Abstract
A macrocyclic tetralactam host is threaded by a highly fluorescent squaraine dye that is flanked by two polyethylene glycol (PEG) chains with nanomolar dissociation constants in water. Furthermore, the rates of bimolecular association are very fast with k(on) ≈ 10(6)-10(7) M(-1) s(-1). The association is effective under cell culture conditions and produces large changes in dye optical properties including turn-on near-infrared fluorescence that can be imaged using cell microscopy. Association constants in water are ∼1000 times higher than those in organic solvents and strongly enthalpically favored at 27 °C. The threading rate is hardly affected by the length of the PEG chains that flank the squaraine dye. For example, macrocycle threading by a dye conjugate with two appended PEG2000 chains is only three times slower than threading by a conjugate with triethylene glycol chains that are 20 times shorter. The results are a promising advance toward synthetic mimics of streptavidin/biotin.
Highlights
The binding pockets within biological receptors usually contain a mixture of hydrophobic and polar residues that act synergistically to drive shape-selective binding of target guest molecules.[1]
One of the classic goals of supramolecular chemistry is to produce synthetic mimics of these remarkable binding systems; there are presently very few uncharged organic host−guest binding partners that associate in water with very high affinity
Binding in Water by an Amphiphilic Cavitya aFormation of host−guest hydrogen bonds compensates for the desolvation of polar groups and combines with desolvation of hydrophobic surfaces to produce strong complexation.[8]
Summary
Communication was the observation of efficient energy transfer from an excited anthracene unit in M2 (ex: 390 nm) to the encapsulated squaraine dye (em: 712 nm) (Figure 2e).[11]. A fluorescence titration experiment at this temperature provided ΔG = −11.3 kcal/mol and TΔS = −0.4 kcal/mol These thermodynamic data support a model where F1 and S3 both form enthalpically favored hydrogen bonds with the four NH residues inside M2, a picture that is supported by computational modeling (Figure S4) and several analogous X-ray crystal structures.[11,16−18] The structure of squaraine S3 is more rigid and more hydrophobic than fumaride F1 which is likely a major reason why complexation of S3 does not suffer as large an entropic penalty.
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