Jennifer S. Hovis

Reactions of substituted aromatic hydrocarbons with the Si(001) surface

The interactions of toluene, para-xylene, meta-xylene and ortho-xylene with the (001) surface of silicon have been investigated using Fourier-transform infrared spectroscopy. Infrared spectra show that these methyl-substituted aromatic hydrocarbons are chemisorbed and oriented on the Si(001) surface at both 110 and 300 K. Peaks in the Si–H stretching region indicate that some dissociation occurs upon adsorption. Comparisons of infrared spectra of these molecules with deuterated and nondeuterated methyl groups reveal that the major source of decomposition is likely from C–H cleavage of the substituent groups, leaving the ring intact. Additionally, the striking similarity of the infrared spectra of benzene, toluene and the xylene isomers suggests that the methyl-substituted aromatic rings interact with the Si(001) surface in much the same way as benzene. Differences in relative peak intensity point to the possibility that the methyl substituent groups may steer the ring into different ratios of specific bon...

Controlled formation of organic layers on semiconductor surfaces

It has been shown that well-defined, ordered organic layers can be formed on the silicon (100) surface. This is achieved through the interaction of unsaturated C=C bonds with the oriented dimers of the reconstructed Si(100)-(2×1) surface. In this article, we present an investigation of the structure and chemical bonding of organic films prepared using different organic precursors. Data were obtained using scanning tunneling microscopy, Fourier-transform infrared spectroscopy, and x-ray photoelectron spectroscopy. The molecules investigated are cyclopentene, 3-pyrroline, and norbornadiene, representing prototypical cyclic, heterocyclic, and bicyclic unsaturated organic molecules, respectively. Each molecule has at least one unsaturated C=C bond.

SCANNING TUNNELING MICROSCOPY OF ORGANIC MOLECULES AND MONOLAYERS ON SILICON AND GERMANIUM (001) SURFACES

High-resolution scanning tunneling microscopy has been used to investigate the adsorption of a variety of unsaturated organic molecules on the (001) surface of silicon and germanium. Results are presented for a number of prototypical alkenes, conjugated and non-conjugated dienes, and aromatic compounds. These include cyclopentene, 1,5-cyclooctadiene, norbornadiene, 2,3-dimethyl-1,3-butadiene, and benzene on the Si(001) surface, and for cyclopentene on the Ge(001) surface. Factors controlling the selectivity of various reaction pathways and the consequences for preparation of ordered organic overlayers are discussed.

Cycloaddition chemistry on germanium(001) surfaces: the adsorption and reaction of cyclopentene and cyclohexene

The adsorption and reaction of two cyclic unsaturated hydrocarbons, cyclopentene and cyclohexene, with the Ge(001) surface has been investigated. The subsequent surface interactions were followed by a variety of surface-sensitive techniques. Bonding configurations were determined by photoelectron spectroscopy, infrared spectroscopy and scanning tunneling microscopy. The strength of surface interactions was also monitored by temperature-programmed desorption. Cyclopentene and cyclohexene react with surface germanium-dimer bonds to yield reaction products that are consistent with a [2+2] cycloaddition reaction. This reaction generates rows of the surface complex oriented along the dimer-bond direction of the Ge(001) surface, which is easily observed by scanning tunneling microscopy.

Adsorption of α-Synuclein on Lipid Bilayers: Modulating the Structure and Stability of Protein Assemblies

The interaction of alpha-synuclein with phospholipid membranes has been examined using supported lipid bilayers and epi-fluorescence microscopy. The membranes contained phosphatidylcholine (PC) and phosphatidic acid (PA), which mix at physiological pH. Upon protein adsorption, the lipids undergo fluid-fluid phase separation into PC-rich and PA-rich regions. The protein preferentially adsorbs to the PA-rich regions. The adsorption and subsequent aggregation of alpha-synuclein was probed by tuning several parameters: the charge on the lipids, the charge on the protein, and the screening environment. Conditions which promoted the greatest extent of adsorption resulted in structurally heterogeneous aggregates, while comparatively homogeneous aggregates were observed under conditions whereby adsorption did not occur as readily. Our observation that different alterations to the system lead to different degrees of aggregation and different aggregate structures poses a challenge for drug discovery. Namely, therapies aimed at neutralizing alpha-synuclein must target a broad range of potentially toxic, membrane-bound assemblies.

α-Synuclein-induced tubule formation in lipid bilayers.

α-Synuclein is a presynaptic protein that binds to phospholipid membranes and is involved in the pathogenesis of Parkinsons disease (PD). In this paper, we describe the effects of adding wild-type α-synuclein (WT) and three familial PD mutants (A53T, A30P, and E46K) to membranes containing 15-35 mol % anionic lipid. Tubules were observed to form in the membranes to an extent that depended on the α-synuclein variant, the anionic lipid content, and the protein concentration. For all four variants, tubule formation decreased with increasing anionic lipid content. Tubules were more readily observed with A30P and E46K than with WT or A53T. The results are consistent with a model wherein the helical content of α-synuclein increases with increasing anionic lipid content, and α-synuclein conformers with low helical content have a high propensity to induce tubule formation. This work, combined with previous work from our laboratory (Pandey et al. Biophys. J. 2009, 96, 540), shows that WT adsorption of the protein has deleterious effects on the membrane when the anionic lipid concentration is less than 30 mol % (tubule formation) or greater than 40 mol % (reorganization of the bilayer, clustering of protein).

Formation of Complex Three-Dimensional Structures in Supported Lipid Bilayers

Cell membranes are continually undergoing a wide range of shape transformations. Here, we demonstrate the formation of several structures in supported bilayers, including tubules, caps, and giant multivesicular structures. The key elements required for these transformations are osmotic pressure imbalances, insertion of lipids with positive curvature, and lipids whose curvature is dependent on the screening environment. With these elements, a wide variety of transformations can be achieved in the absence of protein.

Ultrathin Organic Layers on Silicon Surfaces

Ultrathin organic layers of organic molecules can be produced on the Silicon(001) surface using surface analogs of well-known cycloaddition reactions from organic chemistry. Molecules containing one or more unsaturated C=C can readily bond to the surface at room temperature. Since these reactions form two bonds between the molecule and the surface, the attached molecules are also oriented. More complex molecules containing conjugated pi-electron systems such as styrene can also be bonded to the surface with high selectivity for specific bonds. This manuscript reviews recent progress in forming ultrathin organic layers on silicon, and the current understand of the relevant reaction mechanisms.

Path dependence of three-phase or two-phase end points in fluid binary lipid mixtures.

The phase behavior of anionic/zwitterionic mixtures can be controlled by tuning the charge state of the anionic lipid. In the case of dioleoylphosphatidic acid (DOPA)/dioleoylphosphatidylcholine (DOPC) mixtures, demixing occurs either when DOPA is protonated or when DOPA(2-):Ca(2+) complexes form. Herein it will be shown that the final end point, a three-phase or two-phase system, depends on the order in which the charge state is manipulated. The facile accessibility of different end points is a clear demonstration of the inherent flexibility of biological systems.