Myint Wazo, Ishima Rieko
Department of Structural Biology, University of Pittsburgh School of Medicine, 3501 Fifth Avenue, Pittsburgh, PA 15260, USA.
J Biomol NMR. 2009 Sep;45(1-2):207-16. doi: 10.1007/s10858-009-9344-9. Epub 2009 Jul 19.
In the analysis of the constant-time Carr-Purcell-Meiboom-Gill (CT-CPMG) relaxation dispersion experiment, chemical exchange parameters, such as rate of exchange and population of the exchanging species, are typically optimized using equations that predict experimental relaxation rates recorded as a function of effective field strength. In this process, the effect of chemical exchange during the CPMG pulses is typically assumed to be the same as during the free-precession. This approximation may introduce systematic errors into the analysis of data because the number of CPMG pulses is incremented during the constant-time relaxation period, and the total pulse duration therefore varies as a function of the effective field strength. In order to estimate the size of such errors, we simulate the time-dependence of magnetization during the entire constant time period, explicitly taking into account the effect of the CPMG pulses on the spin relaxation rate. We show that in general the difference in the relaxation dispersion profile calculated using a practical pulse width from that calculated using an extremely short pulse width is small, but under certain circumstances can exceed 1 s(-1). The difference increases significantly when CPMG pulses are miscalibrated.
在对恒时 Carr-Purcell-Meiboom-Gill（CT-CPMG）弛豫色散实验的分析中，化学交换参数，如交换速率和交换物种的丰度，通常使用预测作为有效场强函数记录的实验弛豫速率的方程进行优化。在此过程中，通常假设 CPMG 脉冲期间的化学交换效应与自由进动期间相同。这种近似可能会给数据分析引入系统误差，因为在恒时弛豫期内 CPMG 脉冲的数量会增加，因此总脉冲持续时间会随有效场强而变化。为了估计此类误差的大小，我们模拟了整个恒时期间磁化强度的时间依赖性，并明确考虑了 CPMG 脉冲对自旋弛豫速率的影响。我们表明，一般来说，使用实际脉冲宽度计算的弛豫色散曲线与使用极短脉冲宽度计算的弛豫色散曲线之间的差异很小，但在某些情况下可能超过 1 s⁻¹。当 CPMG 脉冲校准错误时，差异会显著增加。
