Katherine Cimatu

Methyl Groups of Trimethylamine N-Oxide Orient Away from Hydrophobic Interfaces

The molecular orientation of trimethylamine N-oxide (TMAO), a powerful protein stabilizer, was explored at aqueous/hydrophobic interfaces using vibrational sum frequency spectroscopy (VSFS). The systems studied included the octadecyltrichlorosilane (OTS)/water interface, which represents an aqueous solution in direct contact with a hydrophobic medium. Surprisingly, the measurements revealed that the methyl groups of TMAO pointed into the aqueous phase and away from the OTS. This orientation may arise from the more hydrophilic nature of methyl groups attached to a strongly electron-withdrawing atom such as a quaternary nitrogen. Additional studies were performed at the air/water interface. This interface showed a high degree of TMAO alignment, but the dangling OH from water was present even at 5 M TAMO. Moreover, the addition of this osmolyte modestly increased the surface tension of the interface. This meant that this species was somewhat depleted at the interface compared to the bulk solution. These findings may have implications for the stabilizing effect of TMAO on proteins. Specifically, the strong hydration required for the methyl groups as well as the oxide moiety should be responsible for the osmolytes depletion from hydrophobic/aqueous interfaces. Such depletion effects should help stabilize proteins in their folded and native conformations on entropic grounds, although orientational effects may play an additional role.

Chemical Imaging of Corrosion: Sum Frequency Generation Imaging Microscopy of Cyanide on Gold at the Solid−Liquid Interface

Sum frequency generation (SFG) imaging is used to monitor, in situ, the reaction of cyanide ions with gold surface. Spatial and chemical variations across the surface are observed as a function of time. The initial period resulted in the formation of linearly bound cyanide to gold, and continuous exposure of gold film to cyanide solution led to the presence of higher-coordinated gold-cyanide complexes. These species were identified by their specific position in the SFG vibrational spectrum (2105, 2140, 2170, and 2225 cm(-1)). The relative amounts of these gold-cyanide species varied across the surface as resolved by SFG microscopy.

Engineered silica nanoparticles interact differently with lipid monolayers compared to lipid bilayers

Despite a number of fundamental studies, the interactions of engineered nanoparticles with the cell plasma membrane are still not well understood. Membrane models, such as lipid monolayers and bilayers, have been used to provide a mechanistic understanding of nanoparticle effects on the structure and integrity of the cell membrane. However, the role of the membrane model itself in regulating nanoparticle–lipid interactions has generally been overlooked. In an effort to elucidate how changing the membrane model affects the resulting interactions with nanoparticles, the current study investigates the interaction of silica nanoparticles (104 ± 5 nm) with different surface-functional groups: plain (silanol), amine, and poly(ethylene glycol) (PEG), with various molecular weights of 2k, 5k, and 20k Daltons, with lipid monolayers and bilayers of the same composition. Studies with lipid monolayers demonstrated that PEGylated particles, regardless of the PEG molecular weight, and unlike plain and amine-modified particles, were able to penetrate the monolayer and disrupt the structure of lipids as evidenced by tensiometry and atomic force microscopy. In contrast, when interacting with lipid bilayers in suspension (i.e. vesicles), plain, amine-modified, and PEG 20k-modified particles disrupted vesicle structure, causing significant leakage, while PEG 2k- and 5k-modified particles did not affect the integrity of the vesicles. In summary, while the interactions of silica nanoparticles with membrane models is dependent on particle surface properties, a clear difference is observed in particle interactions with lipid monolayers compared to bilayers.

Successive Surface Reactions on Hydrophilic Silica for Modified Magnetic Nanoparticle Attachment Probed by Sum-Frequency Generation Spectroscopy

Successive surface reactions on hydrophilic silica substrates were designed and performed to immobilize ethanolamine-modified magnetic ferrite-based nanoparticle (NP) for surface characterization. The various surfaces were monitored using sum-frequency generation (SFG) spectroscopy. The surface of the hydrophilic quartz substrate was first converted to a vinyl-terminated surface by utilizing a silanization reaction, and then, the surface functional groups were converted to carboxylic-terminated groups via a thiol-ene reaction. The appearance and disappearance of the vinyl (═CH2) peak at ∼2990 cm-1 in the SFG spectra were examined to confirm the success of the silanization and thiol-ene reactions, respectively. Acyl chloride (-COCl) formation from carboxy (-COOH) functional group was then performed for further attachment of magnetic amine-functionalized magnesium ferrite nanoparticles (NPs) via amide bond formation. The scattered NPs attached on the modified silica substrate was then used to study the changes in the spectral profile of the ethanolamine modifier of the NPs for in situ lead(II) (Pb2+) adsorption at the solid-liquid interface using SFG spectroscopy. However, due to the limited number of NPs attached and sensitivity of SFG spectroscopy toward expected change in the modifier spectroscopically, no significant change was observed in the SFG spectrum of the modified silica with magnetic NPs during exposure to Pb2+ solution. Nevertheless, SFG spectroscopy as a surface technique successfully monitored the modifications from a clean fused substrate to -COCl formation that was used to immobilize the decorated magnetic nanoparticles. The method developed in this study can provide a reference for many surface or interfacial studies important for selective attachment of adsorbed organic or inorganic materials or particles.

Ultrafast transient absorption spectra of photoexcited YOYO-1 molecules call for additional investigations of their fluorescence quenching mechanism

In this report, we observed that YOYO-1 immobilized on a glass surface is much brighter when dried (quantum yield 16±4% in the ambient air) or in hexane than in water (quantum yield ~%).YOYO-1 is a typical cyanine dye that has a photo-isomerization reaction upon light illumination. In order to understand this quenching mechanism, we use femtosecond transient absorption spectroscopy to measure YOYO-1s electron dynamics after excitation directly. By deconvoluting the hot-ground-state absorption and the stimulated emission, the dynamics of electronic relaxation and balance are revealed. The results support the intermolecular charge transfer mechanism better than the intramolecular relaxation mechanism that has been widely believed before. We believe that the first step of the relaxation involves a Dexter charge transfer between the photo-excited YOYO-1 molecule and another guest molecule that is directly bound to the YOYO-1 giving two radicals with opposite signs of charges. The charges are recombined either directly between these two molecules, or both molecules start to rotate and separate from each other. Eventually, the two charges recombined non-radiatively via various pathways. These pathways are reflected on the complicated multi-exponential decay curves of YOYO-1 fluorescence lifetime measurements. This charge transfer mechanism suggests that (1) electrical insulation may help improve the quantum yield of YOYO-1 in polar solutions significantly and (2) a steric hindrance for the intramolecular rotation may have a less significant effect.