title: Rebek imides and their adenine complexes: preferences for Hoogsteen binding in the solid state and in solution.
abstract: Rebek imides (3), formed from Kemp's triacid, were developed in the mid-1980's as model receptors for adenine derivatives. We report here the first account of their hydrogen-bonding preferences upon binding 9-ethyladenine (1a) in the solid state. Structural analysis begins with simple imides 3a-e that form discrete dimers, while bis-imide 4 forms ribbon-like structures in the crystalline phase. The hydrogen-bonding interface within each of the representative assemblies features short intermolecular N(3)imide...O(8*)imide* distances (ca. 2.95 A), indicative of two-point hydrogen bonding. Imides 3f-h could be co-crystallized with 1a; single-crystal X-ray analysis of the resulting complexes reveals hydrogen-bonding geometries nearly identical to those observed in nucleobase complexes of adenine and pyrimidine derivatives. Imides 3f and 3g form 2:1 ternary assemblies with 1a; the complex of the former, (3f)2 x 1a, displays both Watson-Crick- and Hoogsteen-type hydrogen bonding, whereas the complex of the latter, (3g)2 x 1a, shows the Hoogsteen motif and imide hydrogen bonding to N(3) of the purine base (N(3)adenine...N(3'')imide = 3.07(1) A). Imide 3h forms a 1:1 complex with 1a (3h x 1a x CHCl3) and displays Hoogsteen binding exclusively. All of the 3 x 1a assemblies show C(adenine)...O(imide) distances (3.38-3.75 A) that are consistent with C-H...O hydrogen bonding. Base-pairing preferences for the Rebek imides are further explored in solution by 1H NMR one-dimensional NOE experiments and by computational means; in all cases the Hoogsteen motif is modestly favored relative to its Watson-Crick counterpart.
authors.affiliation: Laboratorium für Organische Chemie, ETH-Zentrum, Zürich, Switzerland.
authors.forename: Ronald K
authors.initials: RK
authors.lastname: Castellano
authors.forename: Volker
authors.initials: V
authors.lastname: Gramlich
authors.forename: François
authors.initials: F
authors.lastname: Diederich
mesh_terms: D000225:Adenine / Q000031:analogs & derivatives* / Q000737:chemistry*; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D018360:Crystallography, X-Ray; D003500:Cyclization; D004247:DNA / Q000737:chemistry*; D006860:Hydrogen Bonding; D007094:Imides / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D015394:Molecular Structure; D019906:Nuclear Magnetic Resonance, Biomolecular; D013816:Thermodynamics
chemical_list: D007094:Imides; D004247:DNA; D000225:Adenine