Thermodynamic analysis of hydration in human serum heme-albumin

Ferric human serum heme-albumin (heme-HSA) shows a peculiar nuclear magnetic relaxation dispersion (NMRD) behavior that allows to investigate structural and functional properties. Here, we report a thermodynamic analysis of NMRD profiles of heme-HSA between 20 and 60 oC to characterize its hydration. NMRD profiles, all showing two Lorentzian dispersions at 0.3 and 60 MHz, were analyzed in terms of modulation of the zero field splitting tensor for the S = 5/2 manifold. Values of correlation times for tensor fluctuation (τv) and chemical exchange of water molecules (τM) show the expected temperature dependence, with activation enthalpies of -1.94 and -2.46 ± 0.2 kJ mol-1, respectively. The cluster of water molecules located in the close proximity of the heme is progressively reduced in size by increasing the temperature, with ΔH = 68 ± 28 kJ mol-1 and ΔS = 200 ± 80 J mol-1 K-1. These results highlight the role of the water solvent in heme-HSA structure-function relationships.