Anisotropy of collagen fiber orientation in sheep tendon by ¹H double-quantum-filtered NMR signals

The anisotropy of the angular distribution of collagen fibrils in a sheep tendon was investigated by ¹H double-quantum (DQ) filtered NMR signals. Double-quantum build-up curves generated by the five-pulse sequence were measured for different angles between the direction of the static magnetic field and the axis of the tendon plug. Proton residual dipolar couplings determined from the DQ build-up curves in the initial excitation/reconversion time regime which mainly represent the bound water are interpreted in terms of a model of spin-1/2 pairs with their internuclear axes oriented on average along the fibril direction in the presence of proton exchange. The angular distribution of collagen fibrils around the symmetry axis of the tendon measured by the anisotropy of the residual dipolar couplings was described by a Gaussian function with a standard deviation of 12°±1° and with the center of the distribution at 4°±1°. The existence of this distribution is directly reflected in the finite value of the residual dipolar couplings at the magic angle, the value of the angular contrast, and the oscillatory behavior of the DQ build-up curves. The ¹H residual dipolar couplings were also measured from the doublets recorded by the DQ-filtered signals. From the angular dependence of the normalized splitting the angular distribution of the collagen fibrils was evaluated using a Gaussian function with a standard deviation of 19°±1° and with the center of distribution at 2°±1°. The advantages and disadvantages of these approaches are discussed.