Complexation of carboxylate anions with the arginine gas-phase amino acid: Effects of chain length on the geometry of extended ion binding

Photodegradation of Riboflavin under Alkaline Conditions: What Can Gas-Phase Photolysis Tell Us about What Happens in Solution?

The application of electrospray ionisation mass spectrometry (ESI-MS) as a direct method for detecting reactive intermediates is a technique of developing importance in the routine monitoring of solution-phase reaction pathways. Here, we utilise a novel on-line photolysis ESI-MS approach to detect the photoproducts of riboflavin in aqueous solution under mildly alkaline conditions. Riboflavin is a constituent of many food products, so its breakdown processes are of wide interest. Our on-line photolysis setup allows for solution-phase photolysis to occur within a syringe using UVA LEDs, immediately prior to being introduced into the mass spectrometer via ESI. Gas-phase photofragmentation studies via laser-interfaced mass spectrometry of deprotonated riboflavin, [RF - H]-, the dominant solution-phase species under the conditions of our study, are presented alongside the solution-phase photolysis. The results obtained illustrate the extent to which gas-phase photolysis methods can inform our understanding of the corresponding solution-phase photochemistry. We determine that the solution-phase photofragmentation observed for [RF - H]- closely mirrors the gas-phase photochemistry, with the dominant m/z 241 condensed-phase photoproduct also being observed in gas-phase photodissociation. Further gas-phase photoproducts are observed at m/z 255, 212, and 145. The value of exploring both the gas- and solution-phase photochemistry to characterise photochemical reactions is discussed.

Mapping the intrinsic absorption properties and photodegradation pathways of the protonated and deprotonated forms of the sunscreen oxybenzone

Sunscreens provide vital protection against the photodamaging effects of UV radiation, however, many fundamental questions remain about the detailed mechanisms by which they dissipate UV energy. One such issue is the extent to which the pH environment of an organic sunscreen molecule alters its effectiveness, both in terms of ability to absorb UV radiation, and also its potential to photodegrade. Here, we use gas-phase laser photodissociation spectroscopy for the first time to measure the intrinsic UVA-UVC absorption spectra and associated photodegradation products of protonated and deprotonated oxybenzone, away from the complications of bulk mixtures. Our results reveal that protonation state has a dramatic effect on the absorption and photodissociation properties of this sunscreen. While the UV absorption profile of oxybenzone is only modestly affected by protonation across the range from 400-216 nm, deprotonated oxybenzone displays a significantly modified absorption spectrum, with very low photoabsorption between 370-330 nm. Protonated oxybenzone primarily photofragments by rupture of the bonds on either side of the central carbonyl group, producing cationic fragments with m/z 151 and 105. Additional lower mass photofragments (e.g. m/z 95 and 77) are also observed. The production spectra for the photofragments from protonated oxybenzone fall into two distinct categories, which we discuss in the context of different excited state decay pathways. For deprotonated oxybenzone, the major photofragments observed are m/z 211 and 212, which are associated with the ejection of methane and the methyl free radical from the parent ion, respectively. Implications for the suitability of oxybenzone in its protonated and deprotonated forms as an optimum sunscreen molecule are discussed.

UV laser photoactivation of hexachloroplatinate bound to individual nucleobases in vacuo as molecular level probes of a model photopharmaceutical

UV excitation of mass-selected hexachloroplatinate–nucleobase clusters provides detailed insight into the photophysics and photochemistry of a model DNA photopharmaceutical.

Electrospray ionization mass spectrometry of non-covalent complexes formed between N-alkylimidazolium-containing zwitterionic sulfonates and protonated bases

This paper describes non-covalent complexes between zwitterionic 3-(1-alkyl-3N-imidazolio)- propane-1-sulfonates and different amines. Electrospray ionization (ESI) mass spectrometry and collision- induced dissociation were used to measure the stability of such complexes in solution and in the gas phase. Generally, zwitterionic sulfonates formed more abundant complexes with protonated 5-methylcytosine (5-MCH) than with aliphatic amines. The results show that the association constants and half-dissociation threshold energies of these complexes nonlinearly depend on the alkyl chain length of the zwitterion. It is shown that the complexes with the lowest stability exist in acetonitrile solution or in the gas phase. The factors responsible for this complicated behavior are discussed. The structure of the complexes was investigated by quantum chemical calculations using molecular mechanics and density functional theory. Hydrogen bonding is proposed as the main type of interaction responsible for the stability of ion-zwitterion complexes. In summary, the information obtained in this study could be used for the development of the new derivatization reagents for some compounds containing amidinium groups, like 5-MCH, to increase selectivity of ESI-based methods.

Performance of M06, M06-2X, and M06-HF density functionals for conformationally flexible anionic clusters: M06 functionals perform better than B3LYP for a model system with dispersion and ionic hydrogen-bonding interactions

We present a comparative assessment of the performance of the M06 suite of density functionals (M06, M06-2X, and M06-HF) against an MP2 benchmark for calculating the relative energies and geometric structures of the Cl(-)·arginine and Br(-)·arginine halide ion-amino acid clusters. Additional results are presented for the popular B3LYP density functional. The Cl(-)·arginine and Br(-)·arginine complexes are important prototypes for the phenomenon of anion-induced zwitterion formation. Results are presented for the canonical (noncharge separated) and zwitterionic (charge separated) tautomers of the clusters, as well as the numerous conformational isomers of the clusters. We find that all of the M06 functions perform well in terms of predicting the general trends in the conformer relative energies and identifying the global minimum conformer. This is in contrast to the B3LYP functional, which performed significantly less well for the canonical tautomers of the clusters where dispersion interactions contribute more significantly to the conformer energetics. We find that the M06 functional gave the lowest mean unsigned error for the relative energies of the canonical conformers (2.10 and 2.36 kJ/mol for Br(-)·arginine and Cl(-)·arginine), while M06-2X gave the lowest mean unsigned error for the zwitterionic conformers (0.85 and 1.23 kJ/mol for Br(-)·arginine and Cl(-)·arginine), thus providing insight into the types of physical systems where each of these functionals should perform best.

Electrostatically defying cation-cation clusters: can likes attract in a low-polarity environment?

Like-charge ion pairing is commonly observed in protein structures and plays a significant role in biochemical processes. Density functional calculations combined with the conductor-like polarizable continuum model were employed to study the formation possibilities of doubly charged noncovalently linked complexes of a series of model compounds and amino acids in the gas phase and in solution. Hydrogen bond interactions were found to offset the Coulombic repulsion such that cation-cation clusters are minima on the potential energy surfaces and neither counterions nor solvent molecules are needed to hold them together. In the gas phase the dissociation energies are exothermic, and the separation barriers span from 1.7 to 15.6 kcal mol(-1). Liquid-phase computations indicate that the separation enthalpies of the cation-cation complexes become endothermic in water and nonpolar solvents with dielectric constants of ≥7 (i.e., the value for THF). These results reveal that electrostatically defying noncovalent complexes of like-charged ions can overcome their Coulombic repulsion even in low-polarity environments.