Magnesium K-Edge NEXAFS Spectroscopy of Chlorophyll α in Solution

The interaction of the central magnesium atom of chlorophyll a (Chl alpha) with the carbon and nitrogen backbone was investigated by magnesium K near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in fluorescence detection mode. A crude extract of Chl alpha was measured as a 1 X 10(-2) mol/L ethanol solution (which represents an upper limit of concentration without aggregation) and as dried droplets. For the first time, the investigation of Mg bound to Chl alpha in a liquid environment by means of X-ray absorption spectroscopy is demonstrated. A pre-edge feature in the dissolved as well as in dried Chl alpha NEXFAS spectra has been identified as a characteristic transition originating from Mg in the Chl alpha molecule. This result is confirmed by theoretical DFT calculations leading to molecular orbitals (MO) which are mainly situated on the magnesium atom and nitrogen and carbon atoms from the pyrrole rings. The description is the first referring to the MO distribution with respect to the central Mg ion of Chl alpha and the surrounding atoms. On this basis, new approaches for the investigations of dynamic processes of molecules in solution and structure function relationships of photosynthetic pigments and pigment protein complexes in their native environment can be developed.