X-ray absorption spectroscopy of selenium-containing amino acids

The selenium K-edge X-ray absorption spectra of selenomethionine, selenocys teine, selenocystine, and sulfo-selenocystine in solution are compared with the corresponding sulfur K-edge spectra of the sulfur analogues of these c ompounds. The selenium and sulfur spectra follow similar trends, although t he latter are significantly sharper owing to the longer core hole lifetime at the lower energies where sulfur absorbs. The spectra of the selenium com pounds are sufficiently distinct that it is reasonable to expect that curve fitting will allow the speciation of the forms of selenium in complex biol ogical samples.