Complexes of 5,5′-aminoacido-substituted 2,2′-bipyridyl ligands: control of diastereoselectivity with a pH switch and a chloride-responsive combinatorial library

pH-tuned metal coordination and peroxidase activity of a peptide dendrimer enzyme model with a Fe(II)bipyridine at its core

Peptide dendrimer BP1 was obtained by double thioether bond formation between 5,5'-bis(bromomethyl)-2,2'-bipyridine and two equivalents of peptide dendrimer N1 (Ac-Glu-Ser)(8)(Dap-Glu-Ala)(4)(Dap-Amb-Tyr)(2)Dap-Cys-Asp-NH(2) (Dap = branching 2,3-diaminopropanoic acid, Amb = 4-aminomethyl-benzoic acid). At pH 4.0 BP1 bound Fe(ii) to form the expected tris-coordinated complex [Fe(II)(BP1)(3)] (K(f) = 2.1 × 10(15) M(-3)). At pH 6.5 a monocoordinated complex [Fe(II)(BP1)] was formed instead (K(f) = 2.1 × 10(5) M(-1)) due to electrostatic repulsion between the polyanionic dendrimer branches, as confirmed by the behavior of three analogues where glutamates were partially or completely replaced by neutral glutamines or positive lysines. [Fe(II)(BP1)] catalyzed the oxidation of o-phenylenediamine with H(2)O(2) with enzyme-like kinetics (k(cat) = 1.0 min(-1), K(M) = 1.5 mM, k(cat)/k(uncat) = 90 000) and multiple turnover, while Fe(2+) or [Fe(bipy)(3)](2+) were inactive. The labile coordination positions allowing coordination to H(2)O(2) and to the substrate are likely responsible for the enhanced peroxidase activity of the metallopeptide dendrimer.

The use of electrospray mass spectrometry to determine speciation in a dynamic combinatorial library for anion recognition

The composition of a dynamic mixture of similar 2,2'-bipyridine complexes of iron(II) bearing either an amide (5-benzylamido-2,2'-bipyridine and 5-(2-methoxyethane)amido-2,2'-bipyridine) or an ester (2,2'-bipyridine-5-carboxylic acid benzylester and 2,2'-bipyridine-5-carboxylic acid 2-methoxyethane ester) side chain have been evaluated by electrospray mass spectroscopy in acetonitrile. The time taken for the complexes to come to equilibrium appears to be dependent on the counteranion, with chloride causing a rapid redistribution of two preformed heteroleptic complexes (of the order of 1 hour), whereas the time it takes in the presence of tetrafluoroborate salts is in excess of 24 h. Similarly the final distribution of products is dependent on the anion present, with the presence of chloride, and to a lesser extent bromide, preferring three amide-functionalized ligands, and a slight preference for an appended benzyl over a methoxyethyl group. Furthermore, for the first time, this study shows that the distribution of a dynamic library of metal complexes monitored by ESI-MS can adapt following the introduction of a different anion, in this case tetrabutylammonium chloride to give the most favoured heteroleptic complex despite the increasing ionic strength of the solution.

Metal ion-assembled micro-collagen heterotrimers

Collagen mimetic peptides (CMPs) provide critical insight into the assembly, stability, and structure of the triple helical collagen protein. The majority of natural fibrous collagens are aab or abc heterotrimers, yet few examples of heterotrimeric CMPs have been reported. Previously, CMP heterotrimers have only been accessible by total syntheses or by introducing complementary interstrand electrostatic or steric interactions. Here, we describe an abc CMP heterotrimer in which each contributing CMP consists of only three amino acids: glycine, proline and 4-hydroxyproline. Assembly of the heterotrimeric triple helix is directed by a combination of metal-ion coordination to set the relative register of the CMPs, and minimization of valence frustration to direct heterotrimerization. Assembly of the four-component mixture is facile and extremely rapid, and equilibration to the abc heterotrimer occurs within a few hours at modestly elevated temperatures. The melting temperatures of the metal-assembled collagen trimers are higher by some 30°C than the apopeptide assemblies. Two iterations of the design are described, and the outcomes suggest possibilities for designing self-assembling abc and abb heterotrimers.

Supramolecular interactions between library members modulate the behavior of dynamic combinatorial libraries

The presence of a supramolecular network of interactions between library members can lead to very different responses when libraries with identical molecular composition are exposed to the same template. Numerical simulations demonstrate that supramolecular interactions between library members of covalent dynamic combinatorial libraries (DCLs) can affect both degree and selectivity of the response of the library when a template molecule is added.

A new twist in anion binding: metallo-helicate hosts for anionic guests

The condensation of 5-chlorocarbonyl-2,2'-bipyridine with a variety of rigid aromatic diamines, L, gave a series of new bisamido-2,2'-bipyridine based ligands (L = 4,4'-methylenediamine, L1; L = 1,1-bis(4-aminophenyl)cyclohexane, L2; L = 1,1-bis(4-amino-3,5-dimethylphenyl)cyclohexane, L3) capable of forming dinuclear triple helicate complexes on coordination to Fe(II). The reaction of various Fe(II) salts with gave: {[Fe2(L1)3](BF4)4, 1; [Fe2(L1)3](ClO4)4, 2; [Fe2(L1)3]Cl4, 3; [Fe2(L1)3](SO4)2, 4; [Fe2(L2)3](BF4)4, 5; [Fe2(L2)3]Cl4, 6; [Fe2(L3)3](BF4)4; 7; [Fe2(L3)3]Cl4, 8; and [Fe2(L3)3](SO4)2, 9, as determined by UV-Vis, IR and 1H NMR spectroscopy, electrospray mass spectrometry (ESMS) and elemental analyses. A UV-Vis complexometric titration experiment between L3 and Fe(BF4)2 established conclusively a [Fe2(L3)3]4+ product species. 1H NMR spectroscopy showed that the complexes exist as both rac-(helical) and meso-(non-helical) isomers in DMSO-d6 solution at 298 K. L1-L3 were designed such that following complexation, six amide hydrogen atoms would line an inter-strand intrahelical cavity of sufficient size to facilitate the binding of guest species within it. Indeed, ESMS studies showed characteristic peaks typical of complex-anion species in solution. Furthermore, 1H NMR titration experiments showed that anions bind within the intrahelical cavity as titration of 1, 5 and 7 with Bu4NCl showed significant downfield shifts in the amide and bipyridyl H6 proton resonances to yield a species of 1 : 2 host to guest stoichiometry. Moreover, addition of Bu4NCl to 1, 5 and 7 shifted the rac-/meso-species distribution from 1 : 2 in favour of the meso- to 100% in favour of the rac-isomer, showing that Cl- ions favour the formation of the triple helicate species in DMSO-d6 solution.

Effective chiral recognition among ions in polar media

Effective homochiral recognition occurs between the cationic [Fe(eilatin)3]2+ complex and TRISPHAT anions even in polar media such as 90% acetone-CHCl3 (de > or = 89%).

Controlling a recognition-mediated reaction using a pH switch

The selective recognition-mediated reaction between a nitrone bearing a urea recognition site and a maleimide bearing a proton switchable recognition site can be turned 'on' and 'off' by the addition of base and acid respectively.

Highly diastereoselective amplification from a dynamic combinatorial library of macrocyclic oligoimines

Cadmium promoted diastereoselective amplification of a single member from a dynamic combinatorial library of stereoisomeric oligoimines of different sizes allows the efficient preparation of a new macrocyclic polyamine.

Solvent-tunable inversion of chirality transfer from carbon to copper

A change of solvent causes an inversion of the stereochemistry at copper of the chiral Cu(I) complex described herein.

Dynamic covalent and supramolecular direction of the synthesis and reassembly of copper(I) complexes

The technique of subcomponent self-assembly has been applied to the preparation of a set of copper(I) complexes from diamines and aldehydes in aqueous solution. These complexes may be synthesized alongside one another in solution despite the chemical non-orthogonality of their respective starting materials; thermodynamic equilibration eliminates all mixed products. The reactivity of these complexes has been studied, revealing that in certain cases, the substitution of both ligands and ligand subcomponents could be independently carried out. In one particular case, a complex was shown to be inert to ligand substitution but readily underwent ligand subcomponent substitution, creating the possibility of a previously undocumented kind of cascade reaction: Once ligand subcomponent substitution had occurred, ligand exchange could then happen, allowing both reactions to be triggered by a single chemical event.