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.

Hydrolysis of Acetamide on Low-Index CeO2 Surfaces: Ceria as a Deamidation and General De-esterification Catalyst

Using DFT calculations and acetamide as the main example, we show that ceria is a potential catalyst for the hydrolysis of amide and similar bonds. The overall reaction is endergonic in the gas phase, yielding acetic acid and ammonia, but is slightly exergonic in the aqueous phase, which facilitates ionization of the products (CH₃COO^– and NH₄⁺). Neighboring Ce and O sites on the CeO₂(111), (110), and (100) facets are conducive to the formation of an activated metastable tetrahedral intermediate (TI) complex, followed by C–N bond scission. With van der Waals and solvation effects taken into account, the overall reaction energetics is found to be most favorable on the (111) facet as desorption of acetic acid is much more uphill energetically on (110) and (100). We further suggest that the Ce–O–Ce sites on ceria surfaces can activate X(=Y)–Z type bonds in amides, amidines, and carboxylate and phosphate esters, among many others that we term “generalized esters”. A Brønsted-Evans–Polanyi relationship is identified correlating the stability of the transition and final states of the X–Z generalized ester bond scission. A simple descriptor (ΣΔχ) based on the electronegativity of the atoms that constitute the bond (X, Y, Z) versus those of the catalytic site (O, Ce, Ce) captures the trend in the stability of the transition state of generalized ester bond scission and suggests a direction for modifying ceria for targeting specific organic substrates.

Zr-MOF-808 as Catalyst for Amide Esterification

In this work, zirconium-based metal-organic framework Zr-MOF-808-P has been found to be an efficient and versatile catalyst for amide esterification. Comparing with previously reported homogeneous and heterogeneous catalysts, Zr-MOF-808-P can promote the reaction for a wide range of primary, secondary and tertiary amides with n-butanol as nucleophilic agent. Different alcohols have been employed in amide esterification with quantitative yields. Moreover, the catalyst acts as a heterogeneous catalyst and could be reused for at least five consecutive cycles. The amide esterification mechanism has been studied on the Zr-MOF-808 at molecular level by in situ FTIR spectroscopic technique and kinetic study.

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.

Synthesis of a novel chitosan-based Ce(IV) complex with proteolytic activity in vitro toward edible biological proteins

The occurrence of enzymatic activities is attributed to proper spatial organization of functional groups from first principles. A novel chitosan-based Ce(IV) complex (CC[Ce(IV)]), an artificial metalloproteinase, was synthesized by attaching cyclen, Ce(IV), and chlorophyll-Cu(II) to a chitosan-based matrix. The enzymatic hydrolytic efficiency (HE) and the procedure of catalyzing myoglobin (Mb) by CC[Ce(IV)] in vitro were investigated using spectrophotometry, electrophoresis, and liquid chromatography. The results showed that the HE of Mb was up to 60% at 60°C within 24h, displaying a catalytic proficiency. The pseudo-first-order kinetic constant (kobs) for CC[Ce(IV)] treatment within 24h was 3.85×10(-2)h(-1), higher than that for α-chymotrypsin treatment, which was 2.63×10(-2)h(-1). Moreover, the peptide bond derived from Asp-Phe/Phe-Asp in Mb could be specifically cleaved by CC[Ce(IV)], which could simulate the functionality of α-chymotrypsin. This work provides an experimental basis for potential utilization of the chitosan-based Ce(IV) complexes in the food industry.

Mechanistic studies of amide bond scission during acidolytic deprotection of Pip containing peptide

Unusual TFA catalyzed cleavage reaction is reported for peptide containing pipecolic acid residues. Although the use of TFA under standard cleavage conditions is sufficiently mild to prevent degradation of the desired products, the amide bond between consecutive pipecolic acid residues is unexpectedly hydrolyzed by standard TFA treatment. The hydrolysis is proposed to proceed via an oxazolinium ion intermediate. This mechanism is supported by H/D exchange as observed by ESI-MS and NMR experiments.

Interaction of Molybdocene Dichloride with Cysteine-Containing Peptides: Coordination, Regioselective Hydrolysis, and Intramolecular Aminolysis

Reactions of the organometallic compound molybdocene dichloride (Cp2MoCl2, Cp = eta5-cyclopentadienyl) with the cysteine-containing peptides L-cysteinylglycine (Cys-Gly), N-acetyl-L-cysteine (AcCys), glycyl-L-cysteine (Gly-Cys), glycyl-L-cysteinylglycine (Gly-Cys-Gly), and gamma-L-glutamyl-L-cysteinylglycine (glutathione, GSH) have been studied in aqueous solution in the pH range 2-9. The dipeptides Cys-Gly and Gly-Cys and the acetylated amino acid AcCys form 1:1 and 2:1 complexes of composition [Cp2Mo(peptide-S)(OH(2))]n+/- and [Cp2Mo(peptide-S)2]n+/- as well as the chelates [Cp2Mo(AcCys-S,O)], [Cp2Mo(Gly-Cys-S,O)]+, and [Cp2Mo(Cys-Gly-S,N)] with the Cp2Mo2+ unit binding to the deprotonated thiolate group and the free amino or carboxylate group of the cysteine residue. Upon treatment of Gly-Cys-Gly and the naturally occurring tripeptide GSH with Cp2MoCl2 at elevated temperature, release of free glycine was observed. The Cp2Mo2+ entity coordinates to the thiolate group of GSH and mediates regioselective hydrolysis of the Cys-Gly peptide bond by intramolecular metal hydroxide activation. Cp2Mo2+-promoted hydrolysis of GSH was followed at pD 7.4 and 5.2 and 40 and 60 degrees C. By contrast, the Cys-Gly bond in [Cp2Mo(Gly-Cys-Gly-S,N)] is cleaved by intramolecular aminolysis at pD > or = 7.4 and 60 degrees C leading to glycine and the Cp2Mo2+ complex of the 2,5-diketopiperazine derivative cyclo-(Gly-Cys). Chelating coordination of the Cp2Mo2+ moiety to the thiolate group and to the deprotonated amide nitrogen of the tripeptide changes the configuration of the peptide bond from (preferred) trans to cis, thus enabling nucleophilic attack of the primary amino group at the Cys-Gly bond. The reaction product [Cp2Mo{cyclo-(Gly-Cys)}] x 2H2O has been characterized by X-ray crystallography.

Synergic effect of two metal centers in catalytic hydrolysis of methionine-containing peptides promoted by dinuclear palladium(ii) hexaazacyclooctadecane complex

The species obtained by the reaction of [Pd2([18]aneN6)Cl2](ClO4)2(where [18]aneN6 is 1,4,7,10,13,16-hexaazacyclooctadecane) with AgBF4 have been determined by electrospray ionization mass spectrometry (ESI-MS) to be an equilibrium mixture of three major types of dinuclear Pd(II) complex cations, [Pd2(mu-O)([18]aneN6)]2+, [Pd2(mu-OH)([18]aneN6)]3+ and [Pd2(H2O)(OH)([18]aneN6)](3+), in aqueous solution. The hydroxo-group-bridged one, [Pd2(mu-OH)([18]aneN6)]3+, is a dominant species, whose crystal structure has been obtained. The crystal structure of [Pd2(mu-OH)([18]aneN6)](ClO4)3 shows that each Pd(II) ion in the dinuclear complex is tetra-coordinated by three nitrogen atoms and one hydroxo group bridge in a distorted square configuration. The two Pd(II) ions are 3.09 A apart from each other. The dinuclear Pd(II) complex cations [Pd2(mu-OH)([18]aneN6)]3+ and [Pd2(H2O)(OH)([18]aneN6)]3+ can efficiently catalyze hydrolysis of the amide bond involving the carbonyl group of methionine in methionine-containing peptides with turnover number of larger than 20. In these hydrolytic reactions, the two Pd(II) ions are synergic; one Pd(II) ion anchors to the side chain of methionine and the other one delivers hydroxo group or aqua ligand to carbonyl carbon of methionine, or acts as a Lewis acid to activate the carbonyl group of methionine, resulting in cleavage of Met-X bond. The binding constant of dinuclear Pd(II) complex cations with AcMet-Gly and AcMet were determined by 1H NMR titration to be 282 +/- 2 M(-1) and 366 +/- 4 M(-1), respectively. The relatively low binding constants enable the catalytic cycle and the possible catalytic mechanism is proposed. This is the first artificial mimic of metallopeptidases with two metal active centers.

Unprecedented Acceleration of Zirconium(IV)-Assisted Peptide Hydrolysis at Neutral pH

4,13-Diaza-18-crown-6 substantially increases the rate of zirconium(IV) hydrolysis of unactivated peptide amide bonds under near-physiological conditions of temperature and pH. In the presence of this azacrown ether, ZrCl(4) efficiently hydrolyses both neutral and negatively charged peptides (pH 7.0-7.3, 37-60 degrees C).

Interaction of Palladium(II) and Platinum(II) Complexes with Microperoxidase-11 Studied by Electrospray Mass Spectrometry and MS/MS Analysis

Interactions of cis-[Pd(en)(H(2)O)(2)](2+) (en, ethylenediamine) and cis-[Pt(NH(3))(2)(H(2)O)(2)](2+) with microperoxidase-11 (MP-11) in a molar ratio of 1:1 or 2:1 at pH 1.4 were investigated via electrospray mass spectrometry and MS/MS analysis at room temperature and at 40 degrees C with an incubation time of 2 or 3 days. The composition of the Pd(II)- and Pt(II)-anchored MP-11 was confirmed on the basis of the precise molecular mass and the simulated isotope distribution pattern. MS/MS analysis revealed that the Pd(II) center anchored to the side chain of Cys7 as Pd(II) and MP-11 were mixed in an equimolar ratio and to side chains of Cys7 and Cys4 as Pd(II) and MP-11 mixed in a 2:1 molar ratio. When Pt(II) and MP-11 were mixed in a 2:1 molar ratio, Pt(II) first anchored to the side chain of Cys7, and then to the side chain of Cys4 with time. The initial coordination of Pd(II) and Pt(II) to the side chain of Cys7 is the essential step for the Pd(II)- and Pt(II)-promoted cleavage of the His8-Thr9 bond in MP-11. These results support the hypothesis that the Pd(II)-mediated cleavage of the His18-Thr19 bond in cytochorome c is due to the identical binding mode.

Nature of cerium(III)- and lanthanum(III)-induced aggregation of human erythrocyte membrane proteins

To clarify the nature of the aggregation of membrane proteins (MP) induced by lanthanide cations (Lns), the interaction of cerium(III) (Ce3+) and lanthanum(III)(La3+) with erythrocyte membrane proteins was studied by means of SDS-PAGE, light scattering measurement, fluorescence, CD and FTIR spectra. The results showed that Ce3+ and La3+ induce protein aggregation not only by Lns non-covalent binding and cross-linking, but also by oxidative cross-linking through disulfide bond formation. As demonstrated by intrinsic fluorescence, CD and FTIR spectra studies, the aggregation was accompanied by the conformation changes with tryptophane residues exposing to more hydrophobic environment and the decreasing alpha-helix and beta-sheet contents. By stopped-flow studies, protein aggregation was shown to be a slow change, which is initiated by rapid Lns binding and then followed by subsequent conformational changes.

Use of a fluorescence microplate reader for the detection and characterization of metal-assisted peptide hydrolysis

Metal ions and complexes that hydrolyze peptides under nondenaturing conditions of temperature and pH hold great promise for use in protein structural studies. However, the extreme stability of the peptide amide bond has placed limits on the number of reagents available. In addition, the development of new cleavage strategies has been hindered by the fact that no facile procedure exists for the detection and characterization of metal-assisted peptide hydrolysis. Here we describe a rapid assay in which a microplate reader is used to detect fluorescence produced by the reaction of fluorescamine with hydrolyzed peptides. We have employed this assay to detect Zn(II) and Pd(II)-assisted peptide hydrolysis in multiple samples and in each case have extended our approach to a successful analysis of reaction kinetics. Aliquots from multiple time points are treated with fluorescamine in a single 96-well plate. Because the plate is scanned in a microplate reader in only 58 s, the assay is very convenient compared to conventional approaches which rely on NMR and HPLC to monitor individual reactions. Using our assay, rate constants and half-lives are easily derived from the kinetic data by means of linear regression curve fits of triplicate runs.

Lability of N-alkylated peptides towards TFA cleavage

Trifluoroacetic acid (TFA) is a common reagent in both solid-phase and solution peptide synthesis. It is used for the deprotection and/or cleavage of the synthesized peptide from the resin. The use of TFA under these standardized conditions is thought to be sufficiently mild, thereby preventing degradation of the desired product. However, peptides of the general structure R1-(N-alkyl X1)-X2-R2 are hydrolyzed by standard TFA solid-phase peptide synthesis (SPPS) cleavage/deprotection conditions providing fragments R1-(N-alkyl X1)-OH and H-X2-R2. The fragmentation is observed during a TFA cleavage both from the resin and in solution. The hydrolysis is proposed to proceed via an oxazolone-like intermediate in which equilibration of the chiral center of the N-alkylated residue occurs. This mechanism is supported by H/D exchange as observed MS and NMR in conjunction with HPLC.