The fate of DCl molecules striking pure glycerol and a 2.6 M NaI-glycerol solution is investigated using scattering, uptake, and residence time measurements. We find that dissolved Na+ and I- ions alter every gas-liquid pathway from the moment of contact of DCl with the surface to its eventual emergence as HCl. In particular, the salt enhances both trapping-desorption of DCl and interfacial DCl --> HCl exchange at the expense of DCl entry into the bulk solution. The reduced entry and enhanced desorption of thermalized DCl molecules are interpreted by assuming that Na+ and I- ions bind to interfacial OH groups and tie up surface sites that would otherwise capture incoming DCl molecules. These ion-glycerol interactions may also be responsible for enhancing interfacial D --> H exchange by disrupting the interfacial hydrogen bond network that carries the newly formed H+ ion away from its Cl- pair. This disruption may increase the fraction of interfacial Cl- and H+ that recombine and desorb immediately as HCl before the ions separate and diffuse deeply into the bulk.

Gas−liquid scattering experiments are used to explore the role of interfacial ions in controlling collisions and reactions of DCl molecules with ∼2.5 M KI, NaI, LiI, and NaBr glycerol solutions. DCl molecules that thermalize upon collision with the surface of each solution follow one of three pathways:  they may desorb immediately as DCl, undergo D → H exchange in the near-interfacial region and desorb as HCl, or dissolve and dissociate in the bulk. We find that each salt amplifies the differences among the channels:  the fraction of DCl molecules that desorb intact increases from 9% for pure glycerol to 18% on average for the salt solutions, interfacial DCl → HCl exchange increases from 3 to 7%, and bulk dissolution drops from 87 to 75%. Dissolved ions may enhance nonreactive DCl desorption and interfacial D → H exchange by tying up glycerol OH groups, which will impede both DCl−OH bonding and migration of D+ and H+ away from Cl- after DCl ionization. Despite differences in the sizes of the ions and their interactions with the solvent, the enhancements are found to be similar for NaI, LiI, and NaBr and slightly smaller for KI. These similarities may arise from an interplay between ion pairing, the strength and number of ion−solvent bonds, and ion exposure at the surface.

Monolayers of an amyloid ␤ peptide (A␤40) and a much smaller model peptide (LSFD) at the air/water interface have been investigated by isotherm, IRRAS and GIXD measurements. Additionally, the LSFD monolayer has been transferred onto solid support and investigated by ATR-FTIR to test the influence of the transfer on the secondary structure of the peptide. Both peptides are surface active and form stable films of ordered ␤-sheet domains on the surface. The same absorption bands characteristic of an anti-parallel ␤-sheet conformation can be seen in the transferred LSFD film indicating that the transfer does not change the secondary structure. On the water surface, the ␤-sheets are oriented mostly parallel to the surface. GIXD experiments show a Bragg peak at characteristic repeat distances of 4.75-4.8Å for both peptides. The full-width at half maximum (fwhm) of this peak shows that the smaller LSFD peptide forms a monolayer film with high degree of order perpendicular to the ␤-strands, whereas A␤40 exhibits a drastically reduced crystallinity.

Abstract:The structure of the antimicrobial peptide NK-2 has been studied at the air-water interface and in different solutions using spectroscopic methods such as circular dichroism (CD) and infrared reflection absorption spectroscopy (IRRAS) as well as specular X-ray reflectivity (XR). NK-2 adopts an unordered structure in water, buffer, and in the presence of monomeric cationic and noncharged amphiphiles. However, it forms a stable α-helix in 2,2,2-trifluoroethanol (TFE) and in micellar solutions of anionic, cationic as well as nonionic amphiphiles, whereas only in sodium dodecyl sulfonate solutions the α-helical structure can also be found below the critical micellar concentration (cmc). The amphiphilic molecule NK-2 is surface active and forms a Gibbs monolayer at the air-buffer interface. In contrast, no adsorption was observed if NK-2 is dissolved in water. During the adsorption process in buffer solutions, NK-2 undergoes a conformational transition from random coil in bulk to α-helix at the interface. This change of the peptide's secondary structure is known to be associated with its antimicrobial activity. A comparison of the experimental IRRA spectra with the simulated spectra indicates that the adsorbed NK-2 α-helix lies flat at the interface. This is confirmed by XR measurements which show that the thickness of the NK-2 layer is ∼17Å, which is the average diameter of a α-helix, indicating that only a monomolecular adsorption layer is formed.