Carbon edge XANES spectroscopy of amino acids and peptides

Carbon edge X-ray absorption near-edge structures (XANES) of amino acids and small peptides have been investigated for the first time. Because of the heat sensitivity of the samples, gas phase spectroscopy of evaporated samples is difficult. Therefore they were analyzed in solid form as thin films. Quantitative absorption measurements require the region of the sample which is illuminated to be of uniform thickness. For this reason a special sample preparation technique was used, and the spectra were taken using the small spot size of the Stony Brook scanning transmission X-ray microscope, which is operated at the National Synchrotron Light Source at Brookhaven National Laboratory. Carbon K-edge XANES spectra of six amino acid monomers, four dipeptides and one tripeptide were measured and compared. The results show that the peptide spectra can be explained as a sum of the spectra of the monomers they consist of. This indicates that the peptide bond has only a weak effect on the spectra. If this is true in general, it should be possible to predict XANES spectra of proteins from the spectra of the constituent amino acids. This could be used in protein specific imaging using X-ray microscopes. (C) 1997 Elsevier Science B.V.