FLUORESCENCE PROPERTIES OF CARMINIC ACID IN RELATION TO AGGREGATION, COMPLEX-FORMATION AND OXYGEN ACTIVATION IN AQUEOUS FOOD MODELS

The fluorescence of carminic acid is moderately dependent on pH in aci dic and neutral aqueous solution and has, at pH 2.0 in 1.0 M NaCl at 2 5-degrees-C, a quantum yield 0.00133 +/- 0-00019 and an emission lifet ime of 0.093 +/- 0.010 ns, as determined by the fluorescence phase and modulation method. The fluorescence is quenched by iodide (diffusion controlled reaction), and the quantum yield decreases for increasing c arminic acid concentration, suggesting a molecular stacking in fast eq uilibrium, as further evidenced by pressure quenching of emission. The fluorescence of carminic acid in dilute aqueous solution is increased by aluminium as a result of formation of a 1 : 1 complex with a stabi lity constant of (6.8 +/- 1.8) x 10(3) litres mol-1 (1.0 m NaCl with p H 4, 25-degrees-C), a fluorescence quantum yield of 0-022 +/- 0.004 an d an emission lifetime of 2.1 +/- 0.1 ns. An excited state kinetic ana lysis based on the photophysical characterization shows an electronic deactivation path in acidic solution for carminic acid which does not involve oxygen activation. The fluorescence of carminic acid, declines rapidly in alkaline solution with concomitant increase in photooxidat ion of carminic acid, and a photoreactive triplet state, capable of ox ygen activation, becomes of increasing importance at higher pH.