Characterization of molecular motions in13C-labeled aortic elastin by13C-1H magnetic double resonance

Purified insoluble elastin samples labeled with [1‐ 13 C]valine, [1‐ 13 C]alanine, and [1‐ 13 C]‐lysine were prepared from chick aorta in culture. The molecular mobility at the labeled sites was investigated using 13 C‐ 1 H magnetic double‐resonance spectroscopy. Linewidths, T 1 , and nuclear Overhauser effect (NOE) values of the labeled carbons alone were obtained from dipolar decoupled difference spectra. Analysis of these parameters together with signal intensity measurements showed that essentially all the valyl residues, ca. 75% of the alanyl residues, and ca. 60% of the lysyl residues were characterized by rapid backbone motions having τ = 65 nsec. Resonances due to the remaining alanyl and lysyl residues were detected in cross‐polarization experiments, which enhance the signals of motionally restricted carbons. Since lysyl and alanyl residues are found in the crosslink regions of elastin, whereas valyl residues are not, we conclude that crosslinks rather than secondary structures in the extensible region of the protein are the main source of motional restrictions in the protein. Elastin chain mobility was monitored by linewidth measurements over the range −90 to +70°C. When the swelling solvent (0.15 M NaCl) was fixed at 0.6 g/g of elastin, a rapid monotonic reduction in chain mobility was observed as the temperature was lowered from 50 to 5°C. Liquidlike mobility was completely lost at 5°C. In contrast, the same sample in contact with excess solvent retained its liquidlike molecular mobility until −13°C, where it abruptly became rigid. The molecular mobility of this sample was temperature insensitive in the physiologically interesting range, 20–40°C, as a consequence of the opposing influences of temperature and swelling. Taken together these nmr data indicate that under physiological conditions, elastin is a network of mobile chains whose motions are strongly influenced by protein–solvent interactions.