Slawomir Sek

STM studies of fusion of cholesterol suspensions and mixed 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/cholesterol vesicles onto a Au(111) electrode surface.

Electrochemical scanning tunneling microscopy (EC-STM) has been applied to study the structure of the film formed by fusion of cholesterol suspensions and mixed dimyristoylphosphatidylcholine (DMPC)/cholesterol vesicles on a Au(111) electrode surface. It has been demonstrated that cholesterol molecules assemble at the gold support into several structures templated by the crystallography of the metal surface and involving flat or edge-on adsorbed molecules. Studies of the film formed by fusion of mixed DMPC/cholesterol vesicles revealed that ordered domains of either pure DMPC or pure cholesterol were formed. These results indicate that, at the metal surface, the molecules released by the rupture of a vesicle initially self-assemble into a well-ordered monolayer. The self-assembly is controlled by the hydrocarbon skeleton-metal surface interaction. In the case of mixed DMPC/cholesterol vesicles, the molecule-metal interactions induce segregation of the two components into single component domains. However, the molecule-metal interaction induced monolayer is a transient phenomenon. When more molecules accumulate at the surface, the molecule-molecule interactions dominate the assembly, and the monolayer is transformed into a bilayer.

Electron transfer across α-helical peptide monolayers: importance of interchain coupling.

Four helical peptides with the general formula (Boc)-Cys-(S-Acm)-(Ala-Leu)(n)-NH-(CH(2))(2)-SH (n = 4-7) were synthesized and further used for the preparation of self-assembled monolayers (SAMs) on gold substrates. The electron-transfer behavior of these systems was probed using current-sensing atomic force microscopy (CS-AFM). It was found that the electron transmission through SAMs of helical peptides trapped between an AFM conductive tip and a gold substrate occurs very efficiently and that the distance dependence obeys the exponential trend with a decay constant of 4.6 nm(-1). This result indicates that the tunneling mechanism is operative in this case. Conductance measurements under mechanical stress show that peptide-mediated electron transmission occurs with the possible contribution of intermolecular electron tunneling between adjacent helices. It was also demonstrated that an external electric field applied between metallic contacts can affect the structure of the peptide SAM by changing its thickness. This explains the asymmetry of the current-voltage response of metal-monolayer-metal junction.

Oxidation of DNA followed by conformational change after OH radical attack.

Examination of the attack of OH radicals produced in the Fenton way on DNA molecules is important from biological, biochemical, and biosensor points of view. Calf thymus DNA was selected for the investigation, since this natural oligonucleotide is often used in examination of drug-DNA interactions. Particularly useful was the coherent application of five techniques: electrochemical quartz crystal microbalance (EQCM), square wave voltammetry (SWV), circular dichroism (CD), atomic force microscopy (AFM), and UV-vis spectroscopy. These techniques differ in sensitivity to radical concentration and layer thickness of DNA. EQCM appeared to be the most sensitive in monitoring the consequences of OH radical actions; radical activities corresponding to nanomolar concentrations of H(2)O(2) could be detected. SWV and AFM detection gave noticeable signal for higher than 1 μM H(2)O(2) concentrations. EQCM data led to a conclusion that at higher than 1 μM H(2)O(2) concentrations the DNA strands were locally disintegrated. The corresponding DNA loss was ca. 16%. It has been shown that in the presence of α-tocopherol, a strong antioxidant, the damage caused by OH radicals was practically prevented.

Kinetics of long-range electron transfer through alkanethiolate monolayers containing amide bonds

Non-electroactive alkanethiolate monolayers containing internal amide bonds were used as model systems for the studies of the effect of structure of the intervening medium on long-range electron transfer. The blocking properties and the kinetics of electron transfer across the monolayers immobilized on gold were studied by voltammetry with the hexachloroiridate(IV) ion as the redox probe in the solution. The electron transfer efficiency was measured over a large potential window. The three types of monolayers studied were simple octadecanethiol and two amide-containing systems with one or two amide moieties in place of selected methylene groups in the main alkyl chain. Enhanced electronic coupling between the redox probe and the metal of the electrode was found for the monolayers with internal amide bonds. We ascribed it to the contribution of a hydrogen bonded network to electron tunneling through the monolayer. In the case of monolayers formed by molecules containing two secondary amide groups, the location of amide moieties inside the monolayer was shown to play an important role in the electron transfer efficiency. The second amide moiety placed in the alkyl chain in the odd position relative to the first amide did not increase electronic coupling in the monolayer. This behavior can be explained as due to larger distances between the amide groups in the external plane of the monolayer leading to difficulty in the formation of the hydrogen bond network. The position of the amide group relative to the electrode surface may be also considered as an important factor determining the efficiency of electron tunneling through the monolayer.

Mechanism of Lipid Vesicles Spreading and Bilayer Formation on a Au(111) Surface.

Spreading of small unilamellar vesicles on solid surfaces is one of the most common ways to obtain supported lipid bilayers. Although the method has been used successfully for many years, the details of this process are still the subject of intense debate. Particularly controversial is the mechanism of bilayer formation on metallic surfaces like gold. In this work, we have employed scanning probe microscopy techniques to evaluate the details of lipid vesicles spreading and formation of the lipid bilayer on a Au(111) surface in a phosphate-buffered saline solution. Nanoscale imaging revealed that the mechanism of this process differs significantly from that usually assumed for hydrophilic surfaces such as mica, glass, and silicon oxide. Formation of the bilayer on gold involves several steps. Initially, the vesicles accumulate on a gold surface and release lipid molecules that adsorb on a Au(111) surface, giving rise to the appearance of highly ordered stripelike domains. The latter serve as a template for the buildup of a hemimicellar film, which contributes to the increased hydrophilicity of the external surface and facilitates further adsorption and rupture of the vesicles. As a result, the bilayer is spread over a hemimicellar film, and then it is followed by slow fusion between coupled layers leading to formation of a single bilayer supported on a gold surface. We believe that the results presented in this work may provide some new insights into the area of research related to supported lipid bilayers.

STM‐Based Molecular Junction of Carbon Nano‐Onion

was almost concurrent with that of the CNTs, progress in thisfield has been very slow. Our current research has been cen-tered mainly on carbon-based materials, such as CNOs, whichrepresent a structural link between the fullerenes and themulti-wall carbon nanotubes. The CNO structures consist ofa hollow spherical fullerene core surrounded by concentric andcurved graphene layers with increasing diameters. The dis-tance between the layers is very close to the interlayer dis-tance in bulk graphite (0.34 nm).

Review peptides and proteins wired into the electrical circuits: An SPM‐based approach

Development of molecular scale electronics stimulates the search for new functional materials and compounds. One of the promising directions of further growth within this field is bioelectronics, which assumes the use of electron transfer (ET)‐mediating properties of bio‐related compounds. Because of the structural and functional versatility, peptides and proteins are particularly well suited to perform as single‐molecule‐based elements of circuits or sensing devices. In this review, it is demonstrated that purposely designed systems such as molecular junctions are excellent platforms for the studies of ET properties of peptides and proteins at nanoscale level. Using experimental approach based on scanning probe microscopy, individual molecules can be directly wired between two metallic contacts, and their conductance can be determined. ET behavior of the peptides and proteins can be analyzed in terms of their applicability as molecular wires, switches, diodes and elements of sensing devices.

Electrochemical and STM studies of 1-thio-β-D-glucose self-assembled on a Au(111) electrode surface.

In this study, a Au(111) electrode is functionalized with a monolayer of 1-thio-β-D-glucose (β-Tg), producing a hydrophilic surface. A monolayer of β-Tg was formed on a Au(111) surface by either (1) potential-assisted deposition with the thiol in a supporting electrolyte or (2) passive incubation of a gold substrate in a thiol-containing solution. For each method, the properties of the β-Tg monolayer were investigated using cyclic voltammetry (CV), differential capacitance (DC), and chronocoulometry. In addition, electrochemical scanning tunneling microscopy (EC-STM) was used to obtain images of the self-assembled monolayer with molecular resolution. Potential-assisted assembly of β-Tg onto a Au(111) electrode surface was found to be complicated by oxidation of β-Tg molecules. The EC-STM images revealed formation of a passive layer containing honeycomb-like domains characteristic of a formation of S(8) rings, indicating the S-C bond may have been cleaved. In contrast, passive self-assembly of thioglucose from a methanol solution was found to produce a stable, disordered monolayer of β-Tg. Since the passive assembly method was not complicated by the presence of a faradaic process, it is the method of choice for modifying the gold surface with a hydrophilic monolayer.

Peptide molecular junctions: Distance dependent electron transmission through oligoprolines

We have investigated the efficiency of electron transmission through thiolated oligoproline derivatives of general formula: Cys-(Pro)(n)-CSA, where CSA is a cystamine linker and n=1-4. The conductance measurements were performed using STM-based molecular junction approach. We have noted that the conductance of the oligoprolines decays exponentially with increasing length of the molecules and the decay constant was 4.3 nm(-1). This indicates that electron transfer is dominated by superexchange mechanism. Based on this observation, we have concluded that the height of the barrier is affected by the specific conformation of the peptide backbone. Such conclusion is supported by the fact that the oligoprolines do not form intramolecular hydrogen bonds, which could provide alternative electron transfer pathways.

EC-STM Study of Potential-Controlled Adsorption of Substituted Pyrimidinethiol on Au(111)

The ability of the pyrimidine derivatives to form numerous complexes and supramolecular assemblies makes them suitable for the construction of new functional surfaces. Therefore, in this paper, the adsorption behavior of 4-hydroxy-6-(trifluoromethyl)pyrimidine-2-thiol (HTPT) on a Au(111) surface has been investigated using electrochemical scanning tunneling microscopy (EC-STM). High-resolution imaging revealed that the HTPT molecules organize on a gold surface producing a highly ordered monolayer consistent with a (4 x radical3)R-30(0) superstructure. It has been observed that the arrangement of the molecules, as well as their orientation with respect to the substrate, remains stable over a relatively broad potential range from -0.40 to 0.55 V. It has been demonstrated that the presence of the functional groups attached to the aromatic ring affects the final structure of the HTPT adlayer, giving rise to the formation of the assembly with a uniform orientation of the molecules on the substrate.