Tamer Shoeib

A comparison of copper(I) and silver(I) complexes of glycine, diglycine and triglycine

Density functional calculations at B3LYP/DZVP were used to obtain structural information, relative free energies of different isomers and binding energies for the following reaction in the gas phase: M+ + (glycyl)nglycine → M–(glycyl)nglycine+, where M = Ag or Cu and n = 0–2. For the complexes with Cu+, optimizations were also performed at B3LYP/6–31++G(d,p) and single-point calculations at MP2(fc)/6–311++G(2df,2p)//B3LYP/DZVP. The calculated binding energies for the Cu+ complexes are all higher than those of the structurally similar Ag+ ions. These calculated binding energy differences become larger as the size of the ligand increases. For all the Cu+ complexes examined, the coordination number of the copper ion does not exceed two, whereas for the silver complexes tri- and tetracoordinate Ag+ structures are calculated to be at low energy minima. Significant structural and relative free energy differences occur between the lowest energy ‘zwitterionic ’ forms of the M–(glycyl)nglycine+ complexes.

Characterization of the product ions from the collision-induced dissociation of argentinated peptides

Tandem mass spectrometry performed on a pool of 18 oligopeptides shows that the product ion spectra of argentinated peptides, the [bn + OH + Ag]+ ions and the [yn − H + Ag]+ ions bearing identical sequences are virtually identical. These observations suggest strongly that these ions have identical structures in the gas phase. The structures of argentinated glycine, glycylglycine, and glycylglycylglycine were calculated using density functional theory (DFT) at the B3LYP/DZVP level of theory; they were independently confirmed using HF/ LANL2DZ. For argentinated glycylglycylglycine, the most stable structure is one in which Ag+ is tetracoordinate and attached to the amino nitrogen and the three carbonyl oxygen atoms. Mechanisms are proposed for the fragmentation of this structure to the [b2 + OH + Ag]+ and the [y2 − H + Ag]+ ions that are consistent with all experimental observations and known calculated structures and energetics. The structures of the [b2 − H + Ag]+ and the [a2 − H + Ag]+ ions of glycylglycylglycine were also calculated using DFT. These results confirm earlier suggestions that the [b2 − H + Ag]+ ion is an argentinated oxazolone and the [a2 − H + Ag]+ an argentinated immonium ion.

Gas-phase fragmentation of the Ag+—phenylalanine complex: Cation—π interactions and radical cation formation

Collision-induced dissociation experiments on the Ag+—phenylalanine complex using several collision energies were shown to yield ten different fragment ions. Unambiguous assignment of these fragment ions were made by careful analysis of deuterium labeling experiments. The losses of H2O, CO, CO2, and AgH were commonly observed; also encountered were the losses of H2, Ag, and H. Deuterium labeling experiments and density functional calculations have been employed to probe fragmentation mechanisms that account for all experimental results.

A study of complexes Mg(NH3)n+· and Ag(NH3)n+, where n = 1–8: competition between direct coordination and solvation through hydrogen bonding

Abstract Density functional calculations at B3LYP/6-31+G(d) and B3LYP/DZVP are reported for Mg(NH 3 ) n +· , where n = 1–6 and for some solvated ions Mg(NH 3 ) n +· … NH 3 ( n = 1–3, 6). After correction for basis set superposition errors, the enthalpies for sequential addition of NH 3 to Mg +· resulting from direct coordination to the metal are 38.1, 26.6, 21.2, 13.7, 12.1, and 11.3 kcal mol −1 . The free energies for these same addition reactions are all negative, although for complexes with n ≥ 4 the values are very small. Attempts at optimising structures with higher coordination numbers all resulted in the formation of solvated octahedral complexes. Enthalpies for solvation through hydrogen bonding to one of the ligated NH 3 molecules are all less than 16 kcal mol −1 and decrease rapidly as the number of ligated NH 3 molecules increases. Molecular orbital calculations at B3LYP/DZVP have been used to optimise structures for ions Ag(NH 3 ) n + , where n = 1–6. The five-coordinate and six-coordinate structures have very small binding enthalpies (4.3 and 2.6 kcal mol −1 ) and the free energies for formation of these ions are positive. The binding energies for the addition of the first and second NH 3 molecules added to Ag + are 40.1 and 36.1 kcal mol −1 , while those for the third and fourth additions are much smaller (15.1 and 11.0 kcal mol −1 ). Adducts up to n = 3 have been detected in electrospray experiments. The first three adducts of Ag + with NH 3 have been formed in the selected ion flow tube apparatus and multicollision induced dissociation experiments show Ag(NH 3 ) 3 + to have a lower binding enthalpy than both Ag(NH 3 ) 2 + and Ag(NH 3 ) + .

Formation of [M − nH + mNa](m−n)+ and [M − nH + mK](m−n)+ ions in electrospray mass spectrometry of peptides and proteins

The [M − nH + mNa](m−n)+ and [M − nH + mK](m−n)+ ions are common in the electrospray mass spectra of proteins and peptides. The feasibility of forming these ions in the gas phase via collision activation and/or ion-molecule reaction is investigated. Sodium and potassium affinities of the N-methylacetamide anion, the acetate anion, and the 1-propanamide anion have been calculated using density functional theory at the B3LYP/6-311+ +G(d,p) level of theory. These anions were chosen as models for the functional groups on a protein or peptide. These affinity values are then used to calculate reaction enthalpies of alkali hydroxides, chlorides, and hydrates with N-methylacetamide, acetic acid, the acetate anion, and 1-propanamine, model reactions that may lead to formation of the [M − nH + mNa](m−n)+ and [M − nH + mK](m−n)+ ions. It is found that a number of these reactions are exothermic or slightly endothermic (ΔH0 < + 20 kcal/mol) and are accessible after collision activation in the lens region. The potential energy hypersurfaces of model reactions between NaOH and formamide as well as NaCl and formamide show relatively flat surfaces devoid of significant barriers.

A hybrid quantum mechanical molecular mechanical method: Application to hydration free energy calculations

A hybrid quantum mechanical molecular mechanical (QMMM) approach is used to study H3O+, H2O, NH4+, NH3, Cl−, HCl, F−, HF, CH3COO−, CH3COOH, Ag+ and glycine in both zwitterionic and nonzwitterionic forms in water. The free energies of hydration of these species are presented and are shown to compare favorably with experimental values. The difference in water–glycine interaction energy between the zwitterionic and nonzwitterionic forms is calculated as a lower limit and is in line with previous findings. The first theoretical examination of the Ag+–glycine complex in solution is presented.

Elimination of AgR (R = H, CH 3 , C 6 H 5 ) from collisionally-activated argentinated amines

Fragmentation of collisionally-activated argentinated amines results in the formation of Ag+ and non-silver-containing ions. The latter are likely immonium ions that are formed after elimination of AgH and, when the ion structures permit, AgCH3 or AgC6H5. The H, CH3 and C6H5 groups are attached to the carbon alpha to the amino nitrogen, and are believed to be cleaved with the Ag in a 1,2-elimination. This hypothesis is supported by potential energy hypersurfaces calculated using density functional theory for the reactions involving methanamine and ethanamine.

When does the Hard and Soft Acid Base principle apply in the gas phase

Abstract We have studied the silver ion affinities of several RCN ligands using density functional theory. It was found that they correlate linearly with experimental proton affinities, a contradiction to the HSAB principle as H + is a ‘hard’ acid, and Ag + is a ‘soft’ acid. This linear correlation between the Ag + and proton affinities diminishes as the number of RCN ligands attached to the Ag + ion increases from one to three. In the third addition step, the silver ion affinities nearly level off in agreement with the expectation that the electron donating or withdrawing properties of the R group become much less important than in the previous two steps, where the positive charge on the metal ion is not well delocalized. Delocalization of the charge of a metal ion occurs only when a sufficiently large number of ligands are attached to the metal ion. When this condition is satisfied, our data suggests that the HSAB principle may be applicable.

Investigations of the gas-phase reactivity of Cu+ and Ag+ glycine complexes towards CO, D2O and NH3

Abstract The room-temperature reactivities of complexes of Cu + and Ag + with glycine have been investigated using an inductively coupled plasma-selected ion flow tube (ICP-SIFT)/multicollision-induced dissociation (CID) tandem mass spectrometer. These complexes were produced in a flow tube, collisionally thermalized and then allowed to react with CO, D 2 O or NH 3 . The measured reactivities of the CuGly + and AgGly + complexes have been compared with those of the bare metal cations towards the same neutral reagents. All observed reactions resulted in adduct formation, with helium buffer gas presumably acting as a stabilizing agent. Reaction rate enhancements of up to three orders of magnitude and lower extents of ligation were the main characteristics of the reactions initiated by the metal cation–glycine adducts as compared with those initiated by the bare metal cations. The kinetic information combined with equilibrium analyses and CID results suggest that, irrespective of the reagent, ligation to CuGly + is stronger than to AgGly + and, in both instances, ligation of carbon monoxide and water has comparable strengths, while much stronger coordination is achieved with ammonia. The structures of the complexes have been investigated computationally using density functional theory (DFT) at the B3LYP level employing the DZVP basis set. Optimized structures and the free energy changes associated with ligation have been computed. A good correlation was obtained between the experimental reaction efficiencies and the calculated free energies of bonding. Also, results are presented for the potential energy surfaces for the conversion of the “charge solvated” forms into the “metal salt” forms of CuGly + and AgGly + and of their adducts with CO or NH 3 . The computations indicate a modest catalytic effect of the CO and NH 3 on this conversion.

Fragmentation of singly charged silver/α,Ω-diaminoalkane complexes: competition between the loss of H 2 and AgH molecules

Collision-induced dissociation of complexes, Ag+/NH2CH2(CH2) n CH2NH2, where n has values of 0, 1, 2 and 3, show loss of one H2 molecule at low energies. For the largest complex (n = 3), a second H2 molecule is also lost. For complexes with n = 0 and 1, loss of AgH (the only pathway observed in the fragmentation of Ag+/monoamine complexes) also occurs at low collision energies and this becomes the dominant fragmentation pathway at centre-of-mass energies above 2 eV. For complexes with n = 2 and 3, negligible amounts of product ions resulting from loss of only AgH were formed. However, for these complexes, the major products at higher collision energies result from loss of either AgH plus NH3 or, more likely, H2 plus AgNH2; the product ions are postulated to be cyclic iminium ions. Density functional theory (DFT) calculations (B3LYP with a DZVP basis set) on Ag+/NH2CH2(CH2)2CH2NH2 showed the loss of H2 followed by AgNH2 to have a slightly lower barrier than loss of AgH plus NH3. For complexes with n = 2 and 3, there are minor losses of various combinations of AgH, H2, 2H2 and NH3. Reaction profiles for the losses of H2 and AgH from all four Ag+/α,ω-diamine complexes have been examined computationally using DFT calculations. The second step on the reaction profile, loss of either H2 or AgH from the complex between Ag+ and α-amino-γ-imino-propane, Ag+/NH2CH2CH2CHNH, has also been calculated. The highest energy transition state on the profiles to loss of both H2 and AgH from the smallest complex (n = 0) are identical; on all the other profiles, the barrier for H2 loss is lower than that for AgH loss (by 2.5 kJ mol−1 when n = 1, by 13.4 kJ mol−1 when n = 2 and by 33.0 kJ mol−1 when n = 3). Fragmentation of the ligand backbone occurs most extensively for ions derived from the Ag+/NH2CH2CH2CH2NH2. After loss of H2, the product ion, Ag+/NH2CH2CH2CH = NH fragments via a transition state in which the CH2–CH2 bond is breaking, while a 1,5-hydrogen shift from one nitrogen to the other is occurring. Cleavage of the carbon chain of NH2CH2CH2CH = NH2+ occurs at relatively low energy; DFT calculations provide a mechanism by which H2C = NH plus CH3CH = NH2+ are produced.