The effect of axial Mg2+ ligation and peripheral hydrogen bonding on chlorophyll a

Model systems of the chlorophyll a (Chla) molecule were studied using density functional theory. Dependence of structure variation of Chla on the polarity of ligand was described. The hydrogen bonding (H-bond) donated to the 131-keto carbonyl oxygen favors the ionic resonance structure of the 131-keto carbonyl bond. Such a H-bond causes spin density redistribution in anion, and charge transfer in cation from donor of H-bond to the porphyrin plane of Chla because of orbital admixing, which indicates that the spin density of chlorophyll cation could be easily regulated by its peripheral H-bond.