Near‐edge X‐ray absorption and dichroism in amino acids

Nitrogen-containing organic compounds form a special group of compounds that have become of interest for spectroscopic research owing to their potential technical use and biological importance. Near edge X-ray absorption (NEXAFS) and circular dichroism (XCD) spectroscopies constitute two spectroscopies with capability of giving information on composition and structure of such sampies. In the present work we address a particular set of organic compounds with special relevance for biochemistry, namely the amino acids, and explore in what way NEXAFS and XCD spectra can fingerprint such compounds. We briefly review and discuss recent theoretical results on spectra at the K edges of C, N and O, that were presented in a series of papers (Plashkevych et al., 1998; Carravetta et al., 1998) These studies were also motivated by the observations (Kirtley et al., 1992) that the nitrogen edge NEXAFS spectrum of DNA can be understood as a weighted sum of the polynucleotide spectra, and by the suggestion that differences in the NEXAFS spectra might be used for mapping proteins which differ in their amino acid content (Boese et al., 1997). Except for glycine, all the amino acids investigated alanine, cysteine, serine, valine, glycine, phenylalanine, histidine, tyrosine and tryptophan exist in two optically active forms, namely as Dand Lisomers that are mirror images of one another. The rotatory X-ray absorption intensities have been computed and XCD spectra for this class of molecules are predicted (Plashkevych et al., to be published). The direct atomic orbital static exchange approach STEX is employed. It is based on a separate channel description of the absorption process, which allows an easy analysis of the spectra in terms of contributions from different molecular subunits. The STEX technique is implemented for fully relaxed core hole potentials of the different sites, and for using the double basis set algorithm, allowing for close to basis set limit results.