NMR Ensembles of Models

When a macromolecular structure is determined by nuclear magnetic resonance (NMR) in solution, the result is an ensemble of multiple molecular models, each of which is consistent with the experimental data. This is in contrast to the result of an X-ray crystallographic experiment, which is a single model that best fits the empirical electron density. (In some cases where the resolution is very high, the model may include alternative positions for some atoms.)

The number of NMR models published depends upon the experiment and is up to the authors, and varies between 2 and over 100. The first model in the ensemble has no special significance.

The most representative model is the model closest to the average model. A server called Olderado reports the most representative model.

It is common to average the models from an NMR experiment, but in order for the result to be realistic, it must undergo some energy minimization in order to adjust covalent bond lengths and angles. The result is called a minimized average model.

Solution NMR is unable to determine atomic resolution protein structures for molecules in excess of about 30,000 Daltons. In fact, the median mass of NMR structures published in the Protein Data Bank is about 9 kD.