Insulin is a small protein crucial for regulating the blood glucose level in all animals. Since 1922 it has been used for the treatment of patients with diabetes. Despite consisting of just 51 amino acids, insulin contains 17 of the proteinogenic amino acids, A‐ and B‐chains, three disulfide bridges, and it folds with 3 α‐helices and a short β‐sheet segment. Insulin associates into dimers and further into hexamers with stabilization by Zn2+ and phenolic ligands. Selective chemical modification of proteins is at the forefront of developments in chemical biology and biopharmaceuticals. Insulin's structure has made it amenable to organic and inorganic chemical reactions. This Review provides a synthetic organic chemistry perspective on this small protein. It gives an overview of key chemical and physico‐chemical aspects of the insulin molecule, with a focus on chemoselective reactions. This includes N‐acylations at the N‐termini or at LysB29 by pH control, introduction of protecting groups on insulin, binding of metal ions, ligands to control the nano‐scale assembly of insulin, and more.

The reaction of unprotected carbohydrates with aminooxy reagents to provide oximes is a key method for the construction of glycoconjugates. Aniline and derivatives serve as organocatalysts for the formation of oximes from simple aldehydes, and we have previously reported that aniline also catalyzes the formation of oximes from the more complex aldehydes, carbohydrates. Here, we present a comprehensive study of the effect of aniline analogues on the formation of carbohydrate oximes and related glycoconjugates depending on organocatalyst structure, pH, nucleophile, and carbohydrate, covering more than 150 different reaction conditions. The observed superiority of the 1,4-diaminobenzene (PDA) catalyst at neutral pH is rationalized by NMR analyses and DFT studies of reaction intermediates. Carbohydrate oxime formation at pH 7 is demonstrated by the formation of a bioactive glycoconjugate from a labile, decorated octasaccharide originating from exopolysaccharides of the soil bacterium Mesorhizobium loti. This study of glycoconjugate formation includes the first direct comparison of aniline-catalyzed reaction rates and equilibrium constants for different classes of nucleophiles, including primary oxyamines, secondary N-alkyl oxyamines, as well as aryl and arylsulfonyl hydrazides. We identified 1,4-diaminobenzene as a superior catalyst for the construction of oxime-linked glycoconjugates under mild conditions.

Controlled self-assembly (SA) of proteins offers the possibility to tune their properties or to create new materials. Herein, we present the synthesis of a modified human insulin (HI) with two distinct metal-ion binding sites, one native, the other abiotic, enabling hierarchical SA through coordination with two different metal ions. Selective attachment of an abiotic 2,2'-bipyridine (bipy) ligand to HI, yielding HI-bipy, enabled Zn(II)-binding hexamers to SA into trimers of hexamers, [[HI-bipy]6]3, driven by octahedral coordination to a Fe(II)  ion. The structures were studied in solution by small-angle X-ray scattering and on surfaces with AFM. The abiotic metal ligand had a higher affinity for Fe(II) than Zn(II)  ions, enabling control of the hexamer formation with Zn(II) and the formation of trimers of hexamers with Fe(II)  ions. This precise control of protein SA to give oligomers of oligomers provides nanoscale structures with potential applications in nanomedicine.

Metal ion-induced self-assembly (SA) of proteins into higher-order structures can provide new, dynamic nano-assemblies. Here, the synthesis and characterization of a human insulin (HI) analog modified at LysB29 with the...

Controlled self-assembly (SA) of proteins offers the possibility to tune their properties or to create new materials. Herein, we present the synthesis of am odified human insulin (HI) with two distinct metal-ion binding sites,o ne native,t he other abiotic,e nabling hierarchical SA through coordination with two different metal ions.Selective attachment of an abiotic 2,2'-bipyridine (bipy) ligand to HI, yielding HI-bipy, enabled Zn II -binding hexamers to SA into trimers of hexamers, [[HI-bipy] 6 ] 3 ,driven by octahedral coordination to aF e II ion. The structures were studied in solution by small-angle X-ray scattering and on surfaces with AFM. The abiotic metal ligand had ahigher affinity for Fe II than Zn II ions,enabling control of the hexamer formation with Zn II and the formation of trimers of hexamers with Fe II ions.This precise control of protein SA to give oligomers of oligomers provides nanoscale structures with potential applications in nanomedicine.Metal-ion coordination plays akey role in protein function and self-assembly (SA). [1] Understanding and engineering protein SA is highly relevant, given its role in signaling and function. Abiotic methods to control protein SA offer the prospect of modulating their properties.T ransition-metalbased SA of smaller organic compounds has enabled construction of specific 3D structures such as metal-organic frameworks. [2] Modification of peptides with metal-ion-binding ligands has enabled formation of 3D collagen networks. [3] Examples of linear protein assemblies using asingle metal ion type include astreptavidin protein network with apreorganized terpyridine-biotin linker, [4] and the polymerization of ah eme protein. [5] Another approach is to use native metalion-coordinating amino acids to link protein surfaces.Exam-ples include model systems with cytochrome ct ogether with proteins such as ferritin, [6] T4 lysozyme and maltose-binding protein, [7] them onomeric Rab4-binding domain of rabenosyn, [8] and Ni II -directed formation of glutathione Stransferase (GST) nanorings. [9] Tr ansition-metal coordination was used in enzyme de novo design, [10] for example,inanartificial metalloenzyme combining Zn II and Hg II ions. [11] De novo design has combined 2,2'-bipyridine (bipy) with anative-like Cu-binding site in athree-helix bundle metalloprotein. [12] In contrast, our strategy is to control protein quaternary structure by selective introduction of an abiotic metal ligand which enables SA by metal-ion coordination. [13] Human insulin (HI) is aprotein crucial for regulating the blood glucose level. In solution, HI forms homo-oligomers depending on pH value and concentration. Tw oZ n II ions coordinate to residue HisB10H in HI monomers forming atoroidal hexamer with aheight of about 40 and adiameter of circa 50 . [14] Hexamers with Cd II ,Co II ,Ni II ,Cu II ,Mn II ,Fe II , and Co III ions were observed. [15,16] Control of HI SA and disassembly is essential for tuning the therapeutic profile. Introduction of an additional Zn II binding site using natural side...