Anisotropic orientation of lactate in skeletal muscle observed by dipolar coupling in H-1 NMR spectroscopy

Double quantum (DQ), J-resolved H-1 NMR spectra from rat and bovine skeleta l muscle showed a splitting frequency (similar to 24 Hz) for the lactate me thyl protons that varied with the orientation of the muscle fibers relative to the magnetic field, In contrast, spectra of lactate in solution consist of a J-coupled methyl doublet and a J-coupled methine quartet (J(HH) = 7 H z) with no sensitivity to sample orientation, Spectra acquired in magnetic fields of 4.7, 7, and 11 T showed that the splitting was not due to inhomog eneities in magnetic susceptibility within the muscle, because the magnitud e of the splitting did not scale with the strength of B-0 fields, Triple qu antum coherence (TQC) spectra revealed two distinct transition frequencies on the methyl resonance, These frequencies resulted from intra-methyl and m ethine-methyl couplings in this four spin system (A(3)X), Decoupling experi ments on the triple quantum coherence showed that the observed frequency sp litting was due mainly to the dipolar interactions between the methine and methyl protons of the lactate molecule, Thus, all the proton resonances of the lactate molecules in muscle behave anisotropically in the magnetic fiel d. Adequate design and interpretation of spectroscopic experiments to measu re lactate in muscle, and possibly in any cell and organ which contain asym metric structures, require that both the dipolar coupling described here an d the well-known scalar coupling be taken into account. (C) 1999 Academic P ress.