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Fig. 3.7 (a) Schematic diagram showing increasing lengths of helical mPE oligomer complexed with the rodlike guest 25 (top) and capped guest 26 (bottom). (b) Log of the association constant of guests 25 (blue squares) and 26 (red circles) with series 23 oligomers plotted as a function of the length. Values determined in 40% water in CH3CN. (Reprinted with permission from Ref 112. Copyright 2006, American Chemical Society, Washington, DC.)

Fig. 3.7 (a) Schematic diagram showing increasing lengths of helical mPE oligomer complexed with the rodlike guest 25 (top) and capped guest 26 (bottom). (b) Log of the association constant of guests 25 (blue squares) and 26 (red circles) with series 23 oligomers plotted as a function of the length. Values determined in 40% water in CH3CN. (Reprinted with permission from Ref 112. Copyright 2006, American Chemical Society, Washington, DC.)

CD titration studies confirmed the hypothesis [121]. As seen in Fig. 3.7b, binding free energies increased gradually from the 10-mer through the 20-mer. The 1:1 binding constants of 20-mer and 22-mer were about 30 times larger than that of the 10-mer. Interestingly, there was a small but experimentally reproducible reduction in binding with the 24-mer, in line with the slight mismatch between the dimensions of the host and the guest. To increase selectivity further, Moore and colleagues synthesized guest 26, which was capped with two large tri-arylmethyl groups. Even without the side chain, the diameter (ca. 10.2 A) of the capping group exceeded the cavity width. Selectivity was indeed much higher. As can be seen in Fig. 3.8b, peaking at n = 20 and 22 was much more pronounced for the capped 26. Also, compared to that of the uncapped guest, there was an overall increase in the binding of the capped guest. This was attributed to favorable aromatic interactions between the triarylmethyl caps and the end of the folded oligomer, which served to bury additional solvophobic surfaces. Another interesting discovery was that binding was essentially complete for 2320-25 by the time the first measurement was made at 60 s after mixing, but took more than 1000 s for 2320 26 to reach equilibrium. The difference in binding kinetics most likely happened because the folded oligomer had to unfold in order to bind the dumbbell-shaped guest (Fig. 3.7a).

The methylated series 27n was found to have higher folding stabilities in solution than the parent series 23n of the same chain length [122]. With methyl groups located in the interior of the helix, fewer solvent molecules could enter the cavity and solvophobic surface area was reduced. Smaller internal cavity of 27n was also supported by its binding properties: Under identical conditions, 27i2 bound monoterpene guests with a binding constant two orders of magnitude lower than that of 23i2 [120]. Solvents apparently were essential to the folded he lix in 23n because X-ray powder diffraction studies indicated that, in the liquid crystalline state, it packed into lamellar phases with the interlayer spacing linearly related to chain length. Methylated 27n, on the other hand, had the d spacing determined from diffraction independent of chain length, supporting columnar structures obtained from close packing of the helices [123].

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