Surface Recognition and Fluorescence Sensing of Histone by Dansyl-Appended Cyclophane-Based Resorcinarene Trimer

A cyclophane-based resorcinarene trimer (3) bearing a dansyl moiety as an environmentally sensitive fluorophore was prepared by stepwise condensation of a tetraaza[6.1.6.1]paracyclophane skeleton with a dansyl moiety and three resorcinarene derivatives having heptacarboxylic acid residues in this sequence. The dansyl-appended cyclophane exhibited the following fluorescence properties regarding solvent polarity dependency and histone surface recognition:  With increasing dioxane contents in dioxane/water solvents, the fluorescence intensity originating from the dansyl moiety of 3 increased along with a concomitant blue shift of the fluorescence maximum (λ_em). The microenvironmentally sensitive fluorescence properties of dansyl fluorophore were maintained, even when the dansyl moiety was covalently attached to a cyclophane. Most interestingly, the cyclophane-based resorcinarene trimer exhibited recognition and fluorescence sensing capabilities toward histone, a small basic protein of eukaryotic chromatins. The fluorescence intensity originating from 3 increased along with a concomitant blue shift of λ_em upon the addition of histone, reflecting the formation of 3−histone complexes. A relatively large fluorescence polarization (P) value was obtained for the 3−histone complexes (0.15), reflecting highly restricted conformations of 3, and the obtained P value was much larger than that of 3 alone in aqueous medium (0.07). The binding constant (K) of 3 with histone (unit basis) was estimated to be 2.1 × 10⁶ M^-1. On the other hand, upon the addition of acetylated histone (Ac-histone) to an aqueous solution containing 3, the extent of change in fluorescence intensity originating from the dansyl group of 3 was almost negligible, indicating that the electrostatic interactions between 3 and Ac-histone were weak. In addition, the fluorescence spectral changes were also small or negligible upon the addition of other proteins such as albumin, ovalbumin, peanut agglutinin, myoglobin, concanavalin A, cytochrome c, and lysozyme, having isoelectric points of 4.7, 4.8, 5.7−6.7, 6.8, 7.1, 9, and 11.0, respectively, to an aqueous solution containing 3.