The role of water in stabilizing sites of protonation in small gaseous ions is investigated using electrospray ionization (ESI) coupled with infrared photodissociation spectroscopy and computational chemistry. Protonation of p-aminobenzoic acid (PABA) and p-aminobenzoic acid methyl ester (PABAOMe) occurs at the carbonyl oxygen atom both in isolation and when one water molecule is attached. However, protonation occurs at the amine nitrogen atom, which is the most favorable site in aqueous solution, for PABAOMeH(+)·(H(2)O)(3) and for a significant fraction of PABAH(+)·(H(2)O)(6). Fewer water molecules are necessary to stabilize the solution-phase isomer of PABAOMeH(+) (3) than for PABAH(+) (≥6), indicating that the favorable hydrogen bonding in PABAH(+) is a more important factor than the higher gas-phase basicity of PABAOMeH(+) in stabilizing protonation at the carbonyl oxygen atom. Relative Gibbs free energies (133 K) calculated using B3LYP and MP2 with the 6-311++G** basis set were significantly different from each other, and both are in poor agreement with results from the experiments. ωB97X-D/6-311++G**, which includes empirical dispersion corrections, gave results that were most consistent with the experimental data. The relative stabilities of protonating at the carbonyl oxygen atom for PABAH(+)·(H(2)O)(0-6) and PABAOMeH(+)·(H(2)O)(0-2) can be rationalized by resonance delocalization. These findings provide valuable insights into the solvent interactions that stabilize the location of a charge site and the structural transitions that can occur during the ESI desolvation process.
The binding sites of water molecules to protonated Phe and its derivatives are investigated using infrared photodissociation (IRPD) spectroscopy and kinetics as well as by computational chemistry. Calculated relative energies for hydration of PheH(+) at various sites on the N- and C-termini depend on the type of theory and basis set used, and no one hydration site was consistently calculated to be most favorable. Infrared photodissociation (IRPD) spectra between approximately 2650 and 3850 cm(-1) are reported for PheH(+)(H(2)O)(1-4) at 133 K and compared to calculated absorption spectra of low-energy hydration isomers, which do not resemble the IRPD spectra closely enough to unambiguously assign spectral bands. The IRPD spectra of PheH(+)(H(2)O)(1-4) are instead compared to those of N,N-Me(2)PheH(+)(H(2)O)(1,2), N-MePheH(+)(H(2)O)(1-3), and PheOMeH(+)(H(2)O)(1-3) at 133 K, which makes possible systematic band assignments. A unique band associated with a binding site not previously reported for PheH(+)(H(2)O), in which the water molecule accepts a hydrogen bond from the N-terminus of PheH(+) and donates a weak hydrogen bond to the pi-system of the side chain, is identified in the IRPD spectra. IRPD kinetics at laser frequencies resonant with specific hydration isomers are found to be biexponential for N,N-Me(2)PheH(+)(H(2)O), N-MePheH(+)(H(2)O), and PheH(+)(H(2)O). Relative populations of ions with water molecules attached at various binding sites are determined from fitting these kinetic data, and relative energies for hydration of these competitive binding sites at 133 K are obtained from these experimental values.
The origin of enhanced abundances for some hydrated alkali metal ions, M(+)(H2O)n, where M = Cs, Rb, K, Na, and Li with between 17 and 21 water molecules attached was investigated with infrared photodissociation (IRPD) spectroscopy and by blackbody infrared radiative dissociation (BIRD) at 133 K. The abundances of clusters of Cs(+), Rb(+), and K(+) with 18 and 20 water molecules are anomalously high compared to the corresponding clusters of Na(+), and Li(+) with 20 water molecules has only a slightly enhanced abundance. BIRD results indicate that the anomalous abundance at n = 20 for the larger ions is due to the high stability of this cluster, and the significant instability of the next largest cluster, consistent with a stable core structure with 20 water molecules. IRPD spectra in the free-OH region (∼3600-3800 cm(-1)) for Cs(+), Rb(+), and K(+) with 18 and 20 water molecules indicates that water molecules with a free-OH stretch accept two hydrogen bonds and donate one hydrogen bond (acceptor-acceptor-donor water) to other water molecules. No acceptor-donor (AD) bands are observed, consistent with clathrate structures for these ions. In contrast, the AD band is significant for Na(+), indicating that these clusters adopt different structures. Results for Li(+) indicate a contribution from clathrate structures at n = 20, but not at other cluster sizes. This analysis is supported by the relative intensities of bands in the hydrogen-bonding region for n = 20.
Water exhibits remarkable properties in confined spaces, such as nanometer-sized droplets where hundreds of water molecules are required for crystalline structure to form at low temperature due to surface effects. Here, we investigate how a single ion affects the crystallization of (H2O)n clusters with infrared photodissociation spectroscopy of size-selected La(3+)(H2O)n nanodrops containing up to 550 water molecules. Crystallization in the ion-containing nanodrops occurs at n ≥ 375, which is approximately 100 more water molecules than what has been reported for neutral water clusters. This frustration of crystallinity reveals that La(3+) disrupts the hydrogen-bonding network of water molecules located remotely from the ion, a conclusion that is supported by molecular dynamics simulations. Our findings establish that a trivalent ion can pattern the H-bond network of water molecules beyond the third solvation shell, or to a distance of ∼1 nm from the ion.
The presence of many salts, such as sodium chloride, can adversely affect the performance of native electrospray ionization mass spectrometry for the analysis of proteins and protein complexes by reducing the overall molecular ion abundances and distributing signal for any given charge state into many cationized forms with various numbers of adducts attached. Several solution additives, such as ammonium bromide, ammonium iodide, and NaSbF6, can significantly lower the extent of sodium ion adduction to the molecular ions of proteins and protein complexes. For ubiquitin, addition of 25 mM ammonium bromide or ammonium iodide into aqueous solutions also containing 1.0 mM NaCl results in a factor of 72 and 56 increase, respectively, in the relative abundances of the fully protonated molecular ions compared to when these additives are not present. The effectiveness of this method for reducing sodium ion adduction is related to the low proton affinity (PA) values of the anions. Anions with very low PA also have a propensity to adduct as an acid molecule, but these adducts can be readily dissociated from the molecular ions either by activation in the source or subsequently by collisional activation in the mass spectrometer. This method of reducing sodium ion adduction to proteins is simple and requires no experimental modifications, making it an attractive alternative to other methods for desalting proteins prior to mass spectrometry analysis.
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