CONCENTRATION QUENCHING OF FLUORESCENCE FROM CHLOROPHYLL‐a, PHEOPHYTIN‐a, PYROPHEOPHYTIN‐a AND THEIR COVALENTLY‐LINKED PAIRS*

We have systematically investigated the quenching of pigment fluorescence in pyridine solutions of Chlorophyll‐ a , Pheophytin‐ a , Pyropheophytin‐ a , and their covalently‐linked pairs. In all cases, the fluorescence decay curves could be fit within experimental error to a single exponential. Intermolecular quenching constants were computed by fitting the measured decay rate constant vs concentration curves to a power series in the concentration of the various species. Analysis of the shapes of these quenching curves provided information about the operant quenching mechanisms. For Chlorophyll‐ a and Pheophytin‐ a , the observed decay kinetics are consistent with a quenching mechanism in which singlet excitation is transferred from fluorescent monomers to relatively non‐fluorescent dimers. For Pyropheophytin‐ a , on the other hand, the decay kinetics are consistent with a mechanism involving the collision between an excited monomer and a ground state monomer. For all three covalently‐linked pairs, the decay kinetics are consistent with a mechanism in which singlet excitation is transferred from fluorescent ‘open’ forms to relatively non‐fluorescent ‘closed’ forms. Difference absorption spectroscopy was used to obtain direct evidence for the formation of ground state dimers at higher pigment concentrations. Equilibrium constants for dimer formation computed from the kinetic data and difference spectral data were in good agreement, 3.2 M ‐1 and 1.4 M ‐1 , respectively.