Backbone motions in a crystalline protein from field-dependent 2H-NMR relaxation and line-shape analysis
✍ Scribed by James W. Mack; M. G. Usha; Joanna Long; Robert G. Griffin; R. J. Wittebort
- Publisher
- Wiley (John Wiley & Sons)
- Year
- 2000
- Tongue
- English
- Weight
- 112 KB
- Volume
- 53
- Category
- Article
- ISSN
- 0006-3525
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✦ Synopsis
We have used 2 H-nmr to study backbone dynamics of the 2 H-labeled, slowly exchanging amide sites of fully hydrated, crystalline hen egg white lysozyme. Order parameters are determined from the residual quadrupole coupling and values increase from S 2 ϭ 0.85 at 290 K to S 2 ϭ 0.94 at 200 K. Dynamical rates are determined from spin-lattice relaxation at three nmr frequencies (38.8, 61.5, and 76.7 MHz). The approach used here is thus distinct from solution nmr studies where dynamical amplitudes and rates are both determined from relaxation measurements. At temperatures below 250 K, relaxation is independent of the nmr frequency indicating that backbone motions are fast compared to the nmr frequencies. However, as the temperature is increased above 250 K, relaxation is significantly more efficient at the lowest frequency, which shows, in addition, the presence of motions that are slow compared to the nmr frequencies. Using the values of S 2 determined from the residual quadrupole coupling and a model-free relaxation formalism that allows for fast and slow internal motions, we conclude that these slow motions have correlation times in the range of 0.1 to 1.0 s and are effectively frozen out at 250 K where fast motions of the amide planes with ϳ 15 ps effective correlation times and 9°rms amplitudes dominate relaxation. The fast internal motions increase slightly in amplitude as the temperature rises toward 290 K, but the correlation time, as is also observed in solution nmr studies of RNase H, is approximately constant. These findings are consistent with hypotheses of dynamic glass transitions in hydrated proteins arising from temperature-dependent damping of harmonic modes of motion above the transition point.