Synthesis of N-carboxyalkyl and N-aminoalkyl functionalized dipeptide building units for the assembly of backbone cyclic peptides

Synthesis and Characterization of Carboxyethylated Lignosulfonate

Lignosulfonate is a byproduct of the sulfite pulping process and has limited use in industry. The main objective of this study was to investigate the carboxyethylation of lignosulfonate to increase its charge density to broaden its applications. The carboxyethylation of lignosulfonate was optimized under the conditions of 30 wt % NaOH, 2.0 mol mol^-1 2-chloropropinic acid/lignosulfonate, 90 °C, 0.5 h, and 0.03 mol 2-chloropropinic acid, which produced carboxyethylated lignosulfonate with a charge density and molecular weight of -3.51 meq g^-1 and 46 493 g mol^-1 , respectively. The mechanism of the carboxyethylation reaction using 2-chloropropinic acid by an S_N 1 pathway in an alkaline solution was discussed. Methylation was also used to mask the phenolic hydroxide groups of lignosulfonate to investigate if carboxyethylation occurred on aliphatic hydroxide groups of lignosulfonate. The produced carboxyethylated lignosulfonate was characterized by using FTIR spectroscopy, NMR spectroscopy, gel permeation chromatography, and elemental and functional group analyses. Basic ¹ H-¹ H 2 D COSY NMR spectroscopy was used to record the coupled spins of the carboxyethyl group on carboxyethylated lignosulfonate. The information from 1 D ¹ H NMR and 2 D NMR COSY spectroscopy provided evidence for the existence of a 1-carboxyethyl group on the carboxyethylated lignosulfonate structure.

Amino acid fluorides: viable tools for synthesis of peptides, peptidomimetics and enantiopure heterocycles

This review provides a broad perspective of the uses of amino acid fluorides in the synthesis of peptides and a wide range of other molecules including peptidomimetics, heterocycles and biologically active molecules.

Short and efficient diastereoselective synthesis of pyrrolidinone-containing dipeptide analogues

The pyrrolidine-2,4-diones have been identified as a convenient starting point for the synthesis of peptide analogues. Herein we describe an optimized two-step reductive amination procedure, which provides a small library of pyrrolidinone-containing dipeptide analogues in high yield and excellent diastereoselectivity.

Synthesis of linear and cyclic phosphopeptides as ligands for theN-terminal SH2-domain of protein tyrosine phosphatase SHP-1

Linear and cyclic phosphopeptides related to the pY2267 binding site of the epithelial receptor tyrosine kinase Ros have been synthesized as ligands for the amino-terminal SH2 (src homology) domain of protein tyrosine phosphatase SHP-1. The synthesis was accomplished by Fmoc-based solid-phase methodology using side-chain unprotected phosphotyrosine for the linear and mono-benzyl protected phosphotyrosine for the cyclic peptides. According to molecular modelling, the incorporation of a glycine residue between Lys (position pY-1 relative to phosphotyrosine) and Asp or Glu (position pY+2) was recommended for the cyclic candidates. The preparation of these peptides was successfully performed by the incorporation of a Fmoc-Xxx(Gly-OAll)-OH (Xxx = Asp, Glu) dipeptide building block that was prepared in solution prior to SPPS. The cyclization was achieved with PyBOP following Alloc/OAll-deprotection. This study demonstrates the usefulness of allyl-type protecting groups for the generation of side-chain cyclized phosphopeptides. Alloc/OAll-deprotection and cyclization are compatible with phosphorylated tyrosine.

Efficient synthesis and comparative studies of the arginine and Nomega,Nomega-dimethylarginine forms of the human nucleolin glycine/arginine rich domain

The Gly- and Arg-rich C-terminal region of human nucleolin is a 61-residue long domain involved in a number of protein-protein and protein-nucleic acid interactions. This domain contains 10 aDma residues in the form of aDma-GG repeats interspersed with Phe residues. The exact role of Arg dimethylation is not known, partly because of the lack of efficient synthetic methods. This work describes an effective synthetic strategy, generally applicable to long RGG peptides, based on side-chain protected aDma and backbone protected dipeptide Fmoc-Gly-(Dmob)Gly-OH. This strategy allowed us to synthesize both the unmodified (N61Arg) and the dimethylated (N61aDma) peptides with high yield ( approximately 26%) and purity. As detected by NMR spectroscopy, N61Arg does not possess any stable secondary or tertiary structure in solution and N(omega),N(omega)-dimethylation of the guanidino group does not alter the overall conformational propensity of this peptide. While both peptides bind single-stranded nucleic acids with similar affinities (K(d) = 1.5 x 10(-7) M), they exhibit a different behaviour in ssDNA affinity chromatography consistent with the difference in pK(a) values. It has been previously shown that N61Arg inhibits HIV infection at the stage of HIV attachment to cells. This study demonstrates that Arg-dimethylated C-terminal domain lacks any inhibition activity, raising the question of whether nucleolin expressed on the cell-surface is indeed dimethylated.

Novel Gly building units for backbone cyclization: synthesis and incorporation into model peptides

We report the preparation of novel building units for backbone cyclization that have the general formula Fmoc-Nalpha[CH(R)CO2Al]Gly-OH. These building units were prepared by the reductive alkylation method using allyl esters of several amino acids as starting material and hence, respectively, contain the side chain of these amino acids. These N-alkylated Gly building units were incorporated in model backbone cyclic peptides. The resulting crude backbone cyclic peptides were obtained in high degree of purity according to HPLC and mass spectrometric analyses.

Synthesis and biological activities of new side chain and backbone cyclic bradykinin analogues

A series of conformationally constrained cyclic analogues of the peptide hormone bradykinin (BK, Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg) was synthesized to check different turned structures proposed for the bioactive conformation of BK agonists and antagonists. Cycles differing in the size and direction of the lactam bridge were performed at the C- and N-terminal sequences of the molecule. Glutamic acid and lysine were introduced into the native BK sequence at different positions for cyclization through their side chains. Backbone cyclic analogues were synthesized by incorporation of N-carboxy alkylated and N-amino alkylated amino acids into the peptide chain. Although the coupling of Fmoc-glycine to the N-alkylated phenylalanine derivatives was effected with DIC/HOAt in SPPS, the dipeptide building units with more bulky amino acids were pre-built in solution. For backbone cyclization at the C-terminus an alternative building unit with an acylated reduced peptide bond was preformed in solution. Both types of building units were handled in the SPPS in the same manner as amino acids. The agonistic and antagonistic activities of the cyclic BK analogues were determined in rat uterus (RUT) and guinea-pig ileum (GPI) assays. Additionally, the potentiation of the BK-induced effects was examined. Among the series of cyclic BK agonists only compound 3 with backbone cyclization between positions 2 and 5 shows a significant agonistic activity on RUT. To study the influence of intramolecular ring closure we used an antagonistic analogue with weak activity, [D-Phe7]-BK. Side chain as well as backbone cyclization in the N-terminus of [D-Phe7]-BK resulted in analogues with moderate antagonistic activity on RUT. Also, compound 18 in which a lactam bridge between positions 6 and 9 was achieved via an acylated reduced peptide bond has moderate antagonistic activity on RUT. These results support the hypothesis of turn structures in both parts of the molecule as a requirement for BK antagonism. Certain active and inactive agonists and antagonists are able to potentiate the bradykinin-induced contraction of guinea-pig ileum.

Synthesis of novel protected Nalpha(omega-thioalkyl) amino acid building units and their incorporation in backbone cyclic disulfide and thioetheric bridged peptides

General methods for the preparation of protected Nalpha(omega-thioalkyl) amino acids building units for backbone cyclization using reductive alkylation and on-resin preparation are described. The synthesis of non-Gly Fmoc-protected S-functionalized N-alkylated amino acids is based on the reaction of readily prepared protected omega-thio aldehyde with the appropriate amino acid. Preparation of Fmoc-protected S-functionalized N-alkylated Gly building units was carried out using two methods: reaction of glyoxylic acid with Acm-thioalkylamine and an on-resin reaction of bromoacetyl resin with Trt-thioalkylamines. Three model peptides were prepared using these building units. The GlyS2 building unit was incorporated into a backbone cyclic analog of somatostatin that contains a disulfide bridge. Formation of the disulfide bridge was performed by on-resin oxidation using 12 or Tl(CF3COO-)3. Both methods resulted in the desired product in a high degree of purity in the crude. The AspS3 building unit was also successfully incorporated into a model peptide. In addition, the in situ generation of sulfur containing Gly building units was demonstrated on a Substance P backbone cyclic analog containing a thioether bridge.

Facile synthesis of orthogonally protected amino acid building blocks for combinatorial N-backbone cyclic peptide chemistry

Protected Nalpha-(aminoallyloxycarbonyl) and Nalpha-(carboxyallyl) derivatives of all natural amino acids (except proline), and their chiral inverters, were synthesized using facile and efficient methods and were then used in the synthesis of Nalpha-backbone cyclic peptides. Synthetic pathways for the preparation of the amino acid building units included alkylation, reductive amination and Michael addition using alkylhalides, aldehydes and alpha,beta-unsaturated carbonyl compounds, and the corresponding amino acids. The resulting amino acid prounits were then subjected to Fmoc protection affording optically pure amino acid building units. The appropriate synthetic pathway for each amino acid was chosen according to the nature of the side-chain, resulting in fully orthogonal trifunctional building units for the solid-phase peptide synthesis of small cyclic analogs of peptide loops (SCAPELs). Nalpha-amino groups of building units were protected by Fmoc, functional side-chains were protected by t-Bu/Boc/Trt and N-alkylamino or N-alkylcarboxyl were protected by Alloc or Allyl, respectively. This facile method allows easy production of a large variety of amino acid building units in a short time, and is successfully employed in combinatorial chemistry as well as in large-scale solid-phase peptide synthesis. These building units have significant advantage in the synthesis of peptido-related drugs.

Study on the cyclization tendency of backbone cyclic tetrapeptides

The cyclization kinetics of five backbone-cyclic tetrapeptides was investigated both experimentally and computationally. The aim was to both accurately measure the cyclization rates in solution and develop a method that efficiently estimates the relative cyclization tendencies computationally. Progression of the cyclization reaction was monitored directly, yielding the kinetics of changes in the amounts of the linear precursor and the products. These measurements were used to calculate the reaction rates; the results were consistent with a first-order reaction kinetics. In order to predict the cyclization rates computationally, the conformation space of the linear precursors was mapped and used to construct an approximate partition function. We assumed that the cyclization tendency was correlated with the relative probability of being found in a cyclization-prone conformation of the backbone, this probability was estimated from the partition function. The results supported this assumption and demonstrated that, within reasonable accuracy, we are able to predict the relative cyclization tendencies of the peptides measured.

Synthesis of different types of dipeptide building units containing N- or C-terminal arginine for the assembly of backbone cyclic peptides

Different types of dipeptide building units containing N- or C-terminal arginine were prepared for synthesis of the backbone cyclic analogues of the peptide hormone bradykinin (BK: Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg). For cyclization in the N-terminal sequence N-carboxyalkyl and N-aminoalkyl functionalized dipeptide building units were synthesized. In order to avoid lactam formation during the condensation of the N-terminal arginine to the N-alkylated amino acids at position 2, the guanidino function has to be deprotected. The best results were obtained by coupling Z-Arg(Z)2-OH with TFFH/collidine in DCM. Another dipeptide building unit with an acylated reduced peptide bond containing C-terminal arginine was prepared to synthesize BK-analogues with backbone cyclization in the C-terminus. To achieve complete condensation to the resin and to avoid side reactions during activation of the arginine residue, this dipeptide unit was formed on a hydroxycrotonic acid linker. HYCRAM technology was applied using the Boc-Arg(Alloc)2-OH derivative and the Fmoc group to protect the aminoalkyl function. The reduced peptide bond was prepared by reductive alkylation of the arginine derivative with the Boc-protected amino aldehyde, derived from Boc-Phe-OH. The best results for condensation of the branching chain to the reduced peptide bond were obtained using mixed anhydrides. Both types of dipeptide building units can be used in solid-phase synthesis in the same manner as amino acid derivatives.

Synthesis of differentially protected N-acylated reduced pseudodipeptides as building units for backbone cyclic peptides

Backbone cyclization has become an important method for generating or stabilizing the bioactive conformation of peptides without affecting the amino acid side-chains. Up to now, backbone cyclic peptides were mostly synthesized with bridges between N-amino- and N-carboxy-functionalized peptide bonds. To study the influence of a more flexible backbone on the biological activity, we have developed a new type of backbone cyclization which is achieved via the N-functionalized moieties of acylated reduced peptide bonds. As described in our previous publications, the formation of N-functionalized dipeptide units facilitates the peptide assembly compared with the incorporation of N-alkyl amino acids. Besides the racemization-free synthesis of Fmoc-protected pseudodipeptide esters with reduced peptide bonds, the new type of backbone modification allows the use of a great variety of omega-amino- and alpha,omega-dicarboxylic acids differing in chain length and chemical properties. Best results for the coupling of the omega-amino- and alpha,omega-dicarboxylic acids to the reduced peptide bond were obtained by the formation of mixed anhydrides with alkyl chloroformates. Whereas the protecting group combination of Z/OBzl in the dipeptide unit and Boc/OtBu for the N-functionalized moiety leads to the formation of 2-ketopiperazine during hydrogenation, the combination of Fmoc/OtBu and Alloc/OAll is very suitable for the synthesis of backbone cyclic peptides on solid support.