy-α-L-iduronic acid residues. N-sulfation of these residues by TMA·SO3 complex led to a formal replacement of the original 2-O-sulfate groups of heparin with N-sulfates, without configurational changes. These modified uronic acid residues (no longer amenable to alkaline epoxidation) can be easily N-desulfated. The presence of negative or positive charges at position 2 of the newly generated 2-amino-2-deoxy-α-L-iduronic acid residues influences the in vivo antithrombotic activity and haemorrhagic effects in different ways. A free amino group mainly decreases the haemorrhagic properties of heparin, while a negatively charged N-sulfate group decreases the coagulation parameters.

The M/P conformation distributions of 3-O-derivatives of the 1,5-benzothiazepin ring system of diltiazem. The unique effect of silicon.

Abstract Several 3-O-derivatives of (2S,3S)-5-(2-dimethylaminoethyl)-2,3-dihydro-3-hydroxy-2-(4-methoxyphenyl)-1,5-benzothiazepin-4(5H)-one, the parent compound of diltiazem, have been prepared and the M/P conformation ratios have been determined by 1H NMR spectroscopy in solution, at room temperature. In general, the (M,2S,3S) conformer is the major component; however, a preference for the (P,2S,3S) conformation has been observed for the first time in the 3-silyloxy derivatives.