Soft X-ray excited optical luminescence (XEOL) studies of fluorescein isothiocyanate (FITC) and FITC-labeled proteins

The electronic and optical properties of fluorescein-isothiocyanate (FITC) and FITC labeled proteins, Concanavalin A lectin and goat anti-rabbit immunoglobulin G have been investigated by X-ray excited optical luminescence (XEOL) using tunable soft X-rays from a synchrotron light source, together with X-ray absorption find structures (XAFS) spectroscopy at the C, N, and O K-edges. It is found that the carbon-localized excitation at the C K-edge is coupled to the luminescence chromophore effectively and excitations at the nitrogen and oxygen K-edge are not. The implication is that XEOL is excitation channel specific and that it can be used as a powerful tool in studying the optical properties of biomolecules.