## Abstract

The slowly relaxing local structure (SRLS) approach, developed for NMR spin relaxation analysis in proteins, is applied herein to amide ^{15}N relaxation in deoxy and carbonmonoxy hemoglobin. Experimental data including ^{15}N T_{1}, T_{2} and ^{15}N-{^{1}H} 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 (S_{0}^{2}) and orientation (β_{D}) of an axial local ordering tensor, S, and the principal values (R_{∥} ^{L} and R_{⊥}^{L}) and orientation (β_{O}) of an axial local diffusion tensor, R^{L}. The parameters c_{0}^{2} (the potential coefficient in terms of which S_{0}^{2} is defined), R_{∥}^{L}, β_{D}, and β_{O} are determined by data fitting; R_{⊥}^{L} is set equal to the global motional rate, R ^{C}, found previously to be (5.2-5.8) - 10^{6} 1/s in the temperature range investigated. The principal axis of S is (nearly) parallel to the C_{i-1}^{α}-C_{i}^{α} axis; when the two axes are parallel, β_{D} = -101.3° (in the frame used). The principal axis of R^{L} is (nearly) parallel to the N-H bond; when the two axes are parallel, β_{O} = -101.3°. For 'rigid' N-H bonds located in secondary structure elements the best-fit parameters are S_{0}^{2} = 0.88-0.95 (corresponding to local potentials of 8.6-19.9 k_{B}T), R_{∥}^{L} = 10^{9}-10 ^{10} 1/s, β_{D} = -101.3° ± 2.0°, and β_{O} = -101.3° ± 4°. For flexible N-H bonds located in loops the best-fit values are S_{0}^{2} = 0.75-0.80 (corresponding to local potentials of 4.5-5.5 k_{B}T), R _{∥}^{L} = (1.0-6.3) - 10^{8} 1/s, β_{D} = -101.3° ± 4.0°, and β_{O} = -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).

Original language | English |
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Pages (from-to) | 143-157 |

Number of pages | 15 |

Journal | Journal of Physical Chemistry B |

Volume | 115 |

Issue number | 1 |

DOIs | |

State | Published - 13 Jan 2011 |