Toward new designed proteins derived from bovine pancreatic trypsin inhibitor (BPTI): covalent cross-linking of two 'core modules' by oxime-forming ligation

Peptide Ligation via the Suzuki-Miyaura Cross-Coupling Reaction

Chemoselective ligation of two 28-mer peptides has been accomplished using the Suzuki-Miyaura cross-coupling reaction at or near physiological temperature in an aqueous solution containing sodium dodecyl sulfate in 83% yield. The effects of Pd source, solvent, base, and temperature were investigated, and the optimized reaction conditions were studied for compatibility with naturally present and artificially introduced functional groups in peptides including S-protected thiol and azide. The peptide conjugations were carried out in high yield (90%) with their functional groups intact. This method also allowed for facile introduction of an affinity tag or fluorescent probe into 20-mer peptides in >80% yield. These results suggest that the Suzuki-Miyaura cross-coupling is useful for multiple conjugations of peptides in conjunction with conventional conjugation reactions performed in sequence.

α-Oxo aldehyde or glyoxylyl group chemistry in peptide bioconjugation

Since the 1990s, α-oxo aldehyde or glyoxylic acid chemistry has inspired a vast array of synthetic tools for tailoring peptide or protein structures, for developing peptides endowed with novel physicochemical properties or biological functions, for assembling a large diversity of bioconjugates or hybrid materials, or for designing peptide-based micro or nanosystems. This past decade, important developments have enriched the α-oxo aldehyde synthetic tool box in peptide bioconjugation chemistry and explored novel applications. The aim of this review is to give a large overview of this creative field.

Backbone amide linker (BAL) strategy for Nalpha-9-fluorenylmethoxycarbonyl (Fmoc) solid-phase synthesis of peptide aldehydes

A rapid and efficient strategy has been developed for the general synthesis of complex peptide aldehydes. N(alpha)-Benzyloxycarbonylamino acids were converted to protected aldehyde building blocks for solid-phase synthesis in four steps and moderate overall yields. The aldehydes were protected as 1,3-dioxolanes except for one case where a dimethyl acetal was used. These protected amino aldehyde monomers were then incorporated onto 5-[(2 or 4)-formyl-3,5-dimethoxyphenoxy]butyryl-resin (BAL-PEG-PS) by reductive amination, following which the penultimate residue was introduced by HATU-mediated acylation. The resultant resin-bound dipeptide unit, anchored by a backbone amide linkage (BAL), was extended further by routine Fmoc chemistry procedures. Several model peptide aldehydes were prepared in good yields and purities. Some epimerization of the C-terminal residue occurred (10% to 25%), due to the intrinsic stereolability conferred by the aldehyde functional group, rather than any drawbacks to the synthesis procedure.

Site-Specific Polymer Attachment to a CCL-5 (RANTES) Analogue by Oxime Exchange

A synthetic strategy that allows for the site-specific attachment of polymers such as poly(ethylene glycol) (PEG) to protein pharmaceuticals is described. PEG was attached to a 67-amino acid fully synthetic CCL-5 (RANTES) analogue at its GAG binding site both to reduce aggregation and to increase the circulating lifetime. Effective protection of an Aoaa chemoselective linker during peptide assembly, total chemical protein synthesis, and protein folding was achieved with an isopropylidene group. Mild deprotection of the resulting folded synthetic protein and subsequent polymer attachment occur without interference with the native folded structure and activity.

Preparation of cyclic peptide libraries using intramolecular oxime formation

A new method for the synthesis of cyclic head-to-side chain peptide libraries has been developed in which the key cyclization step involves reaction between a C-terminal ketone and an N-terminal hydroxylamine to form a macrocyclic oxime. This methodology efficiently delivers cyclic products that consist of mixtures of syn and anti isomers.

Hydrogen exchange, core modules, and new designed proteins

A strategy for design of new proteins that mimic folding properties of native proteins is based on peptides modeled on the slow exchange cores of natural proteins. We have synthesized peptides, called core modules, that correspond to the elements of secondary structure that carry the very slowest exchanging amides in a protein. The expectation is that, if soluble in water, core modules will form conformational ensembles that favor native-like structure. Core modules modeled on natural bovine pancreatic trypsin inhibitor have been shown by NMR studies to meet this expectation. The next step toward production of a native state mimic is to further shift the conformational bias of a core module toward more ordered structure by promoting module-module interactions that are mutually stabilizing. For this, two core modules were incorporated into a single molecule by means of a long cross-link. From a panel of several two-module peptides, one very promising lead emerged; it is called BetaCore. BetaCore is monomeric in water and forms a new fold composed of a four-stranded, antiparallel beta-sheet. The single, dominant conformation of BetaCore is characterized by various NMR experiments. Here we compare the individual core module to the two-module BetaCore and discuss the progressive stabilization of intramodule structure and the formation of new intermodule interactions.

BetaCore, a designed water soluble four-stranded antiparallel beta-sheet protein

BetaCore is a designed approximately 50-residue protein in which two BPTI-derived core modules, CM I and CM II, are connected by a 22-atom cross-link. At low temperature and pH 3, homo- and heteronuclear NMR data report a dominant folded ('f') conformation with well-dispersed chemical shifts, i, i+1 periodicity, numerous long-range NOEs, and slowed amide hydrogen isotope exchange patterns that is a four-stranded antiparallel beta-sheet with nonsymmetrical and specific association of CM I and CM II. BetaCore 'f' conformations undergo reversible, global, moderately cooperative, non-two-state thermal transitions to an equilibrium ensemble of unfolded 'u' conformations. There is a significant energy barrier between 'f' and 'u' conformations. This is the first designed four-stranded antiparallel beta-sheet that folds in water.