Peptide cyclization and cyclodimerization by Cu(I)-mediated azide-alkyne cycloaddition

Head-to-tail cyclodimerization of resin-bound oligopeptides bearing azide and alkyne groups occurs readily by 1,3-dipolar cycloaddition upon treatment with Cu(I). The process was found to be independent of peptide sequence, sensitive to the proximity of the alkyne to the resin, sensitive to solvent composition, facile for alpha- and beta-peptides but not for gamma-peptides, and inhibited by the inclusion of tertiary amide linkages. Peptides shorter than hexamers were predominantly converted to cyclic monomers. Oligoglycine and oligo(beta-alanine) chains underwent oligomerization by 1,3-dipolar cycloaddition in the absence of a copper catalyst. These results suggest that cyclodimerization depends on the ability of the azido-alkyne peptide to form in-frame hydrogen bonds between chains in order to place the reacting groups in close proximity and lower the entropic penalty for dimerization. The properties of the resin and solvent are crucial, giving rise to a productive balance between swelling and interstrand H-bonding. These findings allow for the design of optimal substrates for triazole-forming ring closure and for the course of the reaction to be controlled by the choice of conditions.

An alternative solid phase peptide fragment condensation protocol with improved efficiency

The success of solid phase peptide synthesis is often limited by the aggregation of the growing peptide chains on the resin. Working from the results of a study of model coupling reactions in solution between Z-Gly-Phe-OH and H-Phe-OBzl, we have achieved higher efficiency in the repetitive solid phase fragment condensation of VGVAPG, in a 3:1 chloroform-phenol solvent system, using diisopropylcarbodiimide (DIC) as coupling agent, and a combination of 3-hydroxy-3,4-dihydro-4-oxo-1,2,3-benzotriazine (HODhbt) and its tetrabutyl ammonium salt as additive, than in DMF with DIC and HODhbt alone.

Interaction of ferrocenoyl-dipeptides with 3-aminopyrazole derivatives: beta-sheet models? A synthetic, spectroscopic, structural, and electrochemical study

The use of 3-aminopyrazole derivatives as beta-sheet templates is investigated using a series of ferrocenoyl (Fc)-dipeptides (Fc-Gly(2)-OEt, Fc-Ala(2)-OBzl, Fc-Leu-Phe-OMe, Fc-Val-Phe-OMe, Fc-Phe(2)-OMe, Fc-Leu(2)-OMe, Fc-Val(2)-OMe). The synthesis and full characterization are reported. The solid-state structures of Fc-Gly(2)-OMe and Fc-Leu-Phe-OMe show extensive hydrogen bonding of the podand peptide substituents, resulting in the formation of supramolecular Fc-dipeptide assemblies. For Fc-Gly(2)-OMe, this can be described as a parallel beta-sheet, whereas intermolecular interactions in Fc-Leu-Phe-OMe result in the formation of supramolecular helical structures. The saturation titrations of Fc-dipeptides with 3-amino-5-methylpyrazole (3-AMP) and 3-trifluoroacetylamido-5-methylpyrazole (3-TFAc-AMP) show a 1:1 interaction of the Fc-peptide with the aminopyrazole derivatives. IR measurements in solution confirm binding to the top face of the Fc-dipeptide and the involvement of the Fc-C=O and the ester C=O groups in establishing H-bonding interactions with the 3-TFAc-AMP. However, binding constants in chloroform are low and range from 8 to 27 M(-1), which correspond to binding energies of 5-7 kJ mol(-1). In higher polarity solvents, such as acetonitrile or acetone, the binding constants are below 5 M(-1), emphasizing the limited utility of 3-AMP derivatives as beta-sheet templates. Electrochemical measurements confirm the weak interactions between the various Fc-dipeptides and 3-TFAc-AMP. Typical shifts in the redox potential of the Fc moiety are in the range 0-20 mV. Attempts to modify 3-AMP at the 3-position by carbodiimide coupling with amino acid derivatives and, thus, enhance the binding to the Fc-peptides resulted in 2-amino acid substituted 3-AMP derivatives. Substitution at the 2-position blocks the binding site, and no interactions with Fc-dipeptides are observed.

Improvements in automated PNA synthesis using Boc/Z monomers

An optimized automated PNA synthesis protocol is reported. Under optimal conditions the product yield of a test 17-mer PNA is approximately 90%. The average coupling yield is 99.4%. The synthesis strategy is Boc/Z and the deprotected amine is neutralized in situ. The monomers are added in molar excess to HATU and pre-activated for 60 s before delivery to the resin. The concentration of the activated monomers is 0.08 M during the couplings. Heteroselective solvation provides the highest coupling yields. Acetic anhydride is used as capping reagent followed by a piperidine wash. The protocol has been developed in a 5 mumol scale but is easily scaled up to 10-50 mumol scale syntheses on the automated synthesizer (ABI 433A).