Proton exchange rates from amino acid side chains--implications for image contrast

Area of Science:

• Biochemistry
• Biophysics
• Magnetic Resonance Imaging

Background:

• Proton exchange rates are crucial for understanding molecular dynamics and interactions in biological systems.
• Magnetization transfer in Magnetic Resonance Imaging (MRI) is influenced by the exchange of protons between water and biomolecules.

Purpose of the Study:

• To measure proton exchange rates between water and various amino acid residues (threonine, serine, tyrosine, lysine, arginine).
• To investigate the effect of pH, temperature, and common biological catalysts on these exchange rates.
• To determine the role of proton exchange rates in the contrast mechanisms of MRI.

Main Methods:

• Proton exchange rates were measured using Nuclear Magnetic Resonance (NMR) spectroscopy.
• Experiments were conducted over a pH range of 0.5 to 8.5 and temperatures from 4°C to 36°C.
• The catalytic effects of phosphate, carbonate, carboxyl, and amino groups were assessed.

Main Results:

• Intrinsic exchange rates for hydroxyl and amino protons at pH 7 and 36°C ranged from 700 to 10,000 s⁻¹.
• Physiological concentrations of catalysts significantly increased proton exchange rates.
• Proton exchange rates were found to be faster than the rate-limiting step in magnetization transfer.

Conclusions:

• Proton exchange rates are rapid and do not limit magnetization transfer between biomolecules and water.
• The contrast in MRI may serve as an indicator of the quantity of exchangeable protons (from OH and NH groups) in tissues.
• This finding has implications for understanding MRI contrast mechanisms and tissue characterization.