Leaving no element of doubt: analysis of proteins using microPIXE.

Introduction The identification of atoms, anions and cations in protein structure electron density is often inferred from the temperature factor of the atom and from its coordination distances to surrounding amino acids. In many cases unambiguous identification is not possible without the use of additional techniques. Mass spectrometry analysis methods [1] have advanced significantly over the past few years and precise total mass measurements are now achievable. There is often ambiguity in the identification of bound atoms, however, because extra mass can arise from a combination of sources; for example, from covalent modification such as that of cysteine residues or glycosylation, and from partially occupied metal atom sites as opposed to totally occupied lighter atom sites.