| 1. |
- Aziz, Emad F., et al.
(författare)
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Cation-specific interactions with carboxylate in amino acid and acetate aqueous solutions : X-ray absorption and ab initio calculations
- 2008
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 112:40, s. 12567-12570
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Tidskriftsartikel (refereegranskat)abstract
- Relative interaction strengths between cations (X = Li+, Na+, K+, NH4+) and anionic carboxylate groups of acetate and glycine in aqueous solution are determined. These model systems mimic ion pairing of biologically relevant cations with negatively charged groups at protein surfaces. With oxygen ls X-ray absorption spectroscopy, we can distinguish between spectral contributions from H2O and carboxylate, which allows us to probe the electronic structure changes of the atomic site of the carboxylate group being closest to the countercation. From the intensity variations of the COOaq- ls X-ray absorption peak, which quantitatively correlate with the change in the local partial density of states from the carboxylic site, interactions are found to decrease in the sequence Na+ > Li+ > K+ > NH4+. This ordering, as well as the observed bidental nature of the -COOaq- and X-aq(+) interaction, is supported by combined ab initio and molecular dynamics calculations.
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| 2. |
- Aziz, Emad F., et al.
(författare)
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Interaction between liquid water and hydroxide revealed by core-hole de-excitation
- 2008
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 455:7209, s. 89-91
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Tidskriftsartikel (refereegranskat)abstract
- The hydroxide ion plays an important role in many chemical and biochemical processes in aqueous solution(1). But ourmolecular- level understanding of its unusual and fast transport in water, and of the solvation patterns that allow fast transport, is far from complete. One proposal seeks to explain the properties and behaviour of the hydroxide ion by essentially regarding it as a water molecule that is missing a proton(2), and by inferring transport mechanisms and hydration structures from those of the excess proton. A competing proposal invokes instead unique and interchanging hydroxide hydration complexes, particularly the hypercoordinated OH-(H2O)(4) species and tri- coordinated OH-(H2O)(3) that can form a transient hydrogen bond between the H atom of the OH- and a neighbouring water molecule(3-5). Here we report measurements of core- level photoelectron emission and intermolecular Coulombic decay(6-8) for an aqueous hydroxide solution, which show that the hydrated hydroxide ion is capable of transiently donating a hydrogen bond to surrounding watermolecules. In agreement with recent experimental studies of hydroxide solutions(9-12), our finding thus supports the notion that the hydration structure of the hydroxide ion cannot be inferred from that of the hydrated excess proton.
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| 3. |
- Aziz, Emad F, et al.
(författare)
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Probing the electronic structure of the hemoglobin active center in physiological solutions.
- 2009
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 102:6, s. 068103-
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Tidskriftsartikel (refereegranskat)abstract
- Soft-x-ray absorption spectroscopy at the L_{2,3} edge of the iron center in bovine hemoglobin and hemin under physiological conditions is reported for the first time. Spectra of the same compounds in solid form are presented for comparison. Striking differences in the electronic structure of the metalloporphyrin are observed between the liquid and solid compounds. We unambiguously show that hemoglobin and hemin are in a high-spin ferric state in solution, and that the 2p spin-orbit coupling decreases for hemin compared to the hemoglobin, while this is not the case in solids. The spectra were simulated using ligand field multiplet theory, in good agreement with the experiment, allowing quantification of the amount of charge transfer between the porphyrin and Fe3+ ion in hemoglobin and in hemin.
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| 4. |
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| 5. |
- Enarsson, Lars-Erik, et al.
(författare)
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Tailoring the chemistry of polyelectrolytes to control their adsorption on cellulosic surfaces
- 2009
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Ingår i: Colloids and Surfaces A. - : Elsevier BV. - 0927-7757 .- 1873-4359. ; 340:1-3, s. 135-142
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption of two types of cationic polyacrylamides in the presence of a background electrolyte was studied on substrates including silicon oxide, kraft pulp fibres and thin films of regenerated cellulose. One of the polyelectrolytes was a conventional cationic polyacrylamide (MeCPAM) and the other was a benzyl-containing derivative of cationic polyacrylamide (BzCPAM). It was found that the aromatic substituents of BzCPAM strongly enhanced the adsorption on substrates of hydrophobically modified silicon oxide when a background electrolyte was present. A similar effect was also seen in the case of BzCPAM adsorption on unbleached pulp fibres, but in this case MeCPAM also exhibited a strong adsorption at high background electrolyte concentrations. On bleached pulp fibres, BzCPAM maintained a high adsorption up to a concentration of 100 mM NaCl whereas MeCPAM adsorption showed a significant decrease at this salt concentration. On model cellulose films, the adsorption of BzCPAM was similar to that of MeCPAM and the non-ionic interactions with cellulose appeared weak, since there was no adsorption of either polyelectrolyte in 300 mM NaCl.
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| 6. |
- Nolting, Dirk, et al.
(författare)
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pH-induced protonation of lysine in aqueous solution causes chemical shifts in X-ray photoelectron spectroscopy.
- 2007
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 129:45, s. 14068-73
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate the applicability of X-ray photoelectron spectroscopy to obtain charge- and site-specific electronic structural information of biomolecules in aqueous solution. Changing the pH of an aqueous solution of lysine from basic to acidic results in nitrogen 1s and carbon 1s chemical shifts to higher binding energies. These shifts are associated with the sequential protonation of the two amino groups, which affects both charge state and hydrogen bonding to the surrounding water molecules. The N1s chemical shift is 2.2 eV, and for carbon atoms directly neighboring a nitrogen the shift for C1s is approximately 0.4 eV. The experimental binding energies agree reasonably with our calculated energies of lysine(aq) for different pH values.
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| 7. |
- Nolting, Dirk, et al.
(författare)
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Pseudoequivalent nitrogen atoms in aqueous imidazole distinguished by chemical shifts in photoelectron spectroscopy.
- 2008
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 130:26, s. 8150-1
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Tidskriftsartikel (refereegranskat)abstract
- The photoelectron spectra of aqueous imidazole are presented, and the N 1s and C 1s binding energies are assigned aided by density functional theory calculations. The chemical equivalency of the two nitrogens of the cationic form is directly identified by the occurrence of a single N 1s photoelectron peak, which results from the delocalization of the positive charge over the molecule as a consequence of the Cv symmetry of the system. In contrast to NMR measurements, the photoemission process is faster than the rapid proton exchange in the aqueous environment, making the pseudoequivalent nitrogens of the neutral state clearly distinguishable with a N 1s binding energy shift of 1.7 eV.
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| 8. |
- Ottosson, Niklas, et al.
(författare)
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An electronic signature of hydrolysation in the X-ray absorption spectrum of aqueous formaldehyde
- 2008
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 460:4-6, s. 540-542
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Tidskriftsartikel (refereegranskat)abstract
- Formaldehyde in aqueous solution is hydrolysed and forms methanediol. Using X-ray absorption spectroscopy we show that the hydrolysation product can be identified by a distinct electronic signature in the spectra. This is manifested by the disappearance of the oxygen 1s -> ; pi* absorption line. The X-ray absorption spectrum of aqueous formaldehyde is compared with those of the structurally similar formamide and urea, which are in contrast not hydrolysed in aqueous solution. We thereby demonstrate the exceptional sensitivity and simplicity of the technique to monitor this fundamental process in the aqueous phase.
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| 9. |
- Ottosson, Niklas, et al.
(författare)
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Electronic rearrangement upon the hydrolyzation of aqueous formaldehyde studied by core-electron spectroscopies
- 2008
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 112:51, s. 16642-16646
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Tidskriftsartikel (refereegranskat)abstract
- We have combined near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS) to study the electronic rearrangement associated with the hydrolyzation of formaldehyde to methanediol in aqueous solution. The spectra are contrasted against those of aqueous formamide and urea, which are structurally similar but do not undergo hydrolysis in solution. We have recently demonstrated that the hydrolyzation of formaldehyde is manifested in the oxygen Is NEXAFS spectrum by the disappearance of the oxygen 1s -> pi* absorption line. This is a characteristic signature that the C=O double bond has been broken. In the present study we extend our investigation to include carbon Is NEXAFS and XPS spectra of the three solutions. The carbon NEXAFS spectra show the C 1s -> pi* absorption line for each solute except for formaldehyde. Moreover, the carbon Is photoelectron spectra exhibit a single peak for each solute. These observations point to a near complete hydrolyzation of formaldehyde, whereas formamide and urea remain intact in the solution. The analysis is further supported by density functional theory (DFT) calculations, showing a C Is chemical shift of approximately 1.0 eV between hydrolyzed and nonhydrolyzed forms, which would give distinguishable features in the photoemission spectrum, if coexisting forms were present in the solutions.
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| 10. |
- Ottosson, Niklas, et al.
(författare)
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Large variations in the propensity of aqueous oxychlorine anions for the solution/vapor interface
- 2009
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 131:12, s. 124706-
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Tidskriftsartikel (refereegranskat)abstract
- Core-level photoelectron spectroscopy measurements have been performed of aqueous solutions of NaCl codissolved with NaClOn (n=1-4). Each species has a distinct Cl 2p electron binding energy, which can be exploited for depth-profiling experiments to study the competition between Cl-and ClOn- anions for residing in the outermost layers of the solution/vapor interface. Strongest propensity for the surface is observed for n=4 (perchlorate), followed by n=3 (chlorate), n=2 (chlorite), n=0 (chloride), and n=1 (hypochlorite). Molecular dynamics simulations rationalize the greatest surface propensity of the most oxidized anions in terms of their larger size and polarizability. The anomalous behavior of hypochlorite, being less surface-active than chloride, although it is both larger and more polarizable, is suggested to arise from the charge asymmetry over the anion, increasing its efficiency for bulk solvation. (C) 2009 American Institute of Physics. [doi:10.1063/1.3236805]
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