How to find the number of macromolecules in the asymmetric unit?
A good method for determining the number of macromolecules in the asymmetric unit is by calculating the Matthew’s coefficient. This can be done easily using a jiffy program available in the CCP4i suite of programs and is aptly called MATTHEWS_COEF.
You need the following information to be able to do this calculation:
1) unit cell parameters
2) space group
3) an estimate of the molecular weight of your macromolecule
What to look for? As a general rule about 50 percent of the volume of the unit cell is composed of solvent. Look for the number of macromolecules in the asymmetric unit that comprises about 50 percent volume.
For example:
unit cell: a=b=c 80
space group: P 2 3
molecular weight: 20 kDa
This would yield 42 percent solvent for 1 molecule in the asymmetric unit.
This is a picture of the gui from CCP4i (Program List->matthews_coef) displaying the appropriate inputs and outs.
Check out this site if you want to play around with this calculation without using real data or CCP4.
In addition, the above site also compares your calculation against what has been published in the protein data base.
The original paper written by B. W. Matthews, describing this calculation can be found here.
