Near Edge X-ray Absorption Fine Structure (NEXAFS) Spectroscopy of Molecules of Biological Interest: From Chemically Pure to Complex Samples

Abstract

In Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy, molecular inner-shell (core) electrons are excited to partially filled and empty states. Excitation of core electrons leads to a complex manifold of decay processes involving both radiative (fluorescence emission) and non-radiative processes. Due to the localized nature of core orbitals, features observed in NEXAFS have a definite atomic or element-specific character, allowing its application to the study of a great variety of samples and systems. In this chapter, the fundamentals of this technique are briefly presented and its application to the study of compounds of increasing complexity is discussed. Starting with gas phase sulfur-containing compounds of biological importance such as dimethyl disulfide (DMDS), we proceed to discuss the NEXAFS spectra of a few amino acids and a protein, insulin, obtained at selected core edges. Finally, the use of NEXAFS in the chemical speciation of complex samples is discussed and exemplified with the speciation of sulfur in samples obtained from the Amazonian plant Andira surinamensis.