Determination of the protein content of crystals formed by Mastigocladus laminosus C-phycocyanin, Chroomonas spec. phycocyanin-645 and modified human fibrinogen using an improved Ficoll density gradient method.

We demonstrated for several protein crystals of known protein content that the simple Ficoll density gradient method for crystal density determination as described by Westbrook (1976) often leads to quite erroneous results. In particular, the apparent density of loosely packed crystals can show a tremendous change within the first minutes of measurement. In order to derive the correct protein content the apparent crystal density must be followed as a function of time and has to be extrapolated back to the time of insertion of the crystal into the gradient. The packing densities of four novel protein crystals, formed by Mastigocladus laminosus C-phycocyanin, Chroomonas spec. phycocyanin-645 (two forms), and modified human fibrinogen have been determined and that of proteinase II of Crotalus adamanteus has been corrected. The C-phycocyanin crystals were found to contain (in contrast to earlier results reported by others) only one (alpha beta)-monomer, the phycocyanin-645 crystals two and three (alpha alpha' beta 2)-monomers, respectively, and the fibrinogen crystals one fibrinogen molecule per asymmetric unit.