Structural determination of four manufacturing impurities of D&C Red No. 33.

Four manufacturing impurities of D&C Red No. 33 isolated by counter-current chromatography were analyzed by NMR and ESI mass spectrometry. Three of these impurities were reported previously with minimal details of structural determination. All four are structurally related to the main component of the dye. The fourth exhibited an unusual discrepancy between the NMR structure and its chemical formula suggested by ESI-MS results. Structural determination and assignment of the main component and four impurities are discussed as well as resolution of the discrepancy between the NMR and ESI-MS results of the fourth impurity.

Activation barriers in the growth of molecular clusters derived from sulfuric acid and ammonia.

Unraveling the chemical mechanism of atmospheric new particle formation (NPF) has important implications for the broader understanding of the role of aerosols in global climate. We present computational results of the transition states and activation barriers for growth of atmospherically relevant positively charged molecular clusters containing ammonia and sulfuric acid. Sulfuric acid uptake onto the investigated clusters has a small activation free-energy barrier, consistent with nearly collision-limited uptake. Ammonia uptake requires significant reorganization of ions in the preexisting cluster, which yields an activation barrier on the order of 29-53 kJ/mol for the investigated clusters. For this reason, ammonia uptake onto positively charged clusters may be too slow for cluster growth to proceed by the currently accepted mechanism of stepwise addition of sulfuric acid followed by ammonia. The results presented here may have important implications for modeling atmospheric NPF and nanoparticle growth, which typically does not consider an activation barrier along the growth pathway and usually assumes collision-limited molecular uptake.

Fragmentation energetics of clusters relevant to atmospheric new particle formation.

The exact mechanisms by which small clusters form and grow in the atmosphere are poorly understood, but this process may significantly impact cloud condensation nuclei number concentrations and global climate. Sulfuric acid is the key chemical component to new particle formation (NPF), but basic species such as ammonia are also important. Few laboratory experiments address the kinetics or thermodynamics of acid and base incorporation into small clusters. This work utilizes a Fourier transform ion cyclotron resonance mass spectrometer equipped with surface-induced dissociation to investigate time- and collision-energy-resolved fragmentation of positively charged ammonium bisulfate clusters. Critical energies for dissociation are obtained from Rice-Ramsperger-Kassel-Marcus/quasi-equilibrium theory modeling of the experimental data and are compared to quantum chemical calculations of the thermodynamics of cluster dissociation. Fragmentation of ammonium bisulfate clusters occurs by two pathways: (1) a two-step pathway whereby the cluster sequentially loses ammonia followed by sulfuric acid and (2) a one-step pathway whereby the cluster loses an ammonium bisulfate molecule. Experimental critical energies for loss of an ammonia molecule and loss of an ammonium bisulfate molecule are higher than the thermodynamic values. If cluster growth is considered the reverse of cluster fragmentation, these results require the presence of an activation barrier to describe the incorporation of ammonia into small acidic clusters and suggest that kinetically (i.e., diffusion) limited growth should not be assumed. An important corollary is that models of atmospheric NPF should be revised to consider activation barriers to individual chemical steps along the growth pathway.

Stability of flavin semiquinones in the gas phase: the electron affinity, proton affinity, and hydrogen atom affinity of lumiflavin.

Examination of electron transfer and proton transfer reactions of lumiflavin and proton transfer reactions of the lumiflavin radical anion by Fourier transform ion cyclotron resonance mass spectrometry is described. From the equilibrium constant determined for electron transfer between 1,4-naphthoquinone and lumiflavin the electron affinity of lumiflavin is deduced to be 1.86 ± 0.1 eV. Measurements of the rate constants and efficiencies for proton transfer reactions indicate that the proton affinity of the lumiflavin radical anion is between that of difluoroacetate (331.0 kcal/mol) and p-formyl-phenoxide (333.0 kcal/mol). Combining the electron affinity of lumiflavin with the proton affinity of the lumiflavin radical anion gives a lumiflavin hydrogen atom affinity of 59.7 ± 2.2 kcal/mol. The ΔG298 deduced from these results for adding an H atom to gas phase lumiflavin, 52.1 ± 2.2 kcal/mol, is in good agreement with ΔG298 for adding an H atom to aqueous lumiflavin from electrochemical measurements in the literature, 51.0 kcal/mol, and that from M06-L density functional calculations in the literature, 51.2 kcal/mol, suggesting little, if any, solvent effect on the H atom addition. The proton affinity of lumiflavin deduced from the equilibrium constant for the proton transfer reaction between lumiflavin and 2-picoline is 227.3 ± 2.0 kcal mol(-1). Density functional theory calculations on isomers of protonated lumiflavin provide a basis for assigning the most probable site of protonation as position 1 on the isoalloxazine ring and for estimating the ionization potentials of lumiflavin neutral radicals.

Size-dependent reactions of ammonium bisulfate clusters with dimethylamine.

The reaction kinetics of ammonium bisulfate clusters with dimethylamine (DMA) gas were investigated using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Clusters ranged in size from 1 to 10 bisulfate ions. Although displacement of the first several ammonium ions by DMA occurred with near unit efficiency, displacement of the final ammonium ion was cluster size dependent. For small clusters, all ammonium ions are exposed to incoming DMA molecules, allowing for facile exchange ("surface" exchange). However, with increasing cluster size, an ammonium ion can be trapped in an inaccessible region of the cluster ("core" exchange), thereby rendering exchange difficult. DMA was also observed to add onto existing dimethylaminium bisulfate clusters above a critical size, whereas ammonia did not add onto ammonium bisulfate clusters. The results suggest that as the cluster size increases, di-dimethylaminium sulfate formation becomes more favorable. The results of this study give further evidence to suggest that ambient sub-3 nm diameter particles are likely to contain aminium salts rather than ammonium salts.

Molecular mass determination of saturated hydrocarbons: reactivity of eta5-cyclopentadienylcobalt ion (CpCo*+) and linear alkanes up to C-30.

The present study demonstrates the feasibility of the eta5-cyclopentadienylcobalt ion (CpCo*+) as a suitable cationization reagent for saturated hydrocarbon analysis by mass spectrometry. Ion/molecule reactions of CpCo*+ and three medium chain-length n-alkanes were examined using Fourier-transform ion cyclotron resonance mass spectrometry. Second-order rate constants and reaction efficiencies were determined for the reactions studied. Loss of two hydrogen molecules from the CpCo-alkane ion complex was found to dominate all reactions ( > or = 80%). Furthermore, this dehydrogenation reaction efficiency increases with increasing chain length. These preliminary results suggest that the CpCo*+ ion may be a promising cationization reagent of longer chain saturated hydrocarbons and polyolefins.

Mass spectrometric differentiation of alpha- and beta-aspartic acid in a pseudo-tetrapeptide thrombosis inhibitor and its isomer.

The pseudo-tetrapeptide designated here as RGD (N-ethyl-N-[1-oxo-4-(4-piperidinyl) butyl] glycyl-L-alpha- aspartyl-3-cyclohexyl-L-alaninamide) and its isomer with beta-aspartic acid rather than alpha-aspartic acid were examined using electrospray ionization (ESI) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). RGD has potential as a thrombosis inhibitor and the isomer, designated here as isopeptide, is an inactive instability product; hence, means were sought to distinguish the two. Both isomers give a protonated parent on ESI and fragments typical of peptides on sustained off resonance irradiation collision-induced decomposition (SORI-CID). Cleavage at the aspartic acid (b(3)) is the dominant process in both isomers, although a significant b(2) and smaller a(2)" and c(2)" peaks are also observed. More distinctive are peaks observed at b(3)-H(2)O, b(3)-(CO + CO(2)) and, only in the case of the RGD, b(3) - (H(2)O + CO). SORI CID on the b(3) ion indicates that, of these distinctive peaks, only the b(3)-(CO + CO(2)) comes from decomposition of the b(3) ion. On this basis, a mechanism is suggested for b(3) formation, involving proton transfer from a back-bone carbonyl to the aspartic acid side-chain carboxyl group. Such an intramolecular proton transfer involves rings of different sizes for the two isomers, providing a basis for the different SORI energy dependences. A mechanism suggested for the formation of the b(3)-H(2)O fragments also involves proton transfer to the aspartic acid side chain carboxyl group. This leads to concomitant H(2)O loss and amide bond cleavage, giving the b(3)-H(2)O ions with ketene moieties resulting from the water loss. According to the suggested mechanism, the observed loss of CO (verified by SORI-CID on the b(3)- H(2)O ion) from the RGD b(3)-H(2)O peak results in a secondary carbocation stabilized by an adjacent nitrogen. The unobserved loss of CO from the b(3)-H(2)O ion, formed by the suggested mechanism from the isopeptide, would give an unstable primary carbocation lacking a neighboring nitrogen. The mechanism, thus, only rationalizes the observation of a b(3)-(H(2)O + CO) fragment in RGD and not in the isopeptide. The isomers can be distinguished on the basis of this unique peak or on the basis of the different SORI energy dependence of the formation of the b(3) ions.

Fragmentation mechanisms of oxidized peptides elucidated by SID, RRKM modeling, and molecular dynamics.

The gas-phase fragmentation reactions of singly charged angiotensin II (AngII, DR(+)VYIHPF) and the ozonolysis products AngII+O (DR(+)VY*IHPF), AngII+3O (DR(+)VYIH*PF), and AngII+4O (DR(+)VY*IH*PF) were studied using SID FT-ICR mass spectrometry, RRKM modeling, and molecular dynamics. Oxidation of Tyr (AngII+O) leads to a low-energy charge-remote selective fragmentation channel resulting in the b(4)+O fragment ion. Modification of His (AngII+3O and AngII+4O) leads to a series of new selective dissociation channels. For AngII+3O and AngII+4O, the formation of [MH+3O](+)-45 and [MH+3O](+)-71 are driven by charge-remote processes while it is suggested that b(5) and [MH+3O](+)-88 fragments are a result of charge-directed reactions. Energy-resolved SID experiments and RRKM modeling provide threshold energies and activation entropies for the lowest energy fragmentation channel for each of the parent ions. Fragmentation of the ozonolysis products was found to be controlled by entropic effects. Mechanisms are proposed for each of the new dissociation pathways based on the energies and entropies of activation and parent ion conformations sampled using molecular dynamics.

Gas-phase IR spectroscopy of anionic iron carbonyl clusters.

The first gas-phase vibrational spectra are presented for several anionic iron carbonyl clusters, ranging in size from Fe(CO)4- to Fe5(CO)14- in the CO-stretching region (1600-2100 cm-1). The experimental spectra provide some immediate structural information about the clusters in the form of low-wavenumber (1750-1850 cm-1) bands marking the presence of bridging carbonyl ligands (mu2-COs). Supporting DFT calculations are presented for the smaller clusters (<3 Fe atoms) and give good agreement with the experimental data, allowing structural assignments for these cases. The Fe2(CO)7- spectrum suggests a structure lacking bridging carbonyl ligands, in agreement with the DFT results. For the case of Fe2(CO)8-, there are two possible structures based on the calculations, both with and without bridging carbonyls. The presence of a low-frequency band ( approximately 1770 cm-1) in the experimental spectrum conclusively demonstrates the existence of the bridged form. The ramifications of these data for metal-metal bonding in the clusters are also considered.