Protein mediated magnetic coupling between lactate and water protons.

The magnetic coupling between methyl lactate protons and water protons in samples of cross-linked bovine serum albumin (BSA) is studied. Cross-relaxation spectroscopy shows efficient magnetization transfer from immobilized BSA to both water and methyl lactate protons. Transient and steady-state NOE experiments reveal a negative intermolecular NOE between methyl lactate and water protons. Lactate is indirectly detected by selectively saturating the methyl lactate protons and measuring the decrease in water proton magnetization. Indirect detection of methyl lactate protons is an order of magnitude more sensitive than direct detection in these model systems. Lactate was indirectly imaged, via the water proton resonance, with 1.1-microliter voxels in 2 min. Immobilized BSA reduces the intermolecular correlation time between water and lactate protons into the spin-diffusion limit where the NOE is negative. Possible molecular mechanisms for this coupling and applications to in vivo spectroscopy are discussed.