Fluorescence quenching of chlorophyll-a in non-ionic micelles. Effect of micellar and quencher concentrations on the static and dynamic quenching parameters

Abstract

Fluorescence quenching of chlorophyll-a by two electron acceptors, tetrazolium blue, TB²⁺, (system I) and anthraquinone-2-sulfonate, AQS^–, (system II), in a non-ionic Triton X-100 micellar medium, pH = 7.00, was studied using steady-state and time-resolved fluorescence quenching data, with nanosecond and picosecond resolution. A combined analysis of the steady-state and time-resolved data (ns regime) showed that the excited state interaction via electron transfer is essentially static, with a small diffusion contribution, higher in system II than in system I, in agreement with the lower diffusion coefficient of TB²⁺ as compared with that of AQS^–.

Transient effects play an important role at short times (up to a few ns) in the more concentrated quencher solutions of both systems. An electron tunnelling model (including a diffusion term for long observation times) was fitted to the time-resolved data obtained with ps resolution, and enabled the evaluation of effective concentrations and effective reactional distances when transient effects are operating. In order to obtain information on the micro-heterogeneity of the systems, a spectral–fractal model was also fitted to the time-resolved data, which enabled the evaluation of the spectral dimensions of the systems. The parameters obtained are discussed in terms of the physical meaning of the kinetic formalisms used.