Figure 4.

Longitudinal spin-lattice (T₁) relaxation of ²³Na NMR signals in sodium heparin samples determined through standard inversion-recovery NMR experiments. (a) The ²³Na inversion-recovery data are shown as relative intensity versus relaxation time (duration of relaxation delay) for the following samples: sodium heparin sample without added ion, sodium heparin samples with 100 mM CaCl₂ or MgCl₂ added, and a reference NaCl solution. Upon the addition of Mg²⁺ and especially Ca²⁺ ions, the relaxation-induced recovery of ²³Na signals becomes slower and approaches but does not reach, that of the reference NaCl sample. (b) The obtained ²³Na T₁ relaxation times of the measured samples. The ²³Na T₁ data show partial Ca²⁺ or Mg²⁺ ion-dependent release of sodium ions from heparin, which is stronger in the case of Ca²⁺ ions. (c) The ¹⁷O inversion-recovery data of the above samples show only slight differences in the viscosity-dependent ¹⁷O T₁ of water.