[Metal ion-catalytic cleavage of the (-CO-NH-)-linkage in peptides and N-acetyl-amino acids]

Macrocyclic Ligand Coordinating Amide-Arm Hydrolysis Reaction Activation in Aqueous Solutions: Tetravalent Uranium Does It Better

Chelation of lanthanide and actinide cations within a suitable macrocyclic ligand often results in a rigid, kinetically inert, and thermodynamically stable complex. A benchmark for such cation-ligand suitability are cyclen-derived macrocyclic ligands, frequently used as large cation hosts for various applications. Herein, a comprehensive study of the 1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane ligand (DOTAM) chelates of UIV and CeIII and their properties in aqueous solutions is presented. By employing multiple analysis techniques, including X-ray crystallography, UV-vis absorbance, 1H NMR, UPLC-MS, cyclic voltammetry, and differential pulse voltammetry, the study has revealed that the two aqueous complexes undergo a spontaneous, gradual, and stepwise hydrolysis of each of the coordinated amides toward carboxylates. The coordination of UIV in the studied reaction has been shown to significantly enhance the reaction rate, leading to an acceleration of up to 6 orders of magnitude compared to the natural process of simple aqueous amides at room temperature. An attempt to describe the unusual chelated metal cation amide-activation feature, based on the relatively lower rigidity of the complex structure, is presented. Additionally, the electrochemical properties of the complex series are discussed in detail, along with the limitations of the analytical methods employed.

Amide Bond Activation of Biological Molecules

Amide bonds are the most prevalent structures found in organic molecules and various biomolecules such as peptides, proteins, DNA, and RNA. The unique feature of amide bonds is their ability to form resonating structures, thus, they are highly stable and adopt particular three-dimensional structures, which, in turn, are responsible for their functions. The main focus of this review article is to report the methodologies for the activation of the unactivated amide bonds present in biomolecules, which includes the enzymatic approach, metal complexes, and non-metal based methods. This article also discusses some of the applications of amide bond activation approaches in the sequencing of proteins and the synthesis of peptide acids, esters, amides, and thioesters.

Hydrolysis of Methionine- and Histidine-Containing Peptides Promoted by Dinuclear Platinum(II) Complexes with Benzodiazines as Bridging Ligands: Influence of Ligand Structure on the Catalytic Ability of Platinum(II) Complexes

Dinuclear platinum(II) complexes, [{Pt(en)Cl}₂(μ-qx)]Cl₂·2H₂O (1), [{Pt(en)Cl}₂(μ-qz)](ClO₄)₂ (2), and [{Pt(en)Cl}₂(μ-phtz)]Cl₂·4H₂O (3), were synthesized and characterized by different spectroscopic techniques. The crystal structure of 1 was determined by single-crystal X-ray diffraction analysis, while the DFT M06-2X method was applied in order to optimize the structures of 1-3. The chlorido Pt(II) complexes 1-3 were converted into the corresponding aqua species 1a-3a, and their reactions with an equimolar amount of Ac-L-Met-Gly and Ac-L-His-Gly dipeptides were studied by ¹H NMR spectroscopy in the pH range 2.0 < pH < 2.5 at 37°C. It was found that, in all investigated reactions with the Ac-L-Met-Gly dipeptide, the cleavage of the Met-Gly amide bond had occurred, but complexes 2a and 3a showed lower catalytic activity than 1a. However, in the reactions with Ac-L-His-Gly dipeptide, the hydrolysis of the amide bond involving the carboxylic group of histidine was observed only with complex 1a. The observed disparity in the catalytic activity of these complexes is thought to be due to different relative positioning of nitrogen atoms in the bridging qx, qz, and phtz ligands and consequent variation in the intramolecular separation of the two platinum(II) metal centers.

Site-Selective Peptide/Protein Cleavage

Site-selective peptide/protein degradation through chemical cleavage methods is an important modification of biologically relevant macromolecules which complements enzymatic hydrolysis. In this review, recent progress in chemical, site-selective peptide bond cleavage is overviewed, with an emphasis on postulated mechanisms and their implications on reactivity, selectivity, and substrate scope.