Fluorescence properties of carminic acid in relation to aggregation, complex formation and oxygen activation in aqueous food models
1993
Stapelfeldt, H. | Jun, H. | Skibsted, L.H.
The fluorescence of carminic acid is moderately dependent on pH in acidic and neutral aqueous solution and has, at pH 2.0 in 1.0 m NaCl at 25 degrees C, a quantum yield 0.00133 +/- 0.00019 and an emission lifetime 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 carminic acid concentration, suggesting a molecular stacking in fast equilibrium, 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 stability constant of (6.8 +/- 1.8) X 10(3) litres mol-1 (1.0 M NaCl with pH 4, 25 degrees C), a fluorescence quantum yield of 0.022 +/- 0.004 and an emission lifetime of 2.1 +/- 0.1 ns. An excited state kinetic analysis 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 photooxidation of carminic acid, and a photoreactive triplet state, capable of oxygen activation, becomes of increasing importance at higher pH.
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