Measuring fast hydrogen exchange rates by NMR spectroscopy

Area of Science:

• Biophysical Chemistry
• Nuclear Magnetic Resonance (NMR) Spectroscopy
• Chemical Kinetics

Background:

• Proton exchange rates are crucial for understanding molecular dynamics and interactions.
• Traditional methods for measuring fast proton exchange are often limited in scope or applicability.
• Nuclear spin coherence provides a sensitive probe for dynamic processes in molecules.

Purpose of the Study:

• To introduce a novel NMR-based method for quantifying hydrogen exchange rates.
• To enable the measurement of proton exchange rates exceeding 10^5 s^-1.
• To extend the study of proton exchange dynamics across a wider pH range.

Main Methods:

• Utilizing the coherence of a neighboring spin (e.g., Nitrogen-15) coupled to the exchanging proton.
• Employing Carr-Purcell-Meiboom-Gill (CPMG) pulse sequences to observe coherence decay.
• Comparing coherence decay in the presence and absence of proton decoupling.

Main Results:

• Successfully measured proton exchange rates up to 10^5 s^-1.
• Extended the measurable pH range for studying indole proton exchange in tryptophan.
• Determined exchange constants for cationic, zwitterionic, and anionic forms of tryptophan.

Conclusions:

• The developed NMR method is effective for measuring fast proton exchange rates.
• This technique enhances the study of pH-dependent molecular dynamics.
• It provides valuable insights into the chemical properties of amino acids and other biomolecules.