Native Chemical Ligation of Highly Hydrophobic Peptides in Ionic Liquid-Containing Media

Do Ionic Liquids Exhibit the Required Characteristics to Dissolve, Extract, Stabilize, and Purify Proteins? Past-Present-Future Assessment

Proteins are highly labile molecules, thus requiring the presence of appropriate solvents and excipients in their liquid milieu to keep their stability and biological activity. In this field, ionic liquids (ILs) have gained momentum in the past years, with a relevant number of works reporting their successful use to dissolve, stabilize, extract, and purify proteins. Different approaches in protein-IL systems have been reported, namely, proteins dissolved in (i) neat ILs, (ii) ILs as co-solvents, (iii) ILs as adjuvants, (iv) ILs as surfactants, (v) ILs as phase-forming components of aqueous biphasic systems, and (vi) IL-polymer-protein/peptide conjugates. Herein, we critically analyze the works published to date and provide a comprehensive understanding of the IL-protein interactions affecting the stability, conformational alteration, unfolding, misfolding, and refolding of proteins while providing directions for future studies in view of imminent applications. Overall, it has been found that the stability or purification of proteins by ILs is bispecific and depends on the structure of both the IL and the protein. The most promising IL-protein systems are identified, which is valuable when foreseeing market applications of ILs, e.g., in "protein packaging" and "detergent applications". Future directions and other possibilities of IL-protein systems in light-harvesting and biotechnology/biomedical applications are discussed.

Mimicking Nonribosomal Peptides from the Marine Actinomycete Streptomyces sp. H-KF8 Leads to Antimicrobial Peptides

Microorganisms within the marine environment have been shown to be very effective sources of naturally produced antimicrobial peptides (AMPs). Several nonribosomal peptides were identified based on genome mining predictions of Streptomyces sp. H-KF8, a marine Actinomycetota isolated from a remote Northern Chilean Patagonian fjord. Based on these predictions, a series of eight peptides, including cyclic peptides, were designed and chemically synthesized. Six of these peptides showed antimicrobial activity. Mode of action studies suggest that two of these peptides potentially act on the cell membrane via a novel mechanism allowing the passage of small ions, resulting in the dissipation of the membrane potential. This study shows that though structurally similar peptides, determined by NMR spectroscopy, the incorporation of small sequence mutations results in a dramatic influence on their bioactivity including mode of action. The qualified hit sequence can serve as a basis for more potent AMPs in future studies.

Traceless Click-Assisted Native Chemical Ligation Enabled by Protecting Dibenzocyclooctyne from Acid-Mediated Rearrangement with Copper(I)

The scope of proteins accessible to total chemical synthesis via native chemical ligation (NCL) is often limited by slow ligation kinetics. Here we describe Click-Assisted NCL (CAN), in which peptides are incorporated with traceless "helping hand" lysine linkers that enable addition of dibenzocyclooctyne (DBCO) and azide handles. The resulting strain-promoted alkyne-azide cycloaddition (SPAAC) increases their effective concentration to greatly accelerate ligations. We demonstrate that copper(I) protects DBCO from acid-mediated rearrangement during acidic peptide cleavage, enabling direct production of DBCO synthetic peptides. Excitingly, triazole-linked model peptides ligated rapidly and accumulated little side product due to the fast reaction time. Using the E. coli ribosomal subunit L32 as a model protein, we further demonstrate that SPAAC, ligation, desulfurization, and linker cleavage steps can be performed in one pot. CAN is a useful method for overcoming challenging ligations involving sterically hindered junctions. Additionally, CAN is anticipated to be an important stepping stone toward a multisegment, one-pot, templated ligation system.