Increasing the size of microcrystals by fine sampling of pH limits

A persistent problem in macromolecular crystallization is that of the promotion of easily and rapidly grown microcrystals to a size and quality sufficient for X-ray diffraction analysis. This is often true even for highly purified proteins, viruses and nucleic acids where recrystallization shows no benefit. It has been found that in some cases larger and better-formed crystals can be grown by graduated finely sampled screens of pH at high values in the neighborhood of 8.5, and at low values around 4.0. This was shown to be quite crucial for the crystallization of several proteins. It is suggested that this approach may owe its success to the precise manipulation of the protonation state of only a few critical carboxyl and basic side chains essential in the formation of intermolecular salt bridges.