Water T2 relaxation in sugar solutions.

Area of Science:

• Biophysical Chemistry
• Solution NMR Spectroscopy
• Hydrogen Bonding Dynamics

Background:

• Understanding water-solute interactions is crucial in various chemical and biological systems.
• Proton exchange dynamics play a significant role in nuclear magnetic resonance (NMR) relaxation.
• Quantifying hydroxyl proton exchange provides insights into molecular mobility and interactions.

Purpose of the Study:

• To investigate the contribution of hydroxyl proton exchange to water 1H spin-spin relaxation in aqueous solutions of various polyols and carbohydrates.
• To extract solute-specific parameters characterizing hydroxyl proton exchange using the Carver-Richards model.
• To explore the influence of molecular size, conformation, and concentration on these exchange dynamics.

Main Methods:

• Measurement of 1H spin-spin relaxation times (T2) using the Carr-Purcell-Meiboom-Gill (CPMG) sequence.
• Varying the CPMG pulse spacing (tau) to observe relaxation-time dispersion.
• Analysis of temperature and tau dependencies using nonlinear regression based on the Carver-Richards model.

Main Results:

• Identified significant contribution of exchange between water and labile solute hydroxyl protons to relaxation.
• Extracted key parameters: fraction of exchangeable protons (P), chemical-shift difference (deltaomega), intrinsic spin-spin relaxation time (T2), and chemical exchange rate (k).
• Determined solute-specific parameters related to concentration, identity, mobility, and hydroxyl site lifetime; typical values at 298 K: deltaomega ~1 ppm, T2 ~100 ms, k ~1000 s(-1).

Conclusions:

• The study successfully quantifies hydroxyl proton exchange in diverse aqueous carbohydrate solutions.
• The extracted parameters offer a detailed molecular-level understanding of water-solute interactions.
• Eyring activation parameters indicate the energetic landscape of the proton exchange mechanism.