Backbone Dynamics of Deoxy and Carbonmonoxy Hemoglobin by NMR/SRLS

The slowly relaxing local structure (SRLS) approach, developed for NMR spin relaxation analysis in proteins, is applied herein to amide 15N relaxation in deoxy and carbonmonoxy hemoglobin. Experimental data including 15N T1, T2 and 15N-{1H} NOE, acquired at 11.7 and 14.1 T, and 29 and 34 °C, are analyzed. The restricted local motion of the N−H bond is described in terms of the principal value (S02) and orientation (βD) of an axial local ordering tensor, S, and the principal values (R||L andRL) and orientation (βO) of an axial local diffusion tensor, RL. The parameters c02 (the potential coefficient in terms of which S02 is defined), R||L, βD, and βO are determined by data fitting; RL is set equal to the global motional rate, RC, found previously to be (5.2−5.8) × 106 1/s in the temperature range investigated. The principal axis of S is (nearly) parallel to the Ci−1α−Ciα axis; when the two axes are parallel, βD = −101.3° (in the frame used). The principal axis of RL is (nearly) parallel to the N−H bond; when the two axes are parallel, βO = −101.3°. For “rigid” N−H bonds located in secondary structure elements the best-fit parameters are S02 = 0.88−0.95 (corresponding to local potentials of 8.6−19.9 kBT), R||L = 109−1010 1/s, βD = −101.3° ± 2.0°, and βO = −101.3° ± 4°. For flexible N−H bonds located in loops the best-fit values are S02 = 0.75−0.80 (corresponding to local potentials of 4.5−5.5 kBT), R||L = (1.0−6.3) × 108 1/s, βD = −101.3° ± 4.0°, and βO = −101.3° ± 10°. These results are important in view of their physical clarity, inherent potential for further interpretation, consistency, and new qualitative insights provided (vide infra).