Investigation of 13C relaxation and nuclear Overhauser enhancement parameters for heparinoid systems: comparison with data from dextrans

Data are presented for ¹³C spin-lattice (T₁) relaxation times and nuclear Overhauser enhancement (NOE) values in a range of heparinoid glycosaminoglycans. These paramaters are compared with T₁and NOE values for simple dextrans of $\text{M}{}_{\mathrm{<mtext>W</mtext>}}\text{= 19 900, 40 000}\text{and}\text{150 000}$. For the heparinoid molecules, significant and consistent differences in relaxation times are observed for different ring carbons, for which simple models do not provide an adequate explanation. No such variations are found for the dextrans. Both families of polysaccharides exhibit NOE values significantly reduced from the theoretical maximum. These changes are discussed in terms of molecular rotational correlation times (τ_c) which are similar in magnitude to the resonance frequencies used for n.m.r. measurements. It is concluded that for the n.m.r. investigation of polysaccharides at high field strengths (${}^{13}\text{C resonance frequencies > 25 MHz}$) considerable economy of time will be achieved by the use of INEPT experiments rather than conventional Overhauser signal enhancement via proton-noise decoupling.