Facile Solid-Phase Synthesis of Well-Defined Defect Lysine Dendrimers

An efficient solid-phase method has been reported to prepare well-defined lysine defect dendrimers. Using orthogonally protected lysine residues, pure G2 to G4 lysine defect dendrimers were prepared with 48-95% yields within 13 h. Remarkably, high-purity products were collected via precipitation without further purification steps. This method was applied to prepare a pair of 4-carboxyphenylboronic acid-decorated defect dendrimers (16 and 17), which possessed the same number of boronic acids. The binding affinity of 16, in which the ε-amines of G1 lysine are fractured, for glucose and sorbitol was 4 times that of 17. This investigation indicated the role of allocation and distribution of peripheries for the dendrimer's properties and activity.

Mechanistic study of the solubilization effect of basic amino acids on a poorly water-soluble drug

Amino acids have shown promising abilities to form complexes with poorly water-soluble drugs and improve their physicochemical properties for a better dissolution profile through molecular interactions. Salt formation via ionization between acidic drugs and basic amino acids is known as the major contributor to solubility enhancement. However, the mechanism of solubility enhancement due to non-ionic interactions, which is less pH-dependent, remains unclear. The aim of this study is to evaluate non-ionic interactions between a model acidic drug, indomethacin (IND), and basic amino acids, arginine, lysine and histidine, in water. At low concentrations of amino acids, IND-arginine and IND-lysine complexes have shown a linear relationship (A_L-type phase solubility diagram) between IND solubility and amino acid concentration, producing ∼1 : 1 stoichiometry of drug-amino acid complexes as expected due to the strong electrostatic interactions. However, IND-histidine complexes have shown a nonlinear relationship with lower improvement in IND solubility due to the weaker electrostatic interactions when compared to arginine and lysine. Interestingly, the results have also shown that at high arginine concentrations, the linearity was lost between IND solubility and amino acid concentration with a negative diversion from linearity, following the type-A_N phase solubility. This is indicative that the electrostatic interaction is being interrupted by non-electrostatic interactions, as seen with histidine. The IND-lysine complex, on the other hand, showed a complex curvature phase solubility diagram (type B_S) as lysine self-assembles and polymerizes at higher concentrations. The freeze-dried drug-amino acid solids were further characterized using thermal analysis and infrared spectroscopy, with results showing the involvement of weak non-ionic interactions. This study shows that the solubility improvement of an insoluble drug in the presence of basic amino acids was due to both non-ionic and ionic interactions.

Improved Handling of Peptide Segments Using Side Chain-Based "Helping Hand" Solubilizing Tools

Maintaining high, or even sufficient, solubility of every peptide segment in chemical protein synthesis (CPS) remains a critical challenge; insolubility of just a single peptide segment can thwart a total synthesis venture. Multiple approaches have been used to address this challenge, most commonly by employing a chemical tool to temporarily improve peptide solubility. In this chapter, we discuss chemical tools for introducing semipermanent solubilizing sequences (termed helping hands) at the side chains of Lys and Glu residues. We describe the synthesis, incorporation by Fmoc-SPPS, and cleavage conditions for utilizing these two tools. For Lys sites, we discuss the Fmoc-Ddap-OH dimedone-based linker, which is achiral, synthesized in one step, can be introduced directly at primary amines, and is removed using hydroxylamine (or hydrazine). For Glu sites, we detail the new Fmoc-SPPS building block, Fmoc-Glu(AlHx)-OH, which can be prepared in an efficient process over two purifications. Solubilizing sequences are introduced directly on-resin and later cleaved with palladium-catalyzed transfer under aqueous conditions to restore a native Glu side chain. These two chemical tools are straightforward to prepare and implement, and we anticipate continued usage in "difficult" peptide segments following the protocols described herein.

A trimethyllysine-containing trityl tag for solubilizing hydrophobic peptides

Hydrophobic membrane peptides/proteins having low water solubility are often difficult to prepare. To overcome this issue, temporal introduction of solubilizing tags has been demonstrated to be beneficial. Following our recent work on the solubilization of a difficult target by using a hydrophilic oligo-Lys tag bearing a trityl linker (Trt-K method), this paper describes a comparative study of the solubilizing abilities of several peptidic trityl tags containing Lys, Arg, Glu, Asn, N^ε-tri-Me-Lys or Cys-sulfonate using two hydrophobic model peptides. Among the tags evaluated, that containing N^ε-tri-Me-Lys exhibits superior solubilizing ability.

A Helping Hand to Overcome Solubility Challenges in Chemical Protein Synthesis

Although native chemical ligation (NCL) and related chemoselective ligation approaches provide an elegant method to stitch together unprotected peptides, the handling and purification of insoluble and aggregation-prone peptides and assembly intermediates create a bottleneck to routinely preparing large proteins by completely synthetic means. In this work, we introduce a new general tool, Fmoc-Ddae-OH, N-Fmoc-1-(4,4-dimethyl-2,6-dioxocyclo-hexylidene)-3-[2-(2-aminoethoxy)ethoxy]-propan-1-ol, a heterobifunctional traceless linker for temporarily attaching highly solubilizing peptide sequences ("helping hands") onto insoluble peptides. This tool is implemented in three simple and nearly quantitative steps: (i) on-resin incorporation of the linker at a Lys residue ε-amine, (ii) Fmoc-SPPS elongation of a desired solubilizing sequence, and (iii) in-solution removal of the solubilizing sequence using mild aqueous hydrazine to cleave the Ddae linker after NCL-based assembly. Successful introduction and removal of a Lys6 helping hand is first demonstrated in two model systems (Ebola virus C20 peptide and the 70-residue ribosomal protein L31). It is then applied to the challenging chemical synthesis of the 97-residue co-chaperonin GroES, which contains a highly insoluble C-terminal segment that is rescued by a helping hand. Importantly, the Ddae linker can be cleaved in one pot following NCL or desulfurization. The purity, structure, and chaperone activity of synthetic l-GroES were validated with respect to a recombinant control. Additionally, the helping hand enabled synthesis of d-GroES, which was inactive in a heterochiral mixture with recombinant GroEL, providing additional insight into chaperone specificity. Ultimately, this simple, robust, and easy-to-use tool is expected to be broadly applicable for the synthesis of challenging peptides and proteins.

The road to the synthesis of "difficult peptides"

The last decade has witnessed a renaissance of peptides as drugs. This progress, together with advances in the structural behavior of peptides, has attracted the interest of the pharmaceutical industry in these molecules as potential APIs. In the past, major peptide-based drugs were inspired by sequences extracted from natural structures of low molecular weight. In contrast, nowadays, the peptides being studied by academic and industrial groups comprise more sophisticated sequences. For instance, they consist of long amino acid chains and show a high tendency to form aggregates. Some researchers have claimed that preparing medium-sized proteins is now feasible with chemical ligation techniques, in contrast to medium-sized peptide syntheses. The complexity associated with the synthesis of certain peptides is exemplified by the so-called "difficult peptides", a concept introduced in the 80's. This refers to sequences that show inter- or intra-molecular β-sheet interactions significant enough to form aggregates during peptide synthesis. These structural associations are stabilized and mediated by non-covalent hydrogen bonds that arise on the backbone of the peptide and-depending on the sequence-are favored. The tendency of peptide chains to aggregate is translated into a list of common behavioral features attributed to "difficult peptides" which hinder their synthesis. In this regard, this manuscript summarizes the strategies used to overcome the inherent difficulties associated with the synthesis of known "difficult peptides". Here we evaluate several external factors, as well as methods to incorporate chemical modifications into sequences, in order to describe the strategies that are effective for the synthesis of "difficult peptides". These approaches have been classified and ordered to provide an extensive guide for achieving the synthesis of peptides with the aforementioned features.