Spectroscopical Uv-Vis Implications Of An Intramolecular Eta(2)-Mg Coordination In Bacteriochlorophyll-Alpha From The Fenna-Matthews-Olson Complex

Chlorophylls and their related compounds prominently feature a Mg2+ ion in the center of a porphyrine, with an intermolecular fifth coordination usually observed to place the ion out of the macrocyclic plane. Herein. we assess the role of a potential intramolecular eta(2)-(C = C)Mg interaction and compare it to the intermolecular coordination from the Hystidine groupt to Mg2+ for Bacterichlorophyll-alpha (Bchl-alpha), the main photosynthetic pigment in the Fenna- Matthews-Olson complex present in green and purple bacteria. The influence of this fifth coordination on the UV-Vis spectroscopy (CAM-B3LYP/cc-pVDZ), and the concomitant change in geometry around Mg in Bchl-alpha from planar to pyramidal is assessed by the quantum theory of atoms in molecules based non-covalent interactions scheme and through energetic analysis via natural bond orbital population methods at the M06-2X/cc-pVDZ and compared to the reference multi-hapto compound, magnesocene. Cp2Mg.