Carey C, Chirlian L E, Francl M M, Gange D M
Department of Chemistry; Bryn Mawr College, PA 19010-2899, USA.
Glycoconj J. 1997 Jun;14(4):501-5. doi: 10.1023/a:1018511804498.
The partitioning of the overall molecular charge distribution into atom centered monopole charges, while quantum mechanically ill-defined, is nevertheless a technique which finds applications in several broad classes of chemical problems. Charges derived from fits to electrostatic potentials have an intuitive appeal since, in principle, these could be derived from either theoretical or experimental data. It has been noted, however, that such potential derived charges can be conformationally dependent in ways that do not appear to reflect the changes in the molecular wavefunction. Both the algorithm used for selecting points at which the molecular electrostatic potential will be fit and the density of points used in the fit have been suggested to influence the resultant charges. Recently [Stouch TR, Williams DE (1992) J Comp Chem 13: 622-32; Stouch TR, Williams DE (1993) J Comp Chem 14: 858-66] it has been noted that numerical difficulties may make it impossible to fit all the atomic charges in a molecule. Singular value decomposition (SVD) of the linear least squares matrices used in fitting atom based monopoles to molecular electrostatic potentials provides a tool for evaluating the integrity of the calculated charges. Based on the SVD analysis for a selected group of molecules we have noted particularly that increasing the molecular size reduces the fraction of charges which can be validly assigned. Users of PD derived charges, especially those who are using those charges for tasks other than reproduction of the MEP, should be aware that there is a high probability that a significant portion of those charges are statistically unreliable. Therefore, charges in many biological molecules, such as sugars, prove to be difficult to obtain by potential derived (PD) methods such as CHELP or CHELPG. Results from the SVD can be used to both assess PD charges and to generate an improved, albeit incomplete, set. Improved PD fits are presented for a series of simple saccharides.
将整体分子电荷分布划分为以原子为中心的单极电荷，虽然从量子力学角度来说定义不明确，但仍是一种在几大类化学问题中都有应用的技术。从静电势拟合得到的电荷具有直观的吸引力，因为原则上这些电荷可以从理论或实验数据中得出。然而，已经有人指出，这种由电势导出的电荷可能会以一种似乎不能反映分子波函数变化的方式依赖于构象。有人认为，用于选择拟合分子静电势的点的算法以及拟合中使用的点的密度都会影响最终得到的电荷。最近[斯托奇TR，威廉姆斯DE（1992）《计算化学杂志》13：622 - 32；斯托奇TR，威廉姆斯DE（1993）《计算化学杂志》14：858 - 66]有人指出，数值困难可能使得无法拟合分子中的所有原子电荷。用于将基于原子的单极子拟合到分子静电势的线性最小二乘矩阵的奇异值分解（SVD）为评估计算电荷的完整性提供了一种工具。基于对一组选定分子的SVD分析，我们特别注意到，分子尺寸的增加会降低能够有效分配的电荷比例。使用由电势导出的电荷的人，尤其是那些将这些电荷用于除再现分子静电势之外的其他任务的人，应该意识到这些电荷中有很大一部分在统计上可能不可靠。因此，许多生物分子中的电荷，如糖类中的电荷，很难通过诸如CHELP或CHELPG等电势导出（PD）方法获得。SVD的结果可用于评估PD电荷并生成一组改进的（尽管不完整）电荷。本文给出了一系列简单糖类的改进的PD拟合结果。
