Helix-Rod Host-Guest Complexes with Shuttling Rates Much Faster than Disassembly

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Science  04 Mar 2011:
Vol. 331, Issue 6021, pp. 1172-1175
DOI: 10.1126/science.1200143

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  1. Fig. 1

    Host-guest components and assembly. (A) Formulas of 1, 2, and 3. Single-helical conformations span 4.5, 3.5, and 2.5 turns, respectively. (B) Rod-like guests possessing carbonyl hydrogen bond acceptors (yellow dots) with bulky end groups (4, 6, 7) or without (5). Depending on length (n), each guest matches best with 1, 2, or 3 as a host. (C) Equilibrium between oligomers 1, 2, and 3 as double helices (DH), single helices (SH), or complexes (C) with rod-like guests (R). The formation of a complex with a dumbbell guest requires the unfolding and refolding of the helix. (D to G) Titration of 1 (2 mM) by 4d [from (D) to (G): 0, 0.5, 1, 1.5 equiv. of 4d] monitored by 400-MHz 1H NMR in CDCl3 at 45°C. The high temperature reduces the time needed to reach equilibrium after each addition of 4d. The signals of the starting double helix (1)2 (♦) progressively disappear while the signals of 14d emerge (●). Benzylic protons of the bound 4d appear as a diastereotopic pattern (□), which reflects the chiral environment of the surrounding helix. (H) Constants of formation at 45°C of the complexes between 1 to 3 and 4 to 7 defined as Ka = [C]2/([DH].[R]2). nd: no binding detected.

  2. Fig. 2

    Solid-state structures. (A) Double helix (3)2. (B) Double helix (2)2. (C) Complex 2⊃6. (D) Complex 1⊃7. (E) Complex 1⊃5. (F) Complex 1⊃8. Side chains (O-isobutyl groups) and included solvent molecules have been removed for clarity.

  3. Fig. 3

    Helix shuttling. (A) Formulas of rod-like guests having two adjacent identical (8) or different (9) stations. Because of the proximity between the two stations and the presence of a single carbonyl group in the middle of the rod, only one station at a time can be occupied by a helix. (B) Scheme of degenerate helix sliding along a symmetrical guest. The yellow dots mark the hydrogen bond acceptors. (C) Scheme of helix sliding along a nondegenerate guest possessing a station that can be blocked or unblocked upon protonation or deprotonation, respectively. The green dot symbolizes the amine function of 9, which can be included in the helix cavity. The red dot is the corresponding ammonium, which is not included in the helix cavity. (D and E) Portions of the 1H and 19F NMR spectra of 19 in CDCl3 at equilibrium, after adding excess of an organic acid (2,4-dinitrophenol) and after neutralizing with a base (Et3N). AF and QF designate fluoro-anthracene and fluoro-quinoline, respectively.