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Structural Influences of Noncovalent Interactions in the Gas Phase


The physical properties of molecules in solution, such as basicity and structure, depend on the cooperation and competition of noncovalent intra- and intermolecular interactions. Studying these interactions in the condensed phase is made difficult by the presence of competing influences from counterions and impurities. In the gas phase, however, specific ions, ion complexes and hydration states can be isolated and studied by Fourier transform mass spectrometry coupled with infrared (IR) laser spectroscopy. Using these two techniques, it is possible to isolate specific ions before inducing dissociation via absorption of IR photons. The extent of absorption at a given wavelength correlates to the relative abundance of product ions produced via dissociation, which can be measured using mass spectrometry. The absorption of IR photons only occurs at specific wavelengths depending on which functional groups are present and how their vibrational modes are influenced by interactions such as hydrogen bonding. Structural information is obtained from these spectra by interpreting the presence of certain bands and their frequencies. In addition, information can also be obtained by comparing the spectra from ions of interest to the spectra of reference ions, with known structures, or the simulated spectra of computed geometries. These types of studies provide valuable insight into how noncovalent interactions govern the structure of biomolecules and hydrogen-bonded networks. This dissertation reports experiments utilizing IR spectroscopy to study how water-ion interactions can affect both the structure of an ion solvated by an aqueous nanodrop as well as the hydrogen-bonding network of the nanodrop itself. In addition, the structural effects of ion-peptide interactions, which are relevant to understanding how ions influence biological processes, are also investigated.

In order to study the ability of water to stabilize protonation sites on larger molecules, I investigated the influence of sequential hydration on the structure of protonated p-aminobenzoic acid (PABAH+), which has different preferred aqueous solution and gas-phase protonation sites. The preferred protonation site of PABA is the amine in aqueous solution, but the preferred protonation site is the carbonyl O atom of the carboxylic acid in the gas phase. The spectrum of PABAH+*(H2O)1 contains an absorption band at a particular photon energy indicating that protonation occurs at the carboxylic acid, i.e. there is a spectroscopic signature for the O-protonated structure. This absorption band persists for PABAH+*(H2O)2-6, indicating that these ions have a population of O-protonated isomers as well. Spectra for PABAH+*(H2O)6 are also consistent with presence of a second isomer, in which the amine is protonated. These results indicate that PABAH+ exists in the preferred gas-phase structure for PABAH+*(H2O)1-6, but there is a transition to the preferred solution-phase structure when the ion is solvated by six or more water molecules. In isolation, the excess charge associated with protonation at the carbonyl O atom of the carboxylic acid can be resonantly stabilized and delocalized into the phenyl ring and amine. When six or more water molecules are attached, however, a more favorable hydrogen-bonding network can be formed at the protonated amine than at the carboxylic acid.

In contrast to PABAH+, protonation for m-aminobenzoic acid (MABA) occurs at the amine site even when solvated by only one water molecule due to orientation of the amine and carboxylic acid group. This orientation prevents the positive charge from being delocalized into the amine. Thus, MABAH+ serves as an ideal model for the solvation of the N- and C-termini of a protonated amino acid, for which the N- and C-termini typically interact with each other. The measured spectra for MABAH+*(H2O)1,2 are consistent with the attachment of water to a H atom of the protonated amine. For MABAH+*(H2O)3, the measured spectrum indicates that the dominant isomer has a hydrogen-bonded water bridge between the amine and carbonyl O atom of the carboxylic acid. This result indicates that the formation of this water bridge is more energetically favorable than the formation of a third ionic hydrogen bond to the amine group. The spectra for MABAH+*(H2O)n also indicate that water molecules attach to the carboxylic acid H atom, i.e. the ion is fully hydrogen-bonded when there are ≥6 water molecules attached.

Ion spectroscopy can also be used to study how ion-water interactions influence hydration structures. Certain positive ions are known to induce cage-like clathrate structures when hydrated by 20 water molecules. The hydration of NH4+ as well as selected, protonated primary, secondary and tertiary amines solvated by 19 - 21 water molecules was investigated in order to elucidate details about how amines can stabilize clathrate structures. The spectra of NH4+ as well as monomethyl-, n-heptyl-, and tert-butylammonium+ with 20 water molecules attached are consistent with the nearly exclusive presence of clathrate structures, whereas nonclathrate structures are present for the more highly substituted amines. By comparison, nonclathrate structures are observed for all ions when 19 or 21 water molecules are attached. Spectroscopic evidence for clathrate structures for NH4+*(H2O)20 has been previously reported, but the location of the ion, whether at the surface or the interior, was difficult to determine based on the IR spectrum of this ion alone. Thus, the spectra of NH4+, monomethyl- and n-heptylammonium+ solvated by 20 water molecules were compared to those for Rb+ and tert-butylammonium+, which serve as references for clathrate structures with the ion located in the interior or at the surface, respectively. These comparisons indicate that NH4+ goes to the interior, whereas protonated primary amines are located at the surface, irrespective of the size of the alkyl group.

In addition to ion-water interactions, ion-biomolecule interactions can also be probed by ion spectroscopy. Although there are several studies that have used ion spectroscopy to investigate cations coordinated to amino acids and peptides, there are fewer studies focused on these same biomolecules complexed with anion adducts. The ions Gly3*X-, Ala3*X- and Leu3*X- (X = Cl, Br and I) were studied in order to investigate how the size of anion adducts and alkyl side chains influence the coordination of halide anions to aliphatic peptides. The spectra of Gly3*Cl-, Ala3*Cl- and Leu3*Cl- suggest that all three complexes adopt similar structures, where Cl- coordinates to the peptides by accepting three or four hydrogen bonds from the amides as well as the N- and C-termini. These results indicate that the size of the alkyl chain does not have a significant influence on the coordination geometry of these complexes. These structures are "inverted" in comparison to previously reported structures for Gly3*Na+ and Ala3*Na+, where the Na+ coordinates to lone pair electrons of the N atom of the N-terminus, or the carbonyl O atoms of the amides and C-terminus. The spectra of Gly3*X-, Ala3*X- and Leu3*X- each appear similar to each other within each peptide, indicating that the size of the anion does not significantly affect the coordination geometry.

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