The use of metalloproteins as standards for X-ray microanalysis of biological material

Thin films of metalloprotein, deposited directly onto carbon-formvar-coated electron microscope grids, provide a potentially useful, but little developed, type of standard for biological transmission X-ray microanalysis. The possibility of using various metalloproteins was investigated in terms of their suitability as standards, and compared with an evaporated film of gelatin-salt mixture. The metalloproteins that were selected comprised conalbumin (containing detectable S, Fe), hemocyanin (S, K, Ca, Fe and Cu), insulin (S, Zn), phosvitin (P, Ca, Fe, and Zn) and alpha-lactalbumin (S, Ca). In each case, the metalloprotein preparation was homogeneous at the microlevel (unlike evaporated gelatin-salt mixtures), and was stable in the electron beam up to a current of 120 nA, over a lifetime of 200 s.