Since the sign of V(P,F) in these metal complexes can be assumed to be negative52 from the twodimensional correlation experiments (cf. Figure 4b), it follows that V(F,Fe) is positive. Of particular value is the combination of two-dimensional experiments, e.g. ('H,57Fe) and (31,57Fe) for complex 1. In this way, the relative signs of a variety of scalar couplings in quasitrigonal and -tetragonal complexes can be obtained (cf. Chart IV). To the best of our knowledge, the signs (52) Staplin, D. C.;

To study the dynamics of peptide groups in solid proteins, we have accurately determined the principal components and molecular orientation of the electric field gradient tensor for the exchangeable deuterons in monoclinic N-acetylglycine by single-crystal deuterium nuclear magnetic resonance. These results are compared with the principal components of the amide deuterons in solid poly(gamma-benzyl-L-glutamate) measured in powder samples over a wide temperature range (140-400 K). The comparison indicates that in the solid polypeptide the N-D bonds undergo a small-amplitude torsional reorientation (libration) perpendicular to the peptide plane. To estimate dynamic rates, longitudinal relaxation times (T1 values) are reported for N-acetylglycine and poly(gamma-benzyl-L-glutamate). T1 values for the carboxyl and amide deuterons in N-acetylglycine are approximately 100 s, whereas for the amide deuterons in the polypeptide T1 approximately 1 s, also indicating that the N-D bonds are not stationary in the polypeptide. We determine from the reduced quadrupole coupling tensor the mean-square amplitude for the libration and show that it increases linearly with temperature. A simple qualitative theory for the relaxation times is presented on the basis of the assumption that the N-D reorientation is described either as a diffusion process in a square well or as a damped Langevin oscillator with a harmonic restoring force. The conclusion is that the short relaxation times of the polypeptide amide deuterons result from substantial frictional effects on reorientation that increase with temperature.

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We have used 2H-nmr to study backbone dynamics of the 2H-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 S2 = 0.85 at 290 K to S2 = 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 S2 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 microsecond and are effectively frozen out at 250 K where fast motions of the amide planes with approximately 15 ps effective correlation times and 9 degrees 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.