Proton Nuclear Magnetic Resonance Relaxation in Aqueous Sugar Solutions: Can Low-Field Nuclear Magnetic Resonance Relaxation Measurements Differentiate between “Bound” and “Free” Water?

Understanding water interactions in complex systems is crucial, as they play a key role in fields such as biochemistry, pharmaceutical formulations, and food science. Nuclear magnetic resonance (NMR) relaxation measurements have become one of the widely used methods to visualize various water characteristics owing to their noninvasive nature and ease of use. However, unambiguous data interpretation can be challenging and potentially misleading if not carefully analyzed. One such example is the observation of multiple relaxation times, which is often linked to different water types such as "bound" and "free". In this paper, we present a new approach for the interpretation of proton NMR relaxation data using a second-order reaction kinetics-based model. The case of first-order asymptotic analysis considering fast proton exchange is shown to be of particular relevance. The presented theory is tested using a series of sucrose-water and sucrose-D₂O systems with varying sucrose content. The comparison of these systems reveals a biexponential behavior in both <i>T</i> ₁ and <i>T</i> ₂ relaxation times. These observations are interpreted by considering both nonexchangeable and exchangeable protons in the system, with the corresponding contribution coefficients following trends consistent with the concentrations of these proton types.