Max-Planck-Institut für biophysikalische Chemie, 37070 Göttingen, Germany.
J Magn Reson. 2014 Jan;238:77-81. doi: 10.1016/j.jmr.2013.11.001. Epub 2013 Nov 16.
Methods for determining inter-spin distances between nitroxide spin labels from dipolar couplings in the intermediate range (r12≈1.1-2 nm) by CW-EPR are addressed. For nitroxide powder patterns, the assumption of unlike spins is a better approximation than assuming strong coupling between like spins. Methods that determine the average splitting in dipolar deconvolutions yield the mean value 〈1/r12(3)〉, and those that determine the dipolar contribution to the spectral second moment correspondingly yield 〈1/r12(6)〉. To relate these ensemble averages to the mean inter-spin distance 〈r12〉 requires knowledge of the distribution in r12. Values of the inverse roots reff,n=〈1/r12(n)〉(-1/n) always lie below 〈r12〉. Consistent comparisons of literature data on double-labelled α-helical peptides support this view. Evaluations of reff,n for Gaussian distributions in r12 yield calibrations to determine 〈r12〉 for a given distribution width, σ(r12).
本文介绍了通过连续波 EPR 技术，从中间范围（r12≈1.1-2nm）的偶极耦合来确定氮氧自由基自旋标记物之间的自旋间距（r12）的方法。对于氮氧自由基粉末图谱，假设不同自旋比假设相似自旋之间存在强耦合更为近似。通过解卷积确定平均分裂的方法得到平均分裂值〈1/r12(3)〉，而通过确定谱二阶矩的偶极贡献相应地得到〈1/r12(6)〉。要将这些整体平均值与平均自旋间距〈r12〉相关联，需要知道 r12 的分布情况。倒数根 reff，n=〈1/r12(n)〉(-1/n)的值始终低于〈r12〉。对双标记α-螺旋肽文献数据的一致性比较支持了这一观点。对 r12 中高斯分布的 reff，n 的评估可以校准确定给定分布宽度 σ(r12)下的〈r12〉。
