The interaction of the central magnesium atom of chlorophyll a (Chl a) 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 a was measured as a 1 × 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 a 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 a NEXFAS spectra has been identified as a characteristic transition originating from Mg in the Chl a 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 a 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.

Charles University Faculty of Mathematics and Physics Department of Chemical Physics and Optics 121 16 Prague Czech Republic

Humboldt Universität zu Berlin School of Analytical Sciences Adlershof 10099 Berlin Germany

Max Born Institut für Nichtlineare Optik und Kurzzeitspektroskopie Berlin 12489 Berlin Germany

Physikalisch Technische Bundesanstalt 10587 Berlin Germany

Southern Federal University International Research Center Smart materials Rostov na Donu 344090 Russia

Technische Universität Berlin Institut für Optik und Atomare Physik 10623 Berlin Germany

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