Porphyrin Clips Derived from Diphenylglycoluril. Synthesis, Conformational Analysis, and Binding Properties

Epoxidation of polybutadiene by a topologically linked catalyst

Nature has evolved complex enzyme architectures that facilitate the synthesis and manipulation of the biopolymers DNA and RNA, including enzymes capable of attaching to the biopolymer substrate and performing several rounds of catalysis before dissociating. Many of these 'processive' enzymes have a toroidal shape and completely enclose the biopolymer while moving along its chain, as exemplified by the DNA enzymes T4 DNA polymerase holoenzyme and lambda-exonucleoase. The overall architecture of these systems resembles that of rotaxanes, in which a long molecule or polymer is threaded through a macrocycle. Here we describe a rotaxane that mimics the ability of processive enzymes to catalyse multiple rounds of reaction while the polymer substrate stays bound. The catalyst consists of a substrate binding cavity incorporating a manganese(III) porphyrin complex that oxidizes alkenes within the toroid cavity, provided a ligand has been attached to the outer face of the toroid to both activate the porphyrin complex and shield it from being able to oxidize alkenes outside the cavity. We find that when threaded onto a polybutadiene polymer strand, this catalyst epoxidizes the double bonds of the polymer, thereby acting as a simple analogue of the enzyme systems.

Highly negative homotropic allosteric binding of viologens in a double-cavity porphyrin

The synthesis of a double-cavity porphyrin with interesting allosteric binding properties toward viologens (N,N'-disubstituted 4,4'-bipyridines) is described. The porphyrin host forms very strong 1:2 complexes with viologens, displaying a negative allosteric behavior. The first viologen guest binds exceptionally tight (K > 107 M-1), and the second guest binds much more weakly (DeltaDeltaG = 9-15 kJ mol-1). The allosteric effect, one of the highest reported so far, originates in structural changes upon binding the first ligand, closely following the sequential (or induced-fit) theory of allosteric interactions by Koshland, Némethy, and Filmer (the KNF-model).

Cavitand Zn(II)−Porphyrin Capsules with High Affinities for Pyridines and N-Methylimidazole

Covalent cavitand Zn(II)-porphyrins 17-20 were prepared via multistep syntheses. These host molecules show moderate to excellent binding affinities to N-methylimidazole and pyridine guests. The complexing behavior strongly depends on the spacer's length, number, and rigidity, in addition to the guest size. Cavitand capping and strapping of porphyrins strongly influence the complex formation and result in a 10-700-fold enhancement of the binding strength compared to tetraphenyl Zn(II)-porphyrin.

Molecular recognition of amines and amino esters by zinc porphyrin receptors: binding mechanisms and solvent effects

Zinc porphyrin receptors bearing 12 ester groups in the meso phenyl groups (1-3) were prepared, and binding of amines and alpha-amino esters was studied with emphasis on the binding mechanisms. The X-ray crystallographic analysis of 5,10,15,20-tetrakis(2, 6-bis(carbomethoxymethoxy)-4-carbomethoxyphenyl)porphyrin (free base of 1) showed that the receptor has a binding pocket above the porphyrin plane. UV-visible titration experiments revealed that the zinc porphyrin receptors bound amines and alpha-amino esters with binding constants (K(a)) ranging from 0.5 to 52 700 M(-1) in CH(2)Cl(2) at 25 degrees C. The ester functional groups of 1 assisted the binding of aromatic alpha-amino esters (K(a) = 8 000-23 000 M(-1) in CH(2)Cl(2) at 25 degrees C) and inhibited the binding of bulky aliphatic alpha-amino esters (K(a) = 460 M(-1) for Leu-OMe in CH(2)Cl(2) at 25 degrees C), indicating that CH-pi type interactions and steric repulsions control the selectivity. The binding of amines and alpha-amino esters was tight both in a nonpolar solvent (CH(2)Cl(2)) and in a polar solvent (water) but loose in a solvent of intermediate polarity (H(2)O-MeOH (1:1)), demonstrating that two competitive driving forces are operating: (1) attractive electrostatic forces between host and guest such as coordination of the amino group to the zinc atom, and (2) entropic forces stemming from desolvation as well as enthalpic forces due to the host-guest dispersion forces. The former forces drive the binding in CH(2)Cl(2) while the latter forces drive the binding in water. The enthalpy changes in the binding in CH(2)Cl(2) and those in water range from -50 to -30 kJ mol(-1) and from -35 to 0 kJ mol(-1), respectively. The entropy changes in CH(2)Cl(2) and those in water range from -120 to -60 J K(-1) mol(-1) and from -50 to +60 J K(-1) mol(-1), respectively. Thus the binding in CH(2)Cl(2) is characterized by large negative enthalpy changes, while that in water by less negative entropy changes. These thermodynamic parameters also indicate that host-guest polar interactions (enthalpic forces) drive the binding in CH(2)Cl(2) while both host-guest dispersion interactions (an enthalpic force) and desolvation (an entropic force) drive the binding in water. Enthalpy-entropy compensation observed for the binding in water indicates that the binding of amines and amino esters in water by zinc porphyrins is associated with conformational changes as well as a high degree of dehydration. In CH(2)Cl(2), no clear compensation was observed, consistent with the mechanism that neither desolvation processes nor conformational changes contribute significantly to the binding energetics.