The static magnetic field dependence of chemical exchange linebroadening defines the NMR chemical shift time scale
The static magnetic field dependence of chemical exchange linebroadening defines the NMR chemical shift time scale
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DOI：
10.1021/ja993511y
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发表时间：
2000-03-29
影响因子：
15
通讯作者：
Palmer, AG
中科院分区：
文献类型：
--
作者：
Millet, O;Loria, JP;Palmer, AG
本文从理论和实验上研究了核磁共振谱中化学交换谱线展宽与静磁场的关系。双位点交换（可逆箭头B）被认为是位点A比位点B更高的填充（p（a）> p（B）），位点之间的位移差等于aw，交换速率常数由k（ex）给出。对于更高填充的位点，交换对横向弛豫速率常数的贡献表示为R-ex。R-ex对静磁场强度的依赖性由标度参数α = d In R-ex/d In Delta ω表征，其中，对于p（α）> 0.7，0小于或等于α小于或等于2。α的值取决于交换过程的NMR化学位移时间尺度：对于慢交换（k（ex）/Δ ω < 1），0小于或等于α < 1;对于中间交换（k（ex）/Δ ω> 1），α = 1;对于快交换（k（ex）/Δ ω> 1），1 <α小于或等于2。因此，R-ex的静态磁场依赖性定义了交换过程的化学位移时间尺度，即使布居是如此高度偏斜（p（a）远大于p（B）），以至于在慢交换极限中无法观察到次要共振。在11.7和14.1T的静磁场和300和313 K的温度下测量了蛋白质碱性胰蛋白酶抑制剂的~ 5 N横向弛豫速率常数，验证了理论结果。在静磁场和温度的每种组合下，使用Carr-Purcel-Meiboom-Gill和Hahn回波技术测量速率常数，自旋回波延迟范围为1.0至63.5 ms。由于化学交换，Cys 14-Cys 38二硫键区域中残基的5 N共振变宽。从弛豫速率常数获得的a值范围为0.26 +/- 0.17（Arg 39，300 K）至1.96 +/- 0.25（Cys 38，313 K）。对于Cys 38和Arg 39，这两个受化学交换影响最大的残基，通过对弛豫速率常数的全局分析，在300 K下确定k（ex）值为380 +/- 70 s（-1）和530 +/- 90 s（-1），在313 K下确定k（ex）值为1300 +/- 290 s（-1）和1370 +/- 160 s（-1）。定标参数ex表明，碱性胰蛋白酶抑制剂中大多数残基的化学交换不满足k（ex）/Δ ω远大于1。因此，蛋白质和其他大分子中交换增宽的快限二次标度的假设可能是不正确的，即使观察到核自旋的单个增宽共振。NMR谱中化学交换谱线展宽与静磁场关系的理论结果也适用于其它核和其它测量化学交换谱线展宽的技术。
The static magnetic field dependence of chemical exchange linebroadening in NMR spectroscopy is investigated theoretically and experimentally. Two-site exchange (A reversible arrow B) is considered with site A more highly populated than site B (p(a) > p(b)), a shift difference between sites equal to aw, and an exchange rate constant given by k(ex). The exchange contribution to the transverse relaxation rate constant for the more highly populated site is denoted R-ex. The dependence of R-ex,, on the static magnetic field strength is characterized by a scaling parameter alpha = d In R-ex/d In Delta omega in which 0 less than or equal to alpha less than or equal to 2 for p(a) > 0.7. The value of alpha depends on the NMR chemical shift time scale for the exchange process: for slow exchange (k(ex)/Delta omega < 1), 0 less than or equal to alpha < 1; for intermediate exchange (k(ex)/Delta omega > 1), alpha = 1 and for fast exchange (k(ex)/Delta omega > 1), 1 < alpha less than or equal to 2. Consequently, the static magnetic field dependence of R-ex defines the chemical shift time scale for an exchange process even if the populations are so highly skewed (p(a) much greater than p(b)) that the minor resonance is not observable in the slow exchange limit. The theoretical results are verified by measuring '5N transverse relaxation rate constants at static magnetic fields of 11.7 and 14.1 T and temperatures of 300 and 313 K fur the protein basic pancreatic trypsin inhibitor. At each combination of static magnetic field and temperature, the rate constants were measured using Carr-Purcel-Meiboom-Gill and Hahn echo techniques with spin-echo delays ranging from 1.0 to 63.5 ms. '5N resonances for residues in the region of the Cys14-Cys38 disulfide bond are broadened due to chemical exchange. Values of a obtained from the relaxation rate constants range from 0.26 +/- 0.17 for Arg39 at 300 K to 1.96 +/- 0.25 for Cys38 at 313 K. For Cys38 and Arg39, the two residues most strongly affected by chemical exchange, values of k(ex) were determined to be 380 +/- 70 s(-1) and 530 +/- 90 s(-1) at 300 K and 1300 +/- 290 s(-1) and 1370 +/- 160 s(-1) at 313 K by global analysis of the relaxation rate constants. The scaling parameters ex indicate that chemical exchange for most residues in basic pancreatic trypsin inhibitor does not satisfy k(ex) /Delta omega much greater than 1. Consequently, the assumption of fast-limit quadratic scaling of exchange broadening in proteins and other macromolecules may be incorrect, even if a single broadened resonance is observed for a nuclear spin. The theoretical results for the static magnetic field dependence of chemical exchange broadening in NMR spectroscopy are applicable to other nuclei and to other techniques for measuring chemical exchange Linebroadening.