Noninvasive measurement of protein concentration

At selected magnetic field strengths, protein and water proton spin-lattice relaxation rates are sensitive to the concentration of rotationally immobilized peptide nitrogen because of field dependent heteronuclear cross relaxation coupling between protein proton and nitrogen-14 spins that is carried to the water by proton homonuclear cross-relaxation. Measurement of the water proton spin-lattice relaxation time, or a signal amplitude proportional to it, may provide a noninvasive measure of peptide bond concentration, which provides a direct measure of immobilized protein content in most tissues. The approach using protein gels in two magnetic field strengths is demonstrated. At 66.7 mT; the proton Zeeman energy matches one of the peptide nitrogen transitions dominated by the unaveraged nuclear electric quadrupole interaction; cross-relaxation between the protons and nitrogen-14 is efficient. At 77.5 mT, the proton Zeeman energy is not matched with the nitrogen energy and proton-nitrogen cross-relaxation is not efficient. It is shown that the difference in the water proton spin-lattice relaxation rates on and off the energy level match condition is a linear function of the rotationally immobilized protein concentration.