Proton MRD Profile Analysis in Human Serum Albumin Solutions: Two and Three Sites Exchange Model Approaches

The two sites water exchange model (2SWEM) and the three sites exchange model (3SEM) were properly used to describe proton (¹H) magnetic relaxation dispersion (¹HMRD) in human serum albumin (HSA) solutions at 310 K. Lyophilized HSA was obtained from Sigma-Aldrich and diluted in phosphate buffered saline (PBS, pH 7.4) to obtain 10 samples with a concentration of 50 g/l. The ¹HMRD profiles (20–60 MHz) were obtained using a fast field cycling nuclear magnetic resonance relaxometry facility (Stelar FFC 2000 Spinmaster) and two Minispec (Mq20, Mq60) relaxometry facilities from Bruker. The longitudinal ¹H magnetic relaxation time (T₁) was measured employing the inversion recovery pulse sequence and the 1/T₁ was plotted as a function of the frequency of resonance to create the ¹HMRD profiles. The 2 sites water exchange model considering ellipsoidal geometry is the best option to fit the ¹HMRD profiles in diluted HSA solutions, which allows to update the physical model previously presented to describe the theoretical dependence between the transverse proton magnetic relaxation rate and the protein dynamic viscosity in blood plasma and blood serum solutions. The physical parameters obtained from the fit, using this model, describe properly the diluted HSA solutions in comparison with previous experimental reports and theoretical estimations. This result can be improved taking into consideration all the proton–proton dipolar interactions of the protons belonging to the bound water molecules.