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Fluorescence Quenching and Copper Complexation by a Chestnut Leaf Litter Extract: Spectroscopic Evidence

Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92621

Nicola Senesi

Inst. di Chimica Agraria, Univ. di Bari, Bari, Italy

*Corresponding author.

ABSTRACT

Fluorescence quenching on addition of Cu(II) to aqueous solutions of chestnut (Castanea sativa L.) leaf litter extract (LLE) has been interpreted previously in terms of Cu complexation by LLE organic ligands. In order to provide direct evidence for the existence of these complexes, as well as information about their coordination structures, infrared (IR) and electron spin resonance (ESR) spectroscopy were applied to investigate Cu/LLE mixtures that showed increasing fluorescence quenching with increasing pH. The Cu/LLE mixtures were prepared with approximately equimolar concentrations of Cu and LLE binding sites (200 mmol m^–3) at pH values in the range 4 to 7. The IR spectra indicated the existence of Cu complexes involving carboxylate groups, even at pH 4. The more sensitive ESR spectra showed that Cu(II) could displace Fe(III) and Mn(II) from LLE binding sites, and that inner-sphere complexes between Cu and LLE ligands were formed. These complexes involved carboxylate ions and H₂O molecules at pH < 6, and both O- and N-containing ligands at pH 6 and 7. These results confirmed the hypothesis that fluorescence quenching is associated with strong Cu complexation by organic ligands in the LLE.

Contribution from the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside. and the Inst. di Chimica Agraria, Bari.

Received for publication August 24, 1987.

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