Deprotonated Amide Nitrogen Research Articles

Copper(II) complexes of dipeptides containing aspartyl, glutamyl, and histidyl residues in the side chain

Copper(II) complexes of various dipeptides containing carboxylate and imidazole-N3 donors (α- and γ-GluVal, α- and β-AspGly, β-AspHis, γ-GluHis, and homocarnosine) were studied by potentiometric and spectroscopic methods in solution. It was found that the presence of an α-carboxylate group in the N-terminal part of a peptide molecule significantly enhances the metal-binding ability of the ligands as a consequence of stable bis complex formation involving amino acid-like coordination. The interaction of copper(II) with β-AspHis was characterized by the formation of an imidazole-bridged dimeric species, while the formation of bis complexes was detected in the case of γ-GluHis. Binding of the imidazole N3 and deprotonated amide nitrogen was presumed in the copper(II)-homocarnosine system.

Inhibition of carbonic anhydrases I and II by N-unsubstituted carbamate esters.

We previously showed that the zinc metalloenzyme carbonic anhydrases (CA I and II isozymes) bind "neutral" amides and related compounds as anions through coordination of their deprotonated amide nitrogen to the active site zinc (Rogers, J. I., Mukherjee, J., and Khalifah, R. G. (1987) Biochemistry 26, 5672-5679). Urethan, the ethyl carbamate ester, was among such compounds. The present study was designed to test whether other N-unsubstituted carbamate esters of pharmacological interest (as sedatives, hypnotics, anxiolytics, and skeletal muscle relaxants) were capable of binding to CA in the same manner. We studied the interaction of human CA I and II with urethan, phenyl carbamate, ethinamate, meprobamate, and methocarbamol. Phenyl carbamate studies were greatly complicated by its uncatalyzed hydrolysis via an elimination mechanism to form cyanate, a powerful CA inhibitor. In general, the compounds display: 1) slow on-off inhibition binding kinetics in the seconds range, 2) maximal inhibitor affinity at alkaline pH, and 3) characteristic three-band visible spectra of their complexes with cobalt-substituted CA I. These properties are shared with the previously studied amide inhibitors and are taken as evidence that the deprotonated carbamate nitrogen coordinates to the active site metal ion. CA I appeared to bind carbamate esters more tightly than CA II, an unusual 1000-fold selectivity being seen in the case of methocarbamol. The inhibition by these drugs is not sufficiently strong to implicate CA I and II in their mechanism of action. However, it does suggest the possible existence of previously unsuspected similarities between binding to CA and to their physiological receptors or targets, particularly the involvement of zinc.

Immobilized metal ion affinity chromatography of synthetic peptides: Binding via the α-amino group

Peptides synthesized by the solid-phase method can be efficiently purified in a single immobilized metal affinity chromatography step based on interaction with the α-amino group if, after coupling of each amino acid residue, unreacted amino groups are irreversibly blocked by acetylation and if no strongly metal-binding amino acids (His, Trp, Cys) are present in the sequence. A difference in basicity for α- and ε-amino functions of ca. 2 pH units is sufficiently large to allow selective binding of peptides to immobilized metal ions via the unprotonated α-amino group. The binding is pH-dependent: on Cu 2+- and Ni 2+-loaded supports most peptides are maximally retarded at pH values around 7.5 and 8.5, respectively. The decreased binding strength at lower pH values is due to protonation of the α-amino function, whereas the reduced affinity at higher pH is caused by metal ion transfer from the matrix to the peptide. The metal ion is captured in a multidentate chelate when, in addition to the α-amino group, up to three adjacent deprotonated amide nitrogens are coordinated to the metal. If the pH is raised further, additional metal ions may be bound in biuret-like structures. Immobilized Ni 2+, owing to its higher selectivity and affinity, is the preferred chromatographic support if slightly basic conditions can be tolerated.

Strong interactions of anionic peptides and alkaline earth metal ions: bis(peptide) complexes in the gas phase

Small peptides interact with alkaline earth metal ions under conditions of fast atom bombardment to form abundant metal-bis(peptide) complexes of the composition [2pept+Met 2+ -3H + ] - . The structure of these gas-phase bis(peptide) complexes as elucidated by their collisionally activated decompositions (CAD), involves metal ion binding to the deprotonated C-terminal carboxylate groups of both peptides and to a deprotonated amide nitrogen of one of the constituent peptides.

Deprotonated amide nitrogen co-ordination to the cadmium(II) ion in ternary 2,2′-bipyridine complexes with N-sulfonyl amino acids

A solid-state investigation of cadmium(II) and zinc(II) ternary 2,2′-bipyridine (bipy), complexes of N-(benzenesulfonyl)glycine (bsgly) and N-(toluene-p-sulfonyl)glycine (tsgly) is presented. The crystal structure of [{Cd(bipy)(tsglyO)2}2]1, separated from preparations at pH 7, and of [{Cd(bipy)(bsglyNO)}4]·8H2O 2 separated at pH 11 are described. Complex 1 crystallizes in the triclinic space group P, Z= 2, a= 10.474(10), b= 15.902(3), c= 18.202(9)A, α= 98.49(2), β= 91.17(5) and γ= 98.31(3)°. The structure consists of dimeric units in which the cadmium atoms are bridged by two carboxylic oxygens. Metal co-ordination is via the two nitrogens of a bipy molecule and four carboxylate oxygens from the amino acid monoanion. A long Cd ⋯ O contact completes seven co-ordination. The final R index was 0.0509. Complex 2 crystallizes in the monoclinic space group P21/c, Z= 4, a= 12.525(5), b= 15.251(4), c= 21.717(3)A and β= 92.36(3)°. The structure is built up of tetrameric units in which the cadmium atoms are joined by monoatomic bridging carboxylic oxygens. Each cadmium is six-co-ordinated through bipy nitrogens and sulfonamide nitrogen and carboxylate oxygen of the amino acid molecules. The final R index was 0.0398. Infrared data for the homologous zinc(II) complexes, very similar to those of the above derivatives, indicate that zinc is also able to promote sulfonamide nitrogen deprotonation in these amino acid ligands in the solid state.

Metal binding ability of hypermodified nucleosides of t-RNA. Potentiometric and spectroscopic studies on the metal complexes of N-[(9-β-D-ribofuranosylpurin-6-yl)carbamoyl] threonine

Copper(II), nickel(II), zinc(II), manganese(II), and magnesium(II) complexes of t 6A (N-[9-β-D-ribofuranosylpurin-6-yl)carbamoyl] threonine and t 6Ade (N 6(threoninocarbonyl)adenine) were studied by potentiometric and spectroscopic methods. It was found that t 6Ade has three dissociable protons in the accessible pH range (N 1 and N 9 of purine and carboxylate), while only two pK values are characteristic of t 6A. Magnesium(II) and manganese(II) do not interact effectively with these ligands, but copper(II) and nickel(II) ions form very stable complexes with the coordination of purine N 1, deprotonated amide nitrogen, and carboxy late oxygen donors.

Copper(II) and nickel(II) complexes of diastereomeric segments of thymopoietin

Copper(II) and nickel(II) complexes of tri- and tetrapeptide segments of thymopoietin—ArgLysAspVal (TP4), ArgLysAsp (TP3, lll, lld, ldl, dll) and ArgLys-(Arg)Asp (ETP3)—were studied by potentiometric spectrophotometric, CD and EPR methods. The coordination of terminal amino, two deprotonated amide nitrogens and the β-carboxylate of the aspartic acid residue was proved in all cases. Significant stereoselectivity was not observed in the copper complexes of diastereomeric tripeptides.

Metal complexes of the deprotonated form of N, N′-bis(2′-pyridinecarboxamide 1′-oxide)-1,2-ethane, (enpco), a dinegative N 2O 2 quadridentate pyridine N-oxide ligand. X-ray crystal structure of [Cu(enpco-2H)] 2·8H 2O

The magnetic properties and infrared and electronic spectra of nickel(II) and copper(II) complexes with deprotonated enpco are discussed. An X-ray structure analysis shows that the complex [Cu(enpco-2H)] 2·8H 2O is triclinic, P 1 , with a = 6.988(2), b = 11.748(2), c = 12.289(2) Å, α = 63.06(2), β = 81.17(1), γ = 73.02(2)° and Z = 2. The ligand (enpco-2H) 2− coordinates as a distorted square-planar N 2O 2 donor via the deprotonated amide nitrogens and the N-oxide oxygens. An N-oxide oxygen from each (enpco-2H) 2− ligand also coordinates to the copper in an adjacent Cu(enpco-2H) entity, the resulting CuO bond length being 2.916(2) Å. This produces a centrosymmetric dimeric structure with tetragonal pyramidal coordination to each copper. The bridging CuO interaction is compared with that in a series of Cu(salen) complexes (H 2salen is the Schiff base of salicylaldehyde and ethylenediamine).

Sulphonamide nitrogen-containing, nitrogen-protected amino acids interacting with the Cd2+ion. A cadmium-113 nuclear magnetic resonance and potentiometric study. X-Ray diffraction of [Cd(ts-β-alaO)2(H2O4)

α-Alanine and β-alanine, N-protected by a tosyl (toluene-p-sulphonyl) group, bind Cd2+ acting initially as a simple carboxylate at low pH, and, at increasing pH, as N,O-bidentate ligands, through the carboxylic oxygen and the deprotonated amide nitrogen atoms. In the latter case they give rise to five- and six-membered ring chelate species, respectively. Cadmium-113 n.m.r. and potentiometric measurements reveal that the stability of the complexes is almost unaffected by the chelate ring size. The five-membered ring chelate species has a more deshielded chemical shift and only a slightly greater stability than the six-membered species. The crystal and molecular structures of the [Cd(ts-β-alaO)2(H2O)4](ts-β-alao =N-tosyl-β-alaninate monoanion) complex have been solved. Crystal data are: monoclinic, space group P21/c, a= 14.956 (6), b= 5.472(2), c= 16.860(7)Å, β= 107.36(3)°, Z= 2, and R= 0.039 for 1 635 reflections. The amina acid molecule binds through the unidentate carboxylate group and the structure consists of discrete units in which the Cd2+ exhibits a nearly regular octahedral geometry.

Cytotoxic activity of cyclic peptides of marine origin and their derivatives: importance of oxazoline functions

Cyclic peptides, especially those produced by marine cyanobacteria symbionts, are considered to play an important ecological role in host defence. Chemists have long compared the cyclic peptide cavitand architecture with that of macrocyclic ligands, and proposed that they mediate metal‐ion transport. The study presented herein investigated the metal chelation of non‐natural heteroatom‐interchanged (HI) isomers of lissoclinamide 5, by using MS, EPR, and DFT calculations. The latter identified three possible structures for the CuII complex with natural lissoclinamide 5, with the most likely determined to be that with the metal ion bound through the nitrogen donors of the thiazoles and one deprotonated amide. For HI‐lissoclinamide 5 the calculations suggest that the CuII ion is bound in a bidentate manner by the oxazoline nitrogen atom and one deprotonated amide nitrogen atom, with the S donor of the thiazole not involved in coordination. Along with evidence of copper binding these systems also bound halide ions. Evaluation of the anti‐cancer properties demonstrated that the biological activity of HI‐lissoclinamide 5 against T24 bladder cells was eleven‐fold lower as compared to natural lissoclinamide 5. Addition of a CuII salt had no effect on the activity of lissoclinamide 5. Overall, this comprehensive study of the HI concept has demonstrated that small changes propagate dramatic effects in complexation, halide binding, and biological activity.

Isolation and two-dimensional 1H-NMR of peptide [1–24] of dog serum albumin and studies of its complexation with copper and nickel by NMR and CD spectroscopy

The NH 2-terminal peptide fragment [1–24] of dog serum albumin was obtained by controlled peptic digestion of the protein. The peptide was purified to homogeneity by gel filtration and ion-exchange chromatography. The NMR assignments of the protons of the individual amino acid residues were made by using two-dimensional correlation matrix, spin-decoupling experiments and analysis of the titration curves. The polypeptide itself has a random-coil conformation. There is a conformational change as a function of pH, but it does not arise from any direct involvement of the amino acid side chains. Complexation of the peptide fragment with Ni(II) and Cu(II) has been investigated by NMR and CD. The Ni(II) complex is in slow exchange with the free ligand on the NMR time scale. The complexation involves the α-NH 2, three deprotonated amide nitrogens of Ala-2, Tyr-3 and Lys-4 residues. The phenolate oxygen of Tyr-3 is not involved in the metal binding; however, an interaction between the aromatic ring and the metal ion is likely. The CD results of Cu(II)-binding to this peptide suggest that the complexation takes place from the terminal NH 2 and step by step to three deprotonated amide nitrogens. There is no major conformational change of the peptide fragment upon complexation.

Spectroscopic study of the complexes of poly ( [formula omitted]) and its model molecule with nickel (II)

The complexes formed between nickel (II) and a polymeric ligand derived from l-lysine, viz. poly ( N- methacryloyl- l-lysine ) (PNML), have been investigated by potentiometry, electronic absorption and circular dichroism. N- isobutyroyl- l-lysine (NIBL) was also studied for comparison with its polymeric analogue. PNML gives two optically active complexes between the carboxylate and the amine group of one side-chain for the species NiL and of two different side-chains of the polymer for the species NiL 2 (one or two five membered chelates rings). For the model molecule, absorption results suggest a mixture of complexes (NiL + NiL 2) at high pH, but the circular dichroism spectra are typical of a NiL species. Complexes of nickel with NIBL or PNML are glycine-like i.e. the deprotonated amide nitrogen is not involved in complex formation.

The palladium(II) promoted hydrolysis of the methyl esters of glycyl-L-leucine, glycyl-L-alanine and L-alanylglycine

The palladium(II)-promoted hydrolysis of the methyl esters of glycyl-L-leucine, glycyl-L-alanine and L-alanylglycine have been studied at 25 °C and I=0.1 M in the pH range 4–5. At a 1:1 metal to ligand ratio the peptide esters act as tridentate ligands, donation occurring via the terminal amino group, the deprotonated amide nitrogen, and the carbonyl group of the ester. Due to the high Lewis acidity of Pd(II) rapid hydrolysis of the ester function by water and hydroxide ion occurs. Rate constants k OH and k H 2O have been obtained for base hydrolysis and water hydrolysis of the coordinated peptide esters at 25 °C. The rate constants for base hydrolysis are 3.4 X 10 6 M −1 s −1 (L-alaglyOMe), 6.4 X 10 6 M −1 s −1 (gly-L-alaOMe) and 2.3 X 10 7 M −1 s −1 (gly-L-leuOMe). Base hydrolysis of the coordinated peptide esters is at least 10 6 times that of the free unprotonated ligand. Activation parameters have been obtained for both water and base hydrolysis of the Pd(II) complex of methyl L-alanylglycinate and possible mechanisms for the hydrolyses are considered.

The palladium(II) promoted hydrolysis of methyl, ethyl and isopropyl glycylglycylglycinate

The palladium(II) promoted hydrolysis of the ester function of methyl, ethyl and isopropyl glycylglycylglycinate has been studied in detail in the pH range 4–5. The tripeptide esters interact with [PdCl 4] 2− at a 1:1 metal-to-ligand ratio to give the complex I in which the α-amino group, two deprotonated amide nitrogen atoms and the alkoxy carbonyl group of the ester act as donors. Rate constants have been determined for hydrolysis of the ester function by water and hydroxide ion and activation parameters obtained for the hydrolysis of the methyl ester. At 25 °C base hydrolysis of the coordinated ester is 10 6 fold that of the free ester ligand. Mechanisms for the reactions are considered.

The effect of a dansyl group on the co-ordinative ability of N-protected amino acids. Part 1. Behaviour of the copper(II) ion–N-dansylglycinate system in aqueous and methanolic solution

Polarographic, potentiometric, and spectrophotometric measurements show that copper(II) and N-dansylglycine (5-dimethylaminonaphthalene-1-sulphonylglycine) form the same complexes in aqueous and methanolic solution. Mixed-hydroxy complexes are the prevailing species. In aqueous (methanolic) solution up to pH 5(5) no formation of complexes is observed; in the pH range 7–8.5 (8.5–11) a change in the ligand co-ordination from O-co-ordination through the carboxylate group to N,O-bidentate co-ordination through the carboxylate oxygen and the deprotonated amide nitrogen is revealed. There is a low tendency for two N-dansylglycine ligand molecules to coordinate to the CuII ion, probably due to the steric hindrance of the dansyl group. The equilibria in which the prevailing complexes are involved and the reduction processes at the electrode are proposed. The nucleophilic and electrophilic properties of the two solvents are used to explain the solvent effect on the polarographic parameters and on the stability constants of the complexes.

The palladium(II)-promoted hydrolysis of methyl and isopropyl glycylglycinate

The palladium(II)-promoted hydrolysis of methyl glycylglycinate and isopropyl glycylglycinate has been studied at four temperatures (25, 30, 35, and 40 °C) at I= 0.1 mol dm–3 in the pH range 4–5. Under these conditions, and at a 1 : 1 metal to ligand ratio, the peptide esters act as tridentate ligands, donation occurring via the terminal amino-group, the deprotonated amide nitrogen, and the carbonyl group of the ester. Rate constants are reported for hydrolysis of the ester function by water and hydroxide ion, and activation parameters calculated. Base hydrolysis of the co-ordinated peptide esters is ca. 105 times faster than the unprotonated peptide esters. Mechanisms for the reactions are considered.

Studies of copper(II) binding to glycylglycyl- l-Tyrosine- N-methyl amide, a peptide mimicking the NH 2-terminal copper(II)-binding site of dog serum albumin by analytical potentiometry, spectrophotometry, CD, and NMR spectroscopy

Unlike human serum albumin (HSA), dog serum albumin (DSA) does not possess the characteristics of the specific first binding site for Cu(II). In DSA, the important histidine residue in the third position, responsible for the Cu(II)-binding specificity in HSA, is replaced by a tyrosine residue. In order to study the influence of the tyrosine residue in the third position of DSA, a simple model of the NH 2-terminal native sequence tripeptide of DSA, glycylglycyl- l-tyrosine- N-methylamide (GGTNMA) was synthesized and its Cu(II)-binding properties studied by analytical potentiometry, spectrophotometry, CD, and NMR spectroscopy. The species analysis indicated the existence of five mono-complexes at different protonation states: MHA, MA, MH −1A, MH −2A, MH −3A, and only one bis-complex MH −2A −2. The complexing ability of GGTNMA to Cu(II) was found to be weaker than that of the Cu(II) binding peptide models of HSA. The visible absorption spectra of Cu(II)-GGTNMA complexes are similar to those observed in the case of DSA-Cu(II) complexes. The weaker binding and the spectral properties of Cu(II)-GGTNMA complexes are consistent with less specific Cu(II)-binding properties of the peptide of this sequence similar to what was noted with DSA. CD results are in excellent agreement with species analysis and visible spectra where it is clearly evident that Cu(II) binds to GGTNMA starting from the α-NH 2 group and step by step to deprotonated amide nitrogens as the pH is raised. The absence of any charge transfer band around 400 nm strongly indicates that Cu(II) does not bind to the phenolate group. Furthermore, NMR results are consistent with the noninvolvement of the tyrosine residue of GGTNMA in Cu(II) complexation. Thus, it is clear that the low Cu(II)-binding affinity of DSA is due to the genetic substitution of tyrosine for histidine at the NH 2-terminal region of the protein.

Coordination chemistry of peptides

The structure of the title compound displays a nearly planar diketopiperazine ring. The side chain is folded on the ring, the S atom making a significantly short contact with the methionine carbonyl atom. The crystal packing shows an unusual assembly of alternating layers of peptide and crystallization water molecules. The side chain sulfur atom is found to be the primary site for metal coordination of the ligand, while in some systems there is evidence of additional coordination through deprotonated amide nitrogens.

Studies on the metal—amide bond XV. The Molecular structure of chloro{[N-methyl-N-(2′-pyridinecarboxamide)N′-(2′-pyridinecarboxamido)]-1,2-ethane}palladium(II) determined by x-ray diffraction and high-resolution NMR studies

Chloro{[N-methyl-N-(2′-pyridinecarboxamide)-N′(2′-pyridinecarboxamido)]-1,2-ethane}palladium(II), C 15H 15N 4O 2ClPd, is tetragonal, space group I4 1/ a with a = 14.340(8), c = 31.226(10) Å, Z = 16. The crystal structure was refined to R = 0.078 for 1886 photographic reflexions by least-squares calculations The ligand acts as an N 3-tridentate to the Pd atoms via the two pyridine and one deprotonated amide nitrogen atoms [Pd-N(amide) 1.98(1) Å; average Pd- N(pyridine) 2.05(1) Å]. The N-methylated amide group is not coordinated but part of an unusual eight-membered chelate ring. A square-planar coordination is completed by the chlorine atoms [Pd-Cl 2.293(4) Å]. The coordinated picolinamide unit is almost co-planar with the coordination plane, whereas the N-methyl picolinamide unit is non-planar. The plane of this amide group is approximately perpendicular to the plane of the pyridine ring, the latter making an angle of 64.9° with the coordination plane. This unusual molecular geometry is shown to be retained in solution. The complex proton n.m.r. spectrum of the compound in CDCl 3 only may be stimulated a rigid structure similar to that found in the solid state. The hydrogen atoms of the central ethane link are closely eclipsed; observed vicinal coupling constants for the protons show a close correlation with those calculated using the HCCH torsion angles derived from the X-ray analysis. Observed chemical shifts are explained in terms of close intramolecular contacts in the structure as determined.

Crystal and molecular structure of sodium µ-(imidazol-1-yl-NN′)-bis-[(glycylglycinato-NN′O)cuprate(II)] hexahydrate: structure–magnetism relationship in imidazolate-bridged binuclear copper(II) complexes

The structure of the title complex, Na[Cu2(Gly-GlyO)2(im)]·6H2O [Gly-GlyO = glycylglycinate(2–), im = imidazolyl), has been determined from X-ray diffractometer data, to give final discrepancy indices of R= 0.0302, and R′= 0.0391. The complex crystallizes in the triclinic space group P with unit-cell dimensions a= 11.246(4), b= 10.464(3), c= 10.858(4)Å, α= 109.99(3), β= 115.05(3), γ= 85.05(3)°, and Z= 2. Each copper(II) atom has square-planar co-ordination by amino-nitrogen, deprotonated amide nitrogen, carboxylate oxygen of the glycylglycinate ligand, and by nitrogen of bridging imidazolate, to form altogether a binuclear copper(II) complex. The complex is almost planar and has an approximate two-fold axis which is coincident with that of the imidazolate ring. Variable-temperature magnetic susceptibility data measured down to 4.2 K show an antiferromagnetic exchange interaction with a coupling constant J=–19 cm–1. The bridging imidazolate plane and copper(II) co-ordination planes make an angle of 5.8° for Cu(1) and 10.4° for Cu(2). The relative orientation of the bridging imidazolate and the copper(II) co-ordination planes does not seem to be responsible for the magnitude of the magnetic exchange coupling constant (J=–19 cm–1) in this case. It is concluded that a π-exchange mechanism is not important in the imidazolate-bridged binuclear copper(II) complexes.