Investigating the Relaxation Characteristics of 23Na in Variable Dynamics compartments using Diffusion and Relaxation NMR

Sodium ions play a crucial role in biological systems. Sodium MRI is essential to assess brain health since sodium plays a key role in cellular homeostasis and neuronal activity1. Changes in sodium distribution may indicate neuro diseases. Limitations occur in quantifying intra- and extracellular sodium because 23Na is a quadrupolar S=3/2 nucleus, and its relaxation does not reflect the typical protons behavior2. Therefore, it would be useful to derive suitable fitting functions to model 23Na relaxation in different dynamics environments. Sodium behavior in solutions containing polymers such as polyethylene glycol (PEG) is of significant interest. PEG is widely used in biological, pharmaceutical, and chemical applications due to its biocompatibility and ability to modulate solution properties. Adding PEG alters the viscosity and molecular dynamics3, which in turn affects the dynamics and relaxation properties of sodium ions. This study aims to define and test models describing sodium relaxation in different dynamic compartments. We investigate 23Na NMR relaxation in aqueous solutions by adding PEG to modify 23Na dynamics. Water solutions with varying PEG-400 concentrations (0%,20%,40%,50%,60%,80%,90%,100%w/v) were prepared and different quantities of sodium were added (10-140mM). Using a Bruker Avance-400, Pulsed-Field-Gradient (PFG) NMR was used to quantify diffusion of PEG, water protons and 23Na in each solution. Inversion-recovery and CPMG sequences were used to determine T1 and T2 relaxation times. Specific models to fit the relaxation time data of 23Na at different dynamics were tested. Ionmacromolecule interactions, hydration effects, and molecular crowding were analyzed. The findings related to 23Na relaxation in different dynamics of complex aqueous environments may have implications for designing PEG-based formulations in biomedical and industrial applications and for better understanding brain and human tissues 23Na-MRI investigation.