title: Cucurbit[7]uril⋅guest pair with an attomolar dissociation constant.
abstract: Host⋅guest complexes between cucurbit[7] (CB[7]) or CB[8] and diamantane diammonium ion guests 3 or 6 were studied by (1) H NMR spectroscopy and X-ray crystallography. (1) H NMR competition experiments revealed that CB[7]⋅6 is among the tightest monovalent non-covalent complexes ever reported in water with Ka =7.2×10(17)  M(-1) in pure D2 O and 1.9×10(15)  M(-1) in D2 O buffered with NaO2 CCD3 (50 mM). The crystal structure of CB[7]⋅6 allowed us to identify some of the structural features responsible for the ultratight binding, including the distance between the NMe3 (+) groups of 6 (7.78 Å), which allows it to establish 14 optimal ion-dipole interactions with CB[7], the complementarity of the convex van der Waals surface contours of 6 with the corresponding concave surfaces of CB[7], desolvation of the CO portals within the CB[7]⋅6 complex, and the co-linearity of the C7  axis of CB[7] with the N(+) ⋅⋅⋅N(+) line in 6. This work further blurs the lines of distinction between natural and synthetic receptors.
authors.affiliation: Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742 (USA).
authors.forename: Liping
authors.initials: L
authors.lastname: Cao
authors.forename: Marina
authors.initials: M
authors.lastname: Śekutor
authors.forename: Peter Y
authors.initials: PY
authors.lastname: Zavalij
authors.forename: Kata
authors.initials: K
authors.lastname: Mlinarić-Majerski
authors.forename: Robert
authors.initials: R
authors.lastname: Glaser
authors.forename: Lyle
authors.initials: L
authors.lastname: Isaacs
mesh_terms: D001952:Bridged-Ring Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D018360:Crystallography, X-Ray; D007093:Imidazoles / Q000138:chemical synthesis / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D047028:Macrocyclic Compounds; D000099246:Cucurbit(n)urils
chemical_list: D001952:Bridged-Ring Compounds; D007093:Imidazoles; C456276:cucurbit(7)uril; D047028:Macrocyclic Compounds; D000099246:Cucurbit(n)urils
keywords: X-ray diffraction; affinity pairs; avidin-biotin; cucurbit[n]uril; host-guest systems