Cyclodextrin enhanced fluorimetric method for the determination of tryptamine

Abstract

The effect of native cyclodextrins (α-, β- or γ- with six, seven or eight glucose units, respectively), hydroxypropyl-β-cyclodextrin, β-cyclodextrin solubilized in urea, soluble starch and glucose in water solution on the fluorescence behaviour of tryptamine [3-(2-aminoethyl)indole] (T) was determined. In addition, the effect of methanol and propanol with and without β-cyclodextrin or hydroxypropyl-β-cyclodextrin was ascertained. From the fluorescence changes with pH and with β-cyclodextrin or hydroxypropyl-β-cyclodextrin, the values of the pK_a of the ground and excited states and the association constants of T and TH [3-(2-ammoniumethyl)indole] with the two hosts were determined. The values are pK_a = 9.5 ± 0.2 and pK_a^* = 8.4 ± 0.2; K_Assoc^TH = (1.6 ± 0.3) × 10² mol^–1 dm³ and K_Assoc^T = (2.8 ± 0.3) × 10² mol^–1 dm³ with β-cyclodextrin, K_Assoc^TH = (1.8 ± 0.5) × 10² mol^–1 dm³ and K_Assoc^T = (4.9 ± 0.9) × 10² mol^–1 dm³ with hydroxypropyl-β-cyclodextrin. The ratio of the fluorescence quantum yields for the bound and free substrate (Φ^b/Φ^f) were in the range 1.25–1.33. The detection limit for the better conditions where the host–guest interactions produce fluorescence enhancement was 0.454 ± 0.002 ng ml^–1 for the complex T–hydroxypropyl-β-cyclodextrin in water. The method is simpler than others reported previously.