Recognition of Caffeine in Aqueous Solutions

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Abstract
Binding of caffeine in aqueous solutions has been achieved for the first time by using water‐soluble, tetracationic peptide–porphyrin conjugates Zn‐1 as the receptor molecules. The association constant for caffeine with receptor Zn‐1 is in some cases as high as 6000 M−1, only 5–6 times lower than the highest binding constant reported for an artificial caffeine receptor in low polarity aprotic solvents. The binding mechanism has been studied by a combination of experimental techniques such as UV‐visible and NMR spectroscopy and microcalorimetry. Recognition of caffeine involves both stacking with the porphyrin ring and metal coordination. Subtle variations of the receptor structure affect the complexation. Receptors Zn‐1 have also been investigated for the recognition of molecules structurally related to caffeine, for example, 1‐methylimidazole. Selectivity towards oxopurine derivatives (caffeine and theophylline) have been found.

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