Ion Beam Analysis of Metals in Proteins
Geoffrey W. Grime
Analysis of metals in proteins using PIXE

    ☐   Absolute measurement of metal concentration is not easy
    ☐   Metal concentrations are low
    ☐   Proteins must be dissolved in buffer solutions so amount of protein may not be well known

The scale of the problem

Elspeth’s insight in 1993: 

     Most proteins have known numbers of the amino acids methionine and cysteine in            their sequence with each contain one sulphur atom.                                                                                 Use the known number of S atoms as an internal standard!

c is measured concentration
A is atomic weight
N is the number of detected metal atoms of interest per molecule
Z, S refer to the metal atom of interest and sulphur

☐   Using ratios eliminates many sources of error affecting absolute measurement.
☐   Other elements may be used as the reference (e.g. P in nucleic acids, Se following               seleno-methionine substitution.)

Methionine molecule