Volume-specific amino acid analysis: a method for Z_a determination

A general method has been developed for determining the number of molecules per asymmetric unit, Z_a, of a protein crystal. The method, volume-specific amino acid analysis, relies on the direct measurement of the number of protein molecules in a crystal as normalized by the crystal volume. The method is accurate, straightforward to implement and sensitive enough for application to small (0.002 m³) crystals. Although the procedures described here are specific to proteins, extension to nucleic acid crystals should be possible.