Internuclear distances, precision determination

In the situation where the interaction is weak, one of the traditional methods that can be applied to obtain structm-al information (internuclear distances) of the bound ligand is the so-called transferred NOE (trNOE) method. Recently, it became possible to use transferred cross-correlated relaxation (trCCR) to directly measure dihedral angles. The combined use of these two techniques significantly improves the precision of the structure determination of ligands weakly bound to macromolecules. 

Electron-diffraction studies have produced many precise structure determinations of small and medium-sized molecules, and have solved many conformational problems (Bartell, 1985). Internuclear distances can be calculated to within a few hundredths, or even thousandths, of an Angstrom unit. However, rotamer proportions are rarely determined to better than 5%, and barrier heights can often only be estimated, and are subject to uncertainties of 25% or more. 

Nonbonded internuclear distances are tabulated in almost all the electron diffraction papers considered, but only those nonbonded distances that have been determined precisely and that are regarded as especially important are fisted in the tables together with bond distances and angles. 

For the evaluation of the electron diffraction gas method as a precision method for internuclear distance determination, two questions have to be answered (1) What is the reproducibility of the method and (2) What does the determined distance mean ... 

---