title: Upper rim guanidinocalix[4]arenes as artificial phosphodiesterases.
abstract: Calix[4]arene derivatives, blocked in the cone conformation and functionalized with two to four guanidinium units at the upper rim were synthesized and investigated as catalysts in the cleavage of the RNA model compound 2-hydroxypropyl p-nitrophenyl phosphate. When compared with the behavior of a monofunctional model compound, the catalytic superiority of the calix[4]arene derivatives points to a high level of cooperation between catalytic groups. Combination of acidity measurements with the pH dependence of catalytic rates unequivocally shows that a necessary requisite for effective catalysis is the simultaneous presence, on the same molecular framework, of a neutral guanidine acting as a general base and a protonated guanidine acting as an electrophilic activator. The additional guanidinium (guanidine) group in the diprotonated (monoprotonated) trifunctional calix[4]arene acts as a more or less innocent spectator. This is not the case with the tetrasubstituted calix[4]arene, whose mono-, di-, and triprotonated forms are slightly less effective than the corresponding di- and triguanidinocalix[4]arene derivatives, most likely on account of a steric interference with HPNP caused by overcrowding.
authors.affiliation: Dipartimento di Chimica Organica e Industriale, Università degli Studi di Parma, Parco Area delle Scienze 17/A, 43124 Parma, Italy.
authors.forename: Laura
authors.initials: L
authors.lastname: Baldini
authors.forename: Roberta
authors.initials: R
authors.lastname: Cacciapaglia
authors.forename: Alessandro
authors.initials: A
authors.lastname: Casnati
authors.forename: Luigi
authors.initials: L
authors.lastname: Mandolini
authors.forename: Riccardo
authors.initials: R
authors.lastname: Salvio
authors.forename: Francesco
authors.initials: F
authors.lastname: Sansone
authors.forename: Rocco
authors.initials: R
authors.lastname: Ungaro
mesh_terms: D047250:Calixarenes / Q000737:chemistry*; D002384:Catalysis; D019791:Guanidine / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008968:Molecular Conformation; D015394:Molecular Structure; D010636:Phenols / Q000737:chemistry*; D010727:Phosphoric Diester Hydrolases / Q000737:chemistry*; D012313:RNA / Q000737:chemistry* / Q000378:metabolism*; D013237:Stereoisomerism
chemical_list: D010636:Phenols; C121325:calix(4)arene; D047250:Calixarenes; D012313:RNA; D010727:Phosphoric Diester Hydrolases; D019791:Guanidine