Tetrahydrofuran Calpha-tetrasubstituted amino acids: two consecutive beta-turns in a crystalline linear tripeptide

β-Turn Induction by a Diastereopure Azepane-Derived Quaternary Amino Acid

β-Turns are one of the most common secondary structures found in proteins. In the interest of developing novel β-turn inducers, a diastereopure azepane-derived quaternary amino acid has been incorporated into a library of simplified tetrapeptide models in order to assess the effect of the azepane position and peptide sequence on the stabilization of β-turns. The conformational analysis of these peptides by molecular modeling, NMR spectroscopy, and X-ray crystallography showed that this azepane amino acid is an effective β-turn inducer when incorporated at the i + 1 position. Moreover, the analysis of the supramolecular self-assembly of one of the β-turn-containing peptide models in the solid state reveals that it forms a supramolecular helical arrangement while maintaining the β-turn structure. The results here presented provide the basis for the use of this azepane quaternary amino acid as a strong β-turn inducer in the search for novel peptide-based bioactive molecules, catalysts, and biomaterials.

Strategies to improve the physicochemical properties of peptide-based drugs

Peptides are a rapid-growing class of therapeutics with unique and desirable physicochemical properties. Due to disadvantages such as low membrane permeability and susceptibility to proteolytic degradation, peptide-based drugs have limited bioavailability, a short half-life, and rapid in vivo elimination. Various strategies can be applied to improve the physicochemical properties of peptide-based drugs to overcome limitations such as limited tissue residence time, metabolic instability, and low permeability. Applied strategies including backbone modifications, side chain modifications, conjugation with polymers, modification of peptide termini, fusion to albumin, conjugation with the Fc portion of antibodies, cyclization, stapled peptides, pseudopeptides, cell-penetrating peptide conjugates, conjugation with lipids, and encapsulation in nanocarriers are discussed.

Short to ultrashort peptide-based hydrogels as a platform for biomedical applications

Short peptides have attracted significant attention from researchers in the past few years due to their easy design, synthesis and characterization, diverse functionalisation possibilities, low cost, possibility to make a large range of hierarchical nanostructures and most importantly their high biocompatibility and biodegradability. Generally, short peptides are also relatively more stable than their longer variants, non-immunogenic in nature and many of them self-assemble to provide an exciting range of nanostructures, including hydrogels. Thus, the development of short peptide-based hydrogels has become an area of intense investigation. Although these hydrogels have a water content of greater than 90%, they are surprisingly highly stable structures, and thus have been used for various biomedical applications, including cell therapeutics, drug delivery, tissue engineering and regeneration, contact lenses, biosensors, and wound healing, by different researchers. Herein, we review the progress of research in the rapidly expanding field of short to ultrashort peptide-based hydrogels and their possible applications. Special attention is paid to address and review this field with regard to the stability of peptide-based hydrogels, particularly to enzymatic degradation.

Sugar-derived oxazolone pseudotetrapeptide as γ-turn inducer and anion-selective transporter

The intramolecular cyclization of a C-3-tetrasubstituted furanoid sugar amino acid-derived linear tetrapeptide afforded an oxazolone pseudo-peptide with the formation of an oxazole ring at the C-terminus. A conformational study of the oxazolone pseudo-peptide showed intramolecular C=O···HN(II) hydrogen bonding in a seven-membered ring leading to a γ-turn conformation. This fact was supported by a solution-state NMR and molecular modeling studies. The oxazolone pseudotetrapeptide was found to be a better Cl⁻-selective transporter for which an anion–anion antiport mechanism was established.

Design, Synthesis and Biological Evaluation of Isoxazole-Based CK1 Inhibitors Modified with Chiral Pyrrolidine Scaffolds

In this study, we report on the modification of a 3,4-diaryl-isoxazole-based CK1 inhibitor with chiral pyrrolidine scaffolds to develop potent and selective CK1 inhibitors. The pharmacophore of the lead structure was extended towards the ribose pocket of the adenosine triphosphate (ATP) binding site driven by structure-based drug design. For an upscale compatible multigram synthesis of the functionalized pyrrolidine scaffolds, we used a chiral pool synthetic route starting from methionine. Biological evaluation of key compounds in kinase and cellular assays revealed significant effects of the scaffolds towards activity and selectivity, however, the absolute configuration of the chiral moieties only exhibited a limited effect on inhibitory activity. X-ray crystallographic analysis of ligand-CK1δ complexes confirmed the expected binding mode of the 3,4-diaryl-isoxazole inhibitors. Surprisingly, the original compounds underwent spontaneous Pictet-Spengler cyclization with traces of formaldehyde during the co-crystallization process to form highly potent new ligands. Our data suggests chiral “ribose-like” pyrrolidine scaffolds have interesting potential for modifications of pharmacologically active compounds.

Can Helical Peptides Unwind One Turn at a Time? - Controlled Conformational Transitions in α,β(2,3)-Hybrid Peptides

Unfolding of helical trans-β(2,3) -hybrid peptides with (α-β)n α composition, when executed by increasing solvent polarity or temperature, proceeded in a systematic manner with the turns unwinding sequentially; C-terminal region of these peptides were first to unwind and the process propagated towards N terminus with more and more β residues equilibrating from the gauche to the anti rotameric state across Cα-Cβ . This is evidenced by clear change in their Cβ H signal splitting, (3)JCαH-CβH values, and sequential disappearance of i,i+2 NOEs.

Metal-catalyzed derivatization of C(alpha)-tetrasubstituted amino acids and their use in the synthesis of cyclic peptides

C(alpha)-tetrasubstituted amino acids are important building blocks in the design and preparation of novel peptidomimetics. We report on the functionalization of the C(alpha)-tetrasubstituted THF amino acid rac-5 by copper(I) catalyzed N-arylation reactions. The aryl bromide substituent of rac-5 is replaced by a variety of aliphatic and aromatic amines. Intramolecular N-arylation yielded only small amounts of a cyclic tripeptide 2, whereas cyclic tripeptide ethers 4 and 50 were obtained in an enantiomerically pure form from a palladium(0)-catalyzed intramolecular O-arylation.

Stable right- and left-handed peptide helices containing C(alpha)-tetrasubstituted alpha-amino acids

Short peptidomimetics with stable secondary structures in solution are of interest for applications in chemistry, biology, and medicine. One way to rigidify the backbone of a peptide is the use of cyclic C(alpha)-tetrasubstituted alpha-amino acids (TAAs) like compound 14. The structures resulting from the incorporation of this unnatural amino acid into peptides were investigated. In total, 13 different peptides with a length of up to eight residues and alternating sequences of TAA 14 and (S)- or (R)-valine were synthesized. Their structures were characterized by X-ray diffraction analysis and NMR and CD measurements showing that the all-S-backbone-configured peptides 5 and 6 (SS)(2-3) form right-handed 3(10)-helices, while the all-R-configured peptides 11-13 (RR)(2-4) form left-handed 3(10)-helices in the solid state and solution.

Synthesis of new Calpha-tetrasubstituted alpha-amino acids

C^α-Tetrasubstituted α-amino acids are important building blocks for the synthesis of peptidemimetics with stabilized secondary structure, because of their ability to rigidify the peptide backbone. Recently our group reported a new class of cyclic C^α-tetrasubstituted tetrahydrofuran α-amino acids prepared from methionine and aromatic aldehydes. We now report the extension of this methodology to aliphatic aldehydes. Although such aldehydes are prone to give aldol products under the reaction conditions used, we were able to obtain the target cyclic amino acids in low to moderate yields and in some cases with good diastereoselectivity.

Recent Progress on the Stereoselective Synthesis of Cyclic Quaternary alpha-Amino Acids

The most recent papers describing the stereoselective synthesis of cyclic quaternary alpha-amino acids are collected in this review. The diverse synthetic approaches are classified according to the size of the ring and taking into account the bond that is formed to complete the quaternary skeleton.