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 °C 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.