Mikhail V. Rekharsky

A synthetic host-guest system achieves avidin-biotin affinity by overcoming enthalpy–entropy compensation

The molecular host cucurbit[7]uril forms an extremely stable inclusion complex with the dicationic ferrocene derivative bis(trimethylammoniomethyl)ferrocene in aqueous solution. The equilibrium association constant for this host-guest pair is 3 × 1015 M−1 (Kd = 3 × 10−16 M), equivalent to that exhibited by the avidin–biotin pair. Although purely synthetic systems with larger association constants have been reported, the present one is unique because it does not rely on polyvalency. Instead, it achieves its extreme affinity by overcoming the compensatory enthalpy–entropy relationship usually observed in supramolecular complexes. Its disproportionately low entropic cost is traced to extensive host desolvation and to the rigidity of both the host and the guest.

New Ultrahigh Affinity Host−Guest Complexes of Cucurbit[7]uril with Bicyclo[2.2.2]octane and Adamantane Guests: Thermodynamic Analysis and Evaluation of M2 Affinity Calculations

A dicationic ferrocene derivative has previously been shown to bind cucurbit[7]uril (CB[7]) in water with ultrahigh affinity (ΔG(o) = -21 kcal/mol). Here, we describe new compounds that bind aqueous CB[7] equally well, validating our prior suggestion that they, too, would be ultrahigh affinity CB[7] guests. The present guests, which are based upon either a bicyclo[2.2.2]octane or adamantane core, have no metal atoms, so these results also confirm that the remarkably high affinities of the ferrocene-based guest need not be attributed to metal-specific interactions. Because we used the M2 method to compute the affinities of several of the new host-guest systems prior to synthesizing them, the present results also provide for the first blinded evaluation of this computational method. The blinded calculations agree reasonably well with experiment and successfully reproduce the observation that the new adamantane-based guests achieve extremely high affinities, despite the fact that they position a cationic substituent at only one electronegative portal of the CB[7] host. However, there are also significant deviations from experiment, and these lead to the correction of a procedural error and an instructive evaluation of the sensitivity of the calculations to physically reasonable variations in molecular energy parameters. The new experimental and computational results presented here bear on the physical mechanisms of molecular recognition, the accuracy of the M2 method, and the usefulness of host-guest systems as test-beds for computational methods.

Complexation Thermodynamics of Cucurbit[6]uril with Aliphatic Alcohols, Amines, and Diamines

Using the isothermal calorimetric titration technique, we determined the stability constants (K), reaction enthalpy (ΔH°), and entropy (ΔS°) for complexation of cucurbit[6]uril (CB[6]) with a series of aliphatic alcohols, mono- and diamines, as well as spermidine and spermine, in aqueous solutions of alkali metal chlorides. The K for spermine reached 5.4 × 1010 M− 1 in 0.2 M LiCl, which is the largest amongst the values reported for CB[6]. Propylamine forms the strongest 1:1 complex with CB[6] (K = 21000 M− 1) in 0.1 M Na acetate buffer, which is driven exclusively by entropy. A comparison of K for 1,3-propanediamine versus 1,4-butanediamine reveals an extraordinary >60000-fold enhancement in affinity, which is the largest increment/CH2 ever observed in supramolecular chemistry. The present results in combination with our ESI-MS data reported recently unambiguously demonstrate that CB[6] exists in aqueous solution of alkali metal salts exclusively as a dicationic species, e.g. [CB[6]·2Na]2+.

Complexation of Aliphatic Ammonium Ions with a Water‐Soluble Cucurbit[6]uril Derivative in Pure Water: Isothermal Calorimetric, NMR, and X‐ray Crystallographic Study

Complexation of a water-soluble cucurbituril (CB) derivative, cyclohexanocucurbit[6]uril (CB*[6]), the cavity dimensions of which are essentially the same as those of CB[6], with various organic mono- and diammonium ions has been studied by isothermal titration calorimetry and 1H NMR spectroscopy. The binding affinity of CB*[6] with the guest molecules in water is 3-5 and 2-3 orders of magnitude higher than those of CB[6] in 50 % formic acid and in 0.05 M NaCl solution, respectively, which is mainly due to the larger enthalpic gains upon complex formation in the absence of interfering ions, such as protons and Na+. In particular, the binding constant (K) of spermine to CB*[6] was measured to be 3.4 x 10(12) M(-1), which is the highest binding constant ever reported for CB[6] or its derivatives. We also obtained the X-ray crystal structures of alpha,omega-alkanediammonium ions (C(n)DA2+, n=4-8) and spermine complexes with CB[6], in which the aliphatic chains of the guest molecules take an extended or partially bent conformation inside the CB[6] cavity, depending on the chain length. The hexamethylene chain of C6DA2+ takes a twisted conformation, which not only allows strong ion-dipole interactions between the ammonium groups and the carbonyl groups at the portals, but also increases hydrophobic interactions between the alkyl part of the guest and the inner wall of the host, which results in the largest enthalpic gain among alpha,omega-alkanediammonium ions. The thermodynamic parameters associated with the complexation are discussed in relation to the binding modes and conformations of the aliphatic chain of the guest molecules inside the host, which were investigated by 1H NMR spectroscopy and X-ray crystallography.

Sequence recognition and self-sorting of a dipeptide by cucurbit[6]uril and cucurbit[7]uril

Cucurbit[7]uril forms very strong complex with zwitterionic dipeptide Phe-Gly with affinity exceeding 10(7) M(-1) and effectively recognizes peptide sequence of Phe-Gly over Gly-Phe as well as Tyr-Gly over Gly-Tyr and Trp-Gly over Gly-Trp with relative affinities of 23 000, 18 000 and 2000, respectively.