Abstract
This work describes the fluorescence enhancement of the anilinonaphthalene sulfonate probes 1,8-ANS, 2,6-ANS, and 2,6-TNS via complexation with PAMAM dendrimer hosts of Generation 4, 5 and 6. The use of this set of three very closely related probes allows for comparative binding studies, with specific pairs of probes differing only in shape (1,8-ANS and 2,6-ANS), or in the presence of a methyl substituent (2,6-TNS vs. 2,6-ANS). The fluorescence of all three probes was significantly enhanced upon binding with PAMAM dendrimers, however in all cases except one, a very unusual spike was consistently observed in the host fluorescence titration plots (fluorescence enhancement vs. host concentration) at low dendrimer concentration. This unprecedented fluorescence titration curve shape makes fitting the data to a simple model such as 1:1 or 2:1 host: guest complexation very difficult; thus only qualitative comparisons of the relative binding of the three guests could be made based on host titrations. In the case of G4 and G5 dendrimers, the order of binding strength was qualitatively determined to be 1,8-ANS < 2,6-ANS indicating that the more streamlined 2,6-substituted probes are a better match for the dendrimer cavity shape than the bulkier 1,8-substituted probe. This order of binding strength was also indicated by double fluorometric titration experiments, involving both host and guest titrations. Further double fluorometric titration experiments on 2,6-ANS in G4 dendrimer revealed a host concentration-dependent change in the nature of the host: guest complexation, with multiple guests complexed per host molecule at very low host concentrations, but less than one guest per host at higher concentrations.
Highlights
Dendrimers are an interesting class of macromolecules that have received significant attention in recent years due to their highly branched structure and the uniformity of their size and shape [1,2,3,4].They possess high concentrations of functional groups and internal cavities, and their unique structure and size range have a great impact on their physical and chemical properties, as well as their applications [1,2,3,4]
The order of enhancement observed of 2,6-TNS > 1,8-ANS ≥ 2,6-ANS is a reflection of the differing polarity sensitivity of these three guests, as measured in our group using the Polarity
Kb = 5.3 ± 2.7 × 105 M-1 and n = 0.23 ± 0.01. These results indicate that at the [Generation 4 (G4)] concentration at which the spike occurs, the binding of 2,6-ANS is weaker, but many more guests are bound per G4 host, as compared to higher host concentrations
Summary
Dendrimers are an interesting class of macromolecules that have received significant attention in recent years due to their highly branched structure and the uniformity of their size and shape [1,2,3,4]. They used a double fluorometric titration technique, in which the fluorescence is studied both as a function of host concentration at fixed guest concentration and as a function of guest concentration at fixed host concentration [23,24,25,26], and found that binding of 1,8-ANS in these dendrimers is very complex They found that there are two distinctive types of binding centers for G4 and G6 PAMAM dendrimers, which they referred to as low affinity and high affinity, and which correspond to surface and core cavities, respectively [25].
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