NUCLEAR SPIN-RELAXATION STUDIES OF HYDRATED ELASTIN

The nuclear magnetic spin-lattice and transverse relaxation processes for the 1H and 2D nuclei in purified elastin (bovine ligamentum nuchae), exchanged and hydrated with excess D2O, were studied in the temperature range 276.degree.-340.degree. K. The 2D relaxation results clearly show the presence of D2O external to the bulk elastin sample, in spaces within the bulk elastin and as an integral part of the protein on a molecular level. The water content of the protein itself changes from .apprx. 0.8 g D2O/g dry elastin at .apprx. 280.degree. K to .apprx. 0.2 g D2O/g dry elastin at .apprx. 335.degree. K, a decrease of 400%. The D2O content of the interfiber spaces decreases by less than 20% over the same temperature range. This fact throws doubt on the validity of the values of .beta., the thermal expansion coefficient of elastin, used by other workers in discussion of the elastic mechanism in elastin. The elastin proton transverse relaxation shows the presence of 3 regions in elastin having different degrees of molecular mobility. These are assigned to protons associated with the crosslinks, a fairly mobile, hydrophobic and low-water-content region and a more mobile higher water-content region. The temperature variation of the relative proportions of these 3 regions is explained in terms of a hypothetical temperature-composition phase diagram in which the 2 mobile regions are represented as 2 partially miscible phases with different negative temperature coefficients of solubilty in water. The implications of these observations for current views of the nature of elastin are assessed. The spin-relaxation results are consistent with a multiphase structural model for elastin. An approximate sorption isotherm for the water/elastin system is reported and shows the relatively weak nature of the water/elastin interaction.