Quenching of enzyme-generated acetone phosphorescence by indole compounds: stereospecific effects of D- and L-tryptophan. Photochemical-like effects.

Indole compounds efficiently quench the acetone phosphorescence observed during the horseradish peroxidase catalyzed aerobic oxidation of isobutyraldehyde. Different types of Stern-Volmer plots are observed: linear, linear with two slopes, and downward- and upward-curved plots. The complexity probably stems from the operation of both dynamic and static quenching, coupled with different efficiencies of quenching of various acetone triplet populations. Binding to the enzyme may also occur, especially in the case of L-tryptophan. The different Stern-Volmer behavior of D- and L-tryptophan fully confirms that the acetone triplet is generated within the enzyme and not free in solution. This chiral discrimination toward an enzymically generated electronically excited species is novel. It is tentatively postulated that a long-range triplet-triplet exciton transfer occurs; the excited triplet indole then undergoes photochemical-like alterations.