Hello Paul, you can't "calculate" the number of molecules in the AU from the spacegroup and cell dimensions, although you can measure the density of the crystal and then calculate the number of macromolecules using the expected protein and solvent densities and macromolecular MW - this is not commonly done anymore.
What you can do is "estimate" the number, using the Matthews coefficient formula and what you know about the macromolecule (MW, probable oligomerisation state) and what is expected for protein/DNA/RNA crystals, see for instance http://www.ruppweb.org/Mattprob/ and references therein. There are uncertainties: your crystal may have unusually high solvent or macromolecule content (therefore it is an "estimation"), but also the macromolecule in the crystal could degraded, or it could be a contaminant (check if your cell dimensions / spacegroup already occur in the pdb and if that protein could come from your expression host). Greetings, Mark J van Raaij Laboratorio M-4 Dpto de Estructura de Macromoleculas Centro Nacional de Biotecnologia - CSIC c/Darwin 3, Campus Cantoblanco E-28049 Madrid, Spain tel. (+34) 91 585 4616 http://www.cnb.csic.es/content/research/macromolecular/mvraaij On 6 Jun 2011, at 12:50, Paul Lindblom wrote: > Hello everybody, > > I have a very basic question concerning crystallography, but I don´t have > anybody in our lab who could explain it to me: > > How can I calculate the numbers of molecules in the AU? > > I have two spacegroups: > > 1) P212121 with cell dimensions a=50, b=55, c=162 > > 2)P21 with cell dimensions a= 94, b=100, c= 95 > > Can anybody help me an explain the calculation in a way I understand? > > Thanks a lot! > > Reagrds, > > Paul
