Christopher F. Rodriquez

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.

Multiple substitution of protons by sodium ions in sodiated oligoglycines

Abstract Multiply-sodiated ions were observed by electrospraying oligoglycines and their N-acetylated and O-amidated derivatives in the presence of sodium hydroxide. These ions have all their hydrogens in the peptide linkages replaced by sodiums; one hydrogen in the C-terminal amide and the hydrogen in the C-terminal carboxylic group are also replaced. The N-terminal amine hydrogens are unreactive. These results are consistent with earlier postulations [Int. J. Mass Spectrom. 192 (1999) 303, J. Am. Soc. Mass Spectrom. 11 (2000) 967], and apparently confirm the gas-phase origin of these ions (formed in the electrospray source and/or the lens region). Collision-induced dissociation of multiply-sodiated oligoglycines showed that the major product ions are C-terminal ions.

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 possible origin of [M − nH + mX](m−n)+ ions (X = alkali metal ions) in electrospray mass spectrometry of peptides

The [M 2 nH 1 mX] (m2n)1 (X 5 alkali metal ion) are common ions in the mass spectrum of a peptide that is electrosprayed in the presence of an alkali metal salt or hydroxide. The feasibility of forming [M 2 nH 1 mX] (m2n)1 ions in the gas phase including those in the lens region of the mass spectrometer via ion‐molecule reactions and/or reactions between components of collisionally activated adducts was investigated. The Li 1 ion was selected for examination since its salts are computationally the least expensive among alkali metal salts. The lithium ion affinities of the [M 2 H] 2 ions of N-methylacetamide, acetic acid, and 1-propanamine were calculated by means of density functional theory (DFT) at various levels of theory, including B3LYP/6-31111G(d, p). These three compounds were selected as representatives of relevant functional groups on a peptide. The calculated lithium ion affinities, together with evaluated thermochemical data, were used to calculate the enthalpies of reactions between the model compounds and LiOH, LiCl, and Li(H2O) 1 that might lead to the formation of [M 2 nH 1 mX] (m2n)1 . A number of these reactions were found to be exothermic or slightly endothermic (DH 8,1 20 kcal/mol). DFT calculations on the energetics of a model reaction revealed a relatively flat potential energy hypersurface containing a well of approximately 35 kcal/mol in depth and devoid of significant barriers. These results are used to postulate the formation of [M 2 nH 1 mX] (m2n)1 ions in the gas phase in the ion source and/or in the lens region via collisions between an ionic peptide and neutral lithium compounds or collisional activation of lithium‐peptide adducts. (Int J Mass Spectrom 192 (1999) 303‐317)

The proton affinity of SiNH and its formation from SiNH2+ in the gas phase

SiNH2+ has been formed in the gas phase by the rapid reaction of Si+(2P) with NH3. Experimental and theoretical studies of the proton affinity of SiNH indicate that this ion is protonated at N, so that deprotonation produces selectively the isomer SiNH in partially ionized interstellar environments.

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.

A flow-tube study of isomers of C 2 H 4 Cl + : the chloronium and 1-chloroethyl cations

Reactions of the 1-chloroethyl and the chloronium cations with ammonia were observed in the gas phase at room temperature in helium at 0.36 Torr using the selected-ion flow tube (SIFT) technique. The 1-chloroethyl cation in its reaction with ammonia only undergoes proton transfer. The chloronium ion reacts with ammonia by association and the elimination of HCl. Association, which has not been observed in previous ICR studies, is the predominant channel. A value of 556 kJ mol−1 was calculated at MP4SDTQ(fc)/6-311++G(2df,p)//MP2(full)/6-311G(d,p) for the standard enthalpy of formation of the adduct of the chloronium ion with ammonia. The adduct ion was observed to react further with NH3 to eliminate NH2Cl and NH2CH2Cl. A mechanism is proposed for the overall sequential reaction of the chloronium ion with two molecules of ammonia.

Experimental and theoretical studies of the proton affinity of fluoroboroxine

Abstract The proton affinity of fluoroboroxine, FBO, is explored both experimentally and theoretically. Results of selected-ion flow tube measurements are reported for proton transfer reactions which place the proton affinity (PA) of FBO between those of ethylene and acetylene, PA(C 2 H 4 ) PA(FBO) PA( C 2 H 2 ), with PA(FBO) equal to 158 ± 6 kcal mol −1 . FBOH + is produced from the reaction of BF + 2 with H 2 O and is observed to rapidly transfer a proton to H 2 O, ethylene, propylene, benzene, cis -2-butene, iso-butene and styrene but only to add to acetylene to form FBOH + ·C 2 H 2 at 296 ± 2 K. The results of ab initio molecular orbital calculations on FBO and on isomers of FBOH + are reported for the Hartree-Fock and the full second-order Moeller-Plesset levels of theory with the 6-13 G ** basis eset. Structure optimizations were performed at both levels of theory using gradient techniques in the GAUSSIAN 82 program. The proton affinity, corrected for zero-point and thermal energies, and including correlation energy, is computed to be 163.8 kcal mol −1 .