Anisotropic interactions of metabolites in skeletal muscle observed by dipolar coupling in hydrogen NMR spectroscopy

Dipolar coupling interactions of several metabolites from various biological tissues have been shown to strongly influence the structure of NMR spectra. The observation of these interactions has several theoretical and practical consequences for both biological and clinical applications of NMR.; Up to recently, the underlying assumption in biological applications of NMR has been that molecules in tissue move freely as in solutions. Solution NMR spectra lack resonances due to dipolar coupling. This absence of dipolar coupled peaks is due to the isotropic motion of molecules which causes dipolar coupling interactions to average to zero.; The goal of the experiments presented in this work was to investigate dipolar coupling in skeletal muscle due to anisotropic motion of different molecules.; DQF, 1H NMR spectra from these molecules showed a frequency splitting due to dipolar coupling. This observation indicates that in muscle molecules do not tumble freely and isotropically but have, on a time average, a preferred orientation.; The frequency splitting of the dipolar coupled peaks depends on the orientation of the muscle fibers relative to the magnetic field.; Experiments were designed to show how this orientational dependence affects lactate quantification in muscle. The main conclusion was that, in order for lactate to be adequately quantified, both scalar and dipolar coupling need to be taken into account. A direct implication of such a statement is that prior to quantifying lactate in muscle using NMR spectroscopy, one needs to identify the orientation of the muscle fibers relative to the static magnetic field, and, most importantly, determine the T₂ relaxation for both the isotropic and anisotropic pools of lactate.; Several studies have shown that dipolar coupling interactions are a useful tool for probing the microstructure of ordered tissues. However, an underlying theory that explains the presence of these interactions is still lacking.; To this end, experiments were conducted to test how muscle pH, molecular charge and chirality affect the dipolar coupling interactions of various molecules in the tissue. The results showed that muscle pH plays a key role in both the magnitude of the frequency splitting and the intensity of the dipolar coupled peaks.