LeMaster David M, Anderson Janet S, Hernández Griselda
Wadsworth Center, New York State Department of Health and Department of Biomedical Sciences, School of Public Health, University at Albany-SUNY, Empire State Plaza, Albany, New York 12201 USA.
Biophys Chem. 2007 Aug;129(1):43-8. doi: 10.1016/j.bpc.2007.05.004. Epub 2007 May 18.
Experimental pK values of ionizable sidechains provide the most direct test for models representing dielectric shielding within the interior of a protein. However, only the strongly shifted pK values are particularly useful for discriminating among models. NMR titration studies have usually found only one or two such shifted pK values in each protein, so that the fitting of the experimental data to a uniform internal dielectric (epsilon(int)) model is not well constrained. The observed variation among proteins for such epsilon(int) estimates may reflect nonuniformity of dielectric shielding within each protein interior or qualitative differences between individual proteins. The differential amide kinetic acidities for a series of metal-substituted rubredoxins are shown to be consistent with Poisson-Boltzmann predictions of dielectric shielding that is relatively uniform for all of the amides that are sensitive to the metal charge, a region which corresponds to roughly 1/3 of the internal volume. The effective epsilon(int) values near 6 that are found in this study are significantly lower than many such estimates derived from sidechain pK measurements. The differing timeframes in which dielectric relaxation can respond to the highly transient peptide anion as compared to the longer lived states of the charged sidechains offers an explanation for the lower apparent dielectric constant deduced from these measurements.
可电离侧链的实验pK值为蛋白质内部介电屏蔽模型提供了最直接的检验。然而，只有显著偏移的pK值对区分不同模型特别有用。核磁共振滴定研究通常在每种蛋白质中仅发现一两个这样偏移的pK值，因此将实验数据拟合到均匀内部介电常数（ε(int)）模型的约束并不充分。在这些ε(int)估计值中观察到的不同蛋白质之间的差异，可能反映了每种蛋白质内部介电屏蔽的不均匀性，或者不同蛋白质之间的质的差异。一系列金属取代的红素氧还蛋白的酰胺动力学酸度差异表明，其与泊松-玻尔兹曼介电屏蔽预测一致，对于所有对金属电荷敏感的酰胺来说，介电屏蔽相对均匀，该区域约占内部体积的1/3。本研究中发现的接近6的有效ε(int)值显著低于许多从侧链pK测量得出的此类估计值。与带电侧链寿命较长的状态相比，介电弛豫对高度瞬态的肽阴离子作出响应的时间尺度不同，这为从这些测量中推导出较低的表观介电常数提供了解释。
