A study of dipolar interactions and dynamic processes of water molecules in tendon by H-1 and H-2 homonuclear and heteronuclear multiple-quantum-filtered NMR spectroscopy

The effect of proton exchange on the measurement of H-1-H-1, H-1-H-2, and H -2-H-2 residual dipolar interactions in water molecules in bovine Achilles tendons was investigated using double-quantum-filtered (DQF) NMR and new pu lse sequences based on heteronuclear and homonuclear multiple-quantum filte ring (MQF). Derivation of theoretical expressions for these techniques allo wed evaluation of the H-1-H-1 and H-1-H-2 residual dipolar interactions and the proton exchange rate at a temperature of 24 degrees C and above, where no dipolar splitting is evident. The values obtained for these parameters at 24 degrees C were 300 and 50 Hz and 3000 s(-1), respectively. The result s for the residual dipolar interactions were verified by repeating the abov e measurements at a temperature of 1.5 degrees C, where the spectra of the H2O molecules were well resolved, so that the H-1-H-1 dipolar interaction c ould be determined directly from the observed splitting. Analysis of the MQ F experiments at 1.5 degrees C, where the proton exchange was in the interm ediate regime for the H-1-H-2 dipolar interaction, confirmed the result obt ained at 24 degrees C for this interaction. A strong dependence of the inte nsities of the MQF signals on the proton exchange rate, in the intermediate and the fast exchange regimes, was observed and theoretically interpreted. This leads to the conclusion that the MQF techniques are mostly useful for tissues where the residual dipolar interaction is not significantly smalle r than the proton exchange rate. Dependence of the relaxation times and sig nal intensities of the MQF experiments on the orientation of the tendon wit h respect to the magnetic field was observed and analyzed. One of the resul ts of the theoretical analysis is that, in the fast exchange regime, the si gnal decay rates in the MQF experiments as well as in the spin echo or CPMG pulse sequences (T-2) depend on the orientation as the square of the secon d-rank Legendre polynomial. (C) 1999 Academic Press.