Interactions between protein and porphyrin-containing cyclodextrin supramolecular system: a fluorescent sensing approach for albumin

The interactions of meso-tetraphenylporphyrin (TPP), meso-tetraphenylporphyrin cobalt(II) (CoTPP) and protein in the presence of a cyclodextrin derivative, heptakis(2,6-di-O-n-octyl)-β-cyclodextrin (Oc-β-CD), have been investigated. In the presence of Oc-β-CD, significant increase of TPP fluorescence was realized, but the increased fluorescence was quenched by CoTPP. To further investigate the fluorescence-quenched system and explore its potential application in bioanalysis, a strategy has been devised to restore the quenching fluorescence of TPP upon interacting with protein. The restoration of TPP fluorescence in the present system is fast and accomplished upon interaction with bovine serum albumin (BSA) or human serum albumin (HSA). On the basis of the spectroscopic measurement and excited state fluorescence lifetime, the mechanism of TPP fluorescence quenching is attributed to formation of a ground-state complex of TPP and CoTPP, and the fluorescence restoration is attributed to the binding of CoTPP with the protein molecule which destroys the aggregate, releasing the free base porphyrin. With optimized conditions, the calibration equations are linear from 0.80 to 75.4 μg mL−1 BSA and from 3.20 to 93.2 μg mL−1 HSA. The corresponding detection limits are 0.32 μg mL−1 for BSA and 1.06 μg mL−1 for HSA, respectively. The method was used for the direct assay of HSA content in human serum. The result is comparable to that obtained by another method. The recovery from BSA in synthetic sample is also satisfactory.