Journal of Physical Chemistry B, Vol.114, No.8, 2798-2806, 2010
Natural Cyclodextrins as Efficient Boosters of the Chemiluminescence of Luminol and Isoluminol: Exploration of Potential Applications
The chemiluminescent oxidation of luminol (LUM) and isoluminol (ISOL) is notably enhanced, both in intensity and duration, in the presence of natural cyclodextrins (alpha-, beta-, gamma-CD). The experiments have considered some of the most widespread applications of these Compounds: the determination of metal cations and the revealing of bloodstains by oxidation with hydrogen peroxide in alkaline solution in the presence of Co(II), Fe(III), human hemoglobin, and blood, in order to explore potential applications. The largest enhancement in the emitted intensity Occurs for the reaction of LUM with Co(II) in the presence of beta-CD. The use of the more soluble gamma-CD permits to expand the range of concentration and obtain more intense emission, although soluble derivatives of the beta-CD (methyl, hydroxypropyl-beta-CD, and a soluble cross-linked epichlorhydrin polymer) do not improve the chemiluminescence (CL) yield. In the case of hemoglobin and diluted human blood, the CDs aid in producing more light but Only at high concentration of CDs, with a more lasting luminescence, up to three times longer. The changes in CL when glucose is used instead, much lower than with any of the CDs, imply that the cyclic structure of these oligosaccharides plays a key factor in the boosting of the emission. The results are explained in terms of the binding between the luminescent intermediate of the reaction, 3-aminophthalate (3-AP) and the CD, rather than to the luminescent reactant itself. The association constants obtained by steady-state fluorescence by assuming 1:1 stoichiometries reveal that the most stable association Occurs between beta-CD and the intermediate, in accordance with the trend in the chemiluminescence. The topology of the complex deduced via ROESY experiments confirms a shallow inclusion of the double-charged intermediate by the primary rim of the CD, which accounts for the low stability of the complexes.