Nuclear spin relaxation in heterogeneous biological systems

The magnetic coupling between liquid and solid protons in heterogeneous media like tissues may affect spin-lattice relaxation rates profoundly. There are at least two possible magnetic coupling mechanisms: (1) through space dipolar interactions, and (2) chemical exchange. Using the Z-spectrum technique, we studied a series of Z-spectra of protons and deuterons in native and cross-linked bovine serum albumin (BSA) solution at the same resonance frequency. Therefore we were able to sample the same portion of the spectral density profile for each nucleus. For the deuteron case, the dipolar contribution is greatly reduced due to its small magnetic dipole moment. To eliminate the complication of atom exchange, we observed the Z-spectra of dimethyl sulfoxide (DMSO) and deuterated DMSO in the BSA samples since DMSO has no exchangeable protons. The contributions of rotating frame relaxation and through space dipolar interaction are experimentally demonstrated to be minor. The dominant mechanism for the magnetic coupling is shown to be chemical exchange.; Chemical exchange may contribute to an effective relaxation coupling between solid and water protons and also contribute to an effective mixing of transverse relaxation rates in heterogeneous materials. Study of the inter-pulse spacing dependence in spin-echo experiments has permitted analysis of chemical exchange contributions to proton transverse relaxation in BSA samples. We distinguish between atom and whole molecule exchanges with protein sites. Based on several different spin-echo experiments we estimate that the molecular exchange times of DMSO with protein binding sites are shorter than 100 {dollar}mu{dollar}s. The molecular exchange provides an effective mechanism of coupling the solid component dynamics to the liquid transverse relaxation response.; The {dollar}sp{lcub}14{rcub}{dollar}N in biological tissues provides an additional relaxation path for the protons that are coupled to it. Since the {dollar}sp{lcub}14{rcub}{dollar}N nuclei are strongly coupled to the lattice by the nuclear electric quadrupole interaction, the {dollar}sp{lcub}14{rcub}{dollar}N relaxation is efficient. When the magnetic field is adjusted so that the {dollar}sp{lcub}14{rcub}{dollar}N and {dollar}sp1{dollar}H levels match, the {dollar}sp1{dollar}H relaxation rates are augmented by the inter-nuclear cross-relaxation. The effect on the {dollar}sp1{dollar}H relaxation rate is proportional to the {dollar}sp{lcub}14{rcub}{dollar}N concentration and thus provides an approach for quantitative measurement of the nitrogen content based on the proton signal response. Applications of the measurement to nitrogen concentration in biological samples are demonstrated.