Controlling Peptide Structure with Coordination Chemistry: Robust and Reversible Peptide–Dirhodium Ligation

✍ Scribed by Alexander N. Zaykov; Kevin R. MacKenzie; Zachary T. Ball

• Publisher: John Wiley and Sons
• Year: 2009
• Tongue: English
• Weight: 419 KB
• Volume: 15
• Category: Article
• ISSN: 0947-6539
• DOI: 10.1002/chem.200901266

✦ Synopsis

Selective bond formation of biomolecule substrates requires synthetic chemistry in an aqueous, functional-grouprich environment to alter and control the structure, function, and aggregation state of biomolecules. Ideally, new methods would be non-denaturing, would utilize chemistry that is orthogonal to existing methods, [1] and would be reversible in response to external stimuli. We here report a reversible method for addressing and bridging glutamate and aspartate carboxylates under mild aqueous conditions.

One powerful way to affect polypeptide structure is through methods that link, or bridge, two amino-acid side chains to form a cyclic product, but there exist few methods to selectively and reversibly bridge amino acid side chains. Cysteine is commonly used as a handle for selective bond formation through redox-mediated formation of disulfide linkages [2] or selective alkylation reactions, [3] but working with cysteine-containing peptides can be difficult. Other methods to address naturally occurring amino acids include activated esters of organic dicarboxylic acids for cross-linking lysine residues, but reversibility in this case is limited.

Metal ions serve structural roles in metalloproteins, where side chains serve as ligands which are bridged by a metal ion to create a folded metal-binding pocket. Taking a cue from these biological examples, the effects of metal binding on peptide structures is an active area of study. [4][5][6] Peptidemetal interactions have been used to understand metalloprotein folding and energetics and to shed light on potential toxicity pathways. [7] Many transition metals can bind to peptides in aqueous solution, most commonly through cysteine or histidine residues. [5,8] Although metalloproteins can bind extremely tightly to metal ions, smaller designed polypeptides often bind metal ions dynamically in aqueous solution, and systems involving a small number of binding groups often require excesses of metal ion in solution. We set out to develop a robust metal-binding method that would selectively address side chain functional groups in a manner complementary to extant methods. Carboxylates are common, naturally occurring polypeptide side chains, and the dirhodium-carboxylate interaction is sufficiently stable that ligand exchange is not observed under a variety of biologically relevant conditions. Selective binding to polycarboxylate regions has been observed with lanthanides, [9] but well defined bridging of a small number of carboxylate side chains has not been demonstrated.

We set out to explore the ability of dirhodium tetracarboxylates to bind chemoselectively and tightly to side chain carboxylates and hoped to develop a reversible metal ligation protocol to link two carboxylate side chains under nondenaturing conditions. In contrast to equilibrium binding exhibited in many peptide-metal binding interactions the robust carboxylate À dirhodium bond allows us to enforce structural changes of an isolable peptide À metal adduct.

For initial studies, we focussed on the bisA C H T U N G T R E N N U N G (cysteine) hairpin domain from a typical zinc finger protein, ZIF268. [10] We chose to examine the generality of the zinc-binding domain, determining if it could serve as the basis for new dirhodiumbinding domains through amino acid substitutions to position two Asp residues in place of the zinc-binding Cys. The peptide sequence ZF, derived from ZIF268 P62-A73, contains a number of reactive functional groups, necessitating a uniquely selective complexation method (Scheme 1). In addition, the parent zinc finger sequence folds into the common a-helix,b-sheet motif in the presence of three or four ligands, but exhibits no change from a random coil in the presence of only two ligands. [6] Bridging the dirhodium tetracarboxylate core with traditional ligands has proven challenging. Preparative yields of bridged structures have been largely limited m-phenylene structures, [11] and it has been reported that aliphatic a,w-diacids give product mixtures and low yields of chelate prod-[a]