A Comprehensive Analysis of Multifield 15N Relaxation Parameters in Proteins: Determination of 15N Chemical Shift Anisotropies
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- 作者：Daniel Canet ; Philippe Barthe ; Pierre Mutzenhardt ; and Christian Roumestand
- 刊名：Journal of the American Chemical Society
- 出版年：2001
- 出版时间：May 16, 2001
- 年：2001
- 卷：123
- 期：19
- 页码：4567 - 4576
- 全文大小：220K
- 年卷期：v.123,no.19(May 16, 2001)
- ISSN：1520-5126
文摘
This study deals with the exploitation of the three classical 15N relaxation parameters (the longitudinalrelaxation rate, R1, the transverse relaxation rate, R2, and the 1H-15N cross-relaxation rate, 
NH) measured atseveral magnetic fields in uniformly 15N-labeled proteins. Spectral densities involved in R1, R2 and NH areanalyzed according to the functional form A + B/(1 + 
), where 
s is the correlation time associated withslow motions sensed by the NH vector at the level of the residue to which it belongs. The coefficient B providesa realistic view of the backbone dynamics, whereas A is associated with fast local motions. According to the"model free approach", B can be identified with 2sS2 where S is the generalized order parameter. The correlationtime s is determined from the field dependency of the relaxation parameters while A and B are determinedthrough linear equations. This simple data processing is needed for obtaining realistic error bars based on astatistical approach. This proved to be the key point for validating an extended analysis aiming at thedetermination of nitrogen chemical shift anisotropy. The protein C12A-p8MTCP1 has been chosen as a modelfor this study. It will be shown that all data (obtained at five magnetic field strengths corresponding to protonresonance of 400, 500, 600, 700, and 800 MHz) are very consistently fitted provided that a specific effectivecorrelation time associated with slow motions is defined for each residue. This is assessed by small deviationsbetween experimental and recalculated values, which, in all cases, remain within experimental uncertainty.This strategy makes needless elaborate approaches based on the combination of several slow motions or theirpossible anisotropy. Within the core of the protein s fluctuates in a relatively narrow range (with a meanvalue of 6.15 ns and a root-mean-square deviation of 0.36 ns) while it is considerably reduced at the proteinextremities (down to ~3 ns). To a certain extent, these fluctuations are correlated with the protein structure.A is not obtained with sufficient accuracy to be valuably discussed. Conversely, order parameters derivedfrom B exhibit a significant correlation with the protein structure. Finally, the multi-field analysis of the evolutionof longitudinal and transverse relaxation rates has been refined by allowing the 15N chemical shift anisotropy(csa) to vary residue by residue. Within uncertainties (derived here on a statistical basis) an almost constantvalue is obtained. This strongly indicates an absence of correlation between the experimental value of thisparameter obtained for a given residue in the protein, the nature of this residue, and the possible involvementof this residue in a structured area of the protein.
NH) measured atseveral magnetic fields in uniformly 15N-labeled proteins. Spectral densities involved in R1, R2 and NH areanalyzed according to the functional form A + B/(1 + ), where s is the correlation time associated withslow motions sensed by the NH vector at the level of the residue to which it belongs. The coefficient B providesa realistic view of the backbone dynamics, whereas A is associated with fast local motions. According to the"model free approach", B can be identified with 2sS2 where S is the generalized order parameter. The correlationtime s is determined from the field dependency of the relaxation parameters while A and B are determinedthrough linear equations. This simple data processing is needed for obtaining realistic error bars based on astatistical approach. This proved to be the key point for validating an extended analysis aiming at thedetermination of nitrogen chemical shift anisotropy. The protein C12A-p8MTCP1 has been chosen as a modelfor this study. It will be shown that all data (obtained at five magnetic field strengths corresponding to protonresonance of 400, 500, 600, 700, and 800 MHz) are very consistently fitted provided that a specific effectivecorrelation time associated with slow motions is defined for each residue. This is assessed by small deviationsbetween experimental and recalculated values, which, in all cases, remain within experimental uncertainty.This strategy makes needless elaborate approaches based on the combination of several slow motions or theirpossible anisotropy. Within the core of the protein s fluctuates in a relatively narrow range (with a meanvalue of 6.15 ns and a root-mean-square deviation of 0.36 ns) while it is considerably reduced at the proteinextremities (down to ~3 ns). To a certain extent, these fluctuations are correlated with the protein structure.A is not obtained with sufficient accuracy to be valuably discussed. Conversely, order parameters derivedfrom B exhibit a significant correlation with the protein structure. Finally, the multi-field analysis of the evolutionof longitudinal and transverse relaxation rates has been refined by allowing the 15N chemical shift anisotropy(csa) to vary residue by residue. Within uncertainties (derived here on a statistical basis) an almost constantvalue is obtained. This strongly indicates an absence of correlation between the experimental value of thisparameter obtained for a given residue in the protein, the nature of this residue, and the possible involvementof this residue in a structured area of the protein.