CONFORMATIONAL-ANALYSIS OF HEPARIN EPOXIDE IN AQUEOUS-SOLUTION - AN NMR RELAXATION STUDY

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