Green thoughts in a green shade

Abstract

The author gives an autobiographical sketch of his path to chlorophyll research, and describes some results. The discussion is largely focused on long wavelength forms of chlorophyll and how they might be generated by self-assembly. Dimers or oligomers, (Chl)n, result from coordination interactions between the central magnesium atom of one macrocycle and nucleophilic side chains of another i.e., keto C=O⋯Mg in the case of Chl a. Coordination interactions mediated by a water molecule coordinated to Mg in one macrocycle and to a nucleophilic group in another e.g., Mg⋯O(H)H⋯O=C keto, form aggregates with very different structures and properties; where more than one strong nucleophile or hydrogen bonding group is present in the chlorophyll, e.g., the formyl group in Chl b, the acetyl group of Bchl a, or the hydroxyethyl group of Bchl c, they may also participate in direct coordination interactions with Mg as well as hydrogen bonding to water coordinated to Mg. The magnetic resonance properties of Chl a/water aggregates have provided the basis for the special pair concept for the primary electron donor in photosynthesis. Structural information derived from small angle neutron scattering studies on chlorophyll aggregates is now providing an experimental basis for comprehensive models that integrate antenna and photoreaction center chlorophyll functions.