Novel β-cyclodextrin-eosin conjugates

Abstract

Eosin B (EoB) and eosin Y (EoY), two xanthene dye derivatives with photosensitizing ability were prepared in high purity through an improved synthetic route. The dyes were grafted to a 6-monoamino-β-cyclodextrin scaffold under mild reaction conditions through a stable amide linkage using the coupling agent 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride. The molecular conjugates, well soluble in aqueous medium, were extensively characterized by 1D and 2D NMR spectroscopy and mass spectrometry. Preliminary spectroscopic investigations showed that the β-cyclodextrin-EoY conjugate retains both the fluorescence properties and the capability to photogenerate singlet oxygen of the unbound chromophore. In contrast, the corresponding β-cyclodextrin-EoB conjugate did not show either relevant emission or photosensitizing activity probably due to aggregation in aqueous medium, which precludes any response to light excitation.

Keywords: fluorescence; photodynamic therapy; photosensitizers; singlet oxygen; xanthene; β-cyclodextrins.

Figure 1

Reaction scheme for the synthesis of eosin Y (2) and eosin B (4).

Figure 2

Reaction scheme for the synthesis of eosin-appended β-CDs, 2–β-CD and 4–β-CD (NMM: N-methylmorpholine).

Figure 3

TLC analysis of the composition of the crude coupling reaction mixtures.

Figure 4

¹H NMR spectrum of 2–β-CD with partial assignment (DMSO-d₆, 600 MHz, 298 K).

Figure 5

Size distributions of 1 mM aqueous solutions of conjugates 4–β-CD (a) and 2–β-CD (b) at 25.0 °C (pH 7) by intensity (three parallel measurements: blue, green and red lines).

Figure 6

Normalized absorption spectra of aqueous solutions of (a) eosin Y (2) and (b) conjugate 2–β-CD and fluorescence emission spectra of aqueous solutions of (c) 2 and (d) 2–β-CD. λ_exc = 490 nm. The fluorescence spectra were recorded with the two samples having the same absorbance at the excitation wavelength.

Figure 7

Time-resolved fluorescence observed for aqueous solutions of (a) eosin Y (2) and (b) the 2–β-CD conjugate. λ_exc = 455 nm; λ_em = 570 nm.

Figure 8

¹O₂ luminescence detected upon 528 nm light excitation of D₂O solutions of (a) eosin Y (2) and (b) 2–β-CD conjugate having the same absorbance at the excitation wavelength.