Detection of electrostatic molecular binding using the water proton signal

Area of Science:

• Magnetic Resonance Imaging
• Biophysical Chemistry
• Molecular Interactions

Background:

• Saturation transfer MRI utilizes water signal enhancement to study molecular binding.
• Relayed nuclear Overhauser effect (rNOE) mechanisms are key to this signal enhancement.
• Quantifying molecular binding affinity and detecting transient binding are critical challenges.

Purpose of the Study:

• To elucidate rNOE-based mechanisms for MRI signal enhancement in molecular binding.
• To develop a strategy for quantifying molecular binding affinity (dissociation constant, Kd).
• To apply the method for detecting electrostatic binding of charged biomolecules and estimate detection limits for transient binding.

Main Methods:

• Quantitative description of signal enhancement using a three-step magnetization transfer model.
• Numerical simulations to verify the theoretical model.
• Experimental determination of equilibrium dissociation constants (Kd) for arginine, choline, and acetyl-choline binding to anionic resin, varying ligand concentration, pH, and salt content.

Main Results:

• Numerical simulations confirmed the ability to detect sub-millimolar ligands binding to low micromolar targets.
• Measured rNOE signals revealed contributions from intra-ligand and intermolecular pathways.
• Binding strengths (Kd) ranged from 70-160 mM, influenced by pH and salt concentration.

Conclusions:

• The developed MRI strategy enables quantification of molecular binding affinity.
• The method is capable of detecting electrostatic binding of charged small biomolecules.
• This approach facilitates the in vivo detection of micromolar level receptor-substrate binding.