Single MoleculesVolume 3, Issue 1 p. 3-3 EditorialFree Access Editorial: Single Mol. 1/2002 Karl Otto Greulich, Karl Otto GreulichSearch for more papers by this author Karl Otto Greulich, Karl Otto GreulichSearch for more papers by this author First published: 28 March 2002 https://doi.org/10.1002/1438-5171(200204)3:1<3::AID-SIMO3>3.0.CO;2-EAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume3, Issue1April 2002Pages 3-3 RelatedInformation
Analysis of various theories for optical bands of a guest molecule in fluctuating environment is carried out. It is shown that any proper dynamical theory for such a system must include initial conditions for two level systems (TLSs) of a solvent. True choice of the initial conditions can be carried out with the help of quantum mechanics principles. Our dynamic theory gives a theoretical basis for a "sudden jump model" which is used in many stochastic theories. Our theory describes the scan time dependence of single molecule optical lines in more comprehensive fashion than any existing stochastic theory is able to carry out. Experimental data on temporal line broadening of single molecule optical lines are treated with the help of our dynamical theory.
Single MoleculesVolume 3, Issue 2-3 p. 89-89 Editorial Editorial: Changes Ahead Karl Otto Greulich, Karl Otto GreulichSearch for more papers by this author Karl Otto Greulich, Karl Otto GreulichSearch for more papers by this author First published: 26 August 2002 https://doi.org/10.1002/1438-5171(200206)3:2/3<89::AID-SIMO89>3.0.CO;2-IAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume3, Issue2-3June 2002Pages 89-89 RelatedInformation
In a preceeding study, new analogues of phosphatidyl- choline (dithio-PC) and of N -biotinoyl- phosphatidyl- ethanolamine (dithio-PE-biotin) have been synthesized which contain disulfide elements at their hydrophobic ends. Chemisorption of these dithiolipids onto bare gold resulted in a self-assembled monolayer (SAM), and specific binding of streptavidin to the biotin residues was observed by surface plasmon resonance. In the present study, a mixture of dithio-PC and dithio-PE-biotin (80/20) was spread at the air/water interface of an aqueous streptavidin solution. Subsequently, ultraflat gold was appositioned from above and the chemisorbed lipid-protein layer was imaged with dynamic force microscopy, revealing crystalline arrays of streptavidin molecules. In parallel, the same procedure was used to functionalize bare gold chips for monitoring of specific binding of biotinylated proteins by surface plasmon resonance (SPR). The streptavidin-coated chip was further adapted to the binding of His 6 -tagged protein by use of a biotin-nitrilotriacetate conjugate (biotin-NTA). In the presence of detergent, even a water-insoluble membrane protein with a His 6 -tag (KcsA channel) was specifically bound in the presence of Ni 2+ and dissociated with imidazol. In conclusion, the novel dithio-phospholipids provide for new ways of target molecule fixation and protein crystallization in single molecule microscopy, and of capture probe fixation in gold-based biosensors.
Membrane microdomains are involved in numerous cellular processes such as membrane transport and signal transduction. Among these functionally microdomains, rafts are enriched in sphingolipids and cholesterol and their existence is actually mainly explained by a fluid-ordered lipid lateral phase separation. They also concentrate proteins like these with a C-terminal glycosylphosphatidylinositol (GPI) modification that serves as an anchor to the plasma membrane. In order to better understand how GPI proteins partition into microdomains, the insertion of the GPI-anchored alkaline phosphatase (AP), added exogenously into supported phospholipid bilayers, was studied in real-time using atomic force microscopy (AFM). Fluid-gel (lipid ordered) phase-separated bilayers were made by fusion of dioleoylphosphatidylcholine/dipalmitoylphosphatidylcholine (DOPC/DPPC, 1:1) vesicles. Time-dependent insertion of AP through its GPI anchor was observed at the periphery of the gel-phase domains. This demonstrates that GPI proteins can spontaneously localize in lipid ordered phases.
In cells and viruses as well as non-viral gene delivery systems, DNA is complexed with different molecules to form highly condensed structures. A wide range of conditions that cause DNA to collapse into compact structures has been discovered [1]. However, in most of these cases an exact description of these structures cannot be given. Since the complex stability is largely due to electrostatic forces, it can be modulated by varying the salt concentration. Apart from the biological aspects the study of the molecular structure of polyelectrolyte complexes may be used to improve our general understanding of polyelectrolyte interactions. Theoretical models reveal the structure of complexes formed between a stiff charged cylinder and an oppositely charged flexible or semiflexible polymer [2], [3]. Here we determine the influence of salt on the structure of linearized pUC19 plasmid DNA and positively charged dendronized polymers of generation two (PG2). The repeat units of the polymers are styrenes functionalized with dendrons carrying protonated amine groups at the periphery. Starting with the amino-terminated dendronized polystyrene of generation one (PG1), higher generations were obtained by the so called mixed “attach-to” approach [4]. For the analysis of the structure of the polyelectrolyte complexes scanning force microscopy (SFM) was used. The molecules were allowed to adsorb from solution onto mica or poly-L-ornithin coated mica, rinsed three times with water and finally dried under a stream of N 2 . Further details of sample preparation are given elsewhere [5]. Complexes of DNA and dendronized polymers of generation two, deposited from different NaCl solutions (10, 50, 100 and 300 mM) onto poly-L-ornithin coated mica, were visualized via SFM (Fig. 1). For the analysis of the complexes the heights and the contour lengths of both the complex (L C ) and of the DNA that belonged to the complex (L DNA-C ) were determined. While for DNA/PG2 complexes in 0 mM NaCl the average height was (4.0 ± 0.3) nm [5], within the errors the same heights were obtained for the complex using different NaCl solutions (4.2 ± 0.4) nm. The underestimation of the height of molecules in SFM images due to tip-sample interactions (deformation of the sample) is a well known feature. To evaluate the contour lengths of the complex and DNA, their contour was divided into straight segments of 2-5 nm. For their analysis only those complexes were chosen which exhibited a constant height along their contour, and where single DNA strands that came out of this complex belonged clearly to the complex (see [5]). The length of the DNA that contributes to the complex (L DNA-C ) can be obtained by subtracting the measured contour length of the DNA molecule out of the complex (L out ) from the length of the monodisperse DNA (L 0 ) by L DNA-C = L 0 -L out . In Fig. 2 the contour lengths of the complexes (L c ) are plotted versus the DNA that contributes to the complex (L DNA-C ) [5]. In the presence of elevated salt concentration, the obtained data for each NaCl concentration exhibit an overall linear dependence where the slope ( m ) decreased for increasing concentration of NaCl. Using the estimated radius for PG2 (1.6 ± 10 %) [5] and the theoretical diameter for DNA (2 nm), we calculate the DNA length required for one turn around the dendronized polymers ( U ) to be (16.3 ± 1.0). With X i =m i U (i stands for the different salt concentrations), the pitch ( X ) of the wrapped DNA can be calculated. Comparing the results to the case of 0 mM NaCl obtained previously [5], the increase in NaCl concentration lead to a decrease in the pitch separation of DNA which is consistent with the theory [3]. We propose a molecular level structural model for a DNA/dendronized polymer complex, according to which the polyelectrolyte with the smaller linear charge density (DNA) is wrapped around the more highly charged dendronized polymer (PG2) and the pitch (X) depends upon the salt concentration. The dendronized polymers together with DNA are a useful model system to test theories on the interaction of oppositely charged polyelectrolytes.
In the last few years the visualization and tracking of single fluorescent proteins, nanometer-sized RNP particles and viruses within the cellular interior was accomplished. This became feasible by use of photostable fluorescent dyes, extremely low probe concentrations, and wide-field fluorescence microscopic setups equipped with sensitive slow-scan or intensified CCD cameras. This paper reviews the results of the studies performed so far, discusses potential problems and gives an outlook on future applications.
Aldosterone is an important mediator of osmoregulation in vertebrate cells and is thought to be involved in cell differentation. Its signaling pathway starts with the formation of a complex with the mineralocorticoid receptor (MCR) which is then translocated into the cell nucleus where transcription of target genes is initiated. Finally, mRNAs of specifically activated genes appear in the cytoplasm. Using atomic force microscopy (AFM), we were able to visualize the signaling cascade described above. We injected stage VI oocytes of Xenopus laevis with aldosterone and isolated their nuclei after different incubation times. Nuclei obtained 2 min after application of aldosterone clearly exhibited macromolecules (“flags”) attached to the nuclear pore complexes (NPCs) when examined by AFM. The estimated size of the observed particles (80-160 kDa) corresponds with the actual size of the MCR (∼120kDa). NPCs of oocytes injected about 20 min prior to preparation showed large macromolecules (“plugs”) within their central channels. We concluded that these plugs contain transcripts induced by aldosterone. Electrical measurements support these findings; appearance of both flags and plugs is linked to an increase in nuclear envelope electrical resistance (NEER), probably due to partial plugging of NPCs. The increase of NEER parallel to the occurrence of plugs is prevented by adding RNase A or actinomycin D (an inhibitor of transcription). Alterations of NEER and formation of flags and plugs in response to aldosterone are suppressed by coinjection of the MCR-inhibitor spironolactone. Therefore we consider plugs to represent the early genomic response to aldosterone stimulation of Xenopus oocytes. In conclusion, AFM not only allows imaging of structures in the submicrometer range, it may also be used to manipulate them. By applying forces to the AFM tip approximately 10 fold higher than those used for imaging we were able to dislocate plugs from NPCs (see figures). The macromolecules sticking to the AFM tip can be used as substrates for further experiments, for example RT-PCR protocols.
Single MoleculesVolume 3, Issue 5-6 p. 253-254 Editorial Editorial: Spring School on Physics, Chemistry and Biology with Single Molecules, Hofgeismar, April 4th to 8th 2002 Ulrich Kubitscheck, Ulrich KubitscheckSearch for more papers by this authorUrlich Fischer, Urlich FischerSearch for more papers by this author Ulrich Kubitscheck, Ulrich KubitscheckSearch for more papers by this authorUrlich Fischer, Urlich FischerSearch for more papers by this author First published: 19 November 2002 https://doi.org/10.1002/1438-5171(200211)3:5/6<253::AID-SIMO253>3.0.CO;2-GCitations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article.Citing Literature Volume3, Issue5-6November 2002Pages 253-254 RelatedInformation
Scanning Near-field Optical Microscopy (SNOM) allows optical microscopy with highest spatial resolution beyond the diffraction limit. We present a new microscope set-up which uses micro-machined cantilever SNOM sensors and combines in a unique way the advantages of Scanning Near-field Optical Microscopy, Confocal Scanning Microscopy (CSM) and Atomic Force Microscopy (AFM) in one instrument. Results from measurement at different samples system like histological microtome cuts and fluorescent labelled human chromosomes are shown to demonstrate the capabilities of the new set-up.
Single MoleculesVolume 3, Issue 1 p. 75-75 MiscellaneousFree Access Job Offers: Single Mol. 1/2002 First published: 28 March 2002 https://doi.org/10.1002/1438-5171(200204)3:1<75::AID-SIMO75>3.0.CO;2-AAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume3, Issue1April 2002Pages 75-75 RelatedInformation
Fluorescence correlation spectroscopy (FCS) was used to study the diffusion behavior of the glucocorticoid dexamethasone on the cell membrane of the mouse pituitary cell line AtT-20 to investigate the putative membrane glucocorticoid receptor (mGR). This putative membrane receptor is thought to mediate glucocorticoid effects independent from protein synthesis and the well-known intracellular glucocorticoid receptor (iGR), the so-called “nongenomic” effects. The high spatial resolution of the FCS technique allowed for focusing exclusively on the cell membrane; measurements were taken on living cells in solution after incubation with 60 nM fluoresceindexamethasone for 1 h without manipulation of the cells. FCS measurements were performed by moving the position of the laser focus in 1 μm steps through a single cell. In a scan through the cell a component typical for a receptor-bound ligand (diffusion constant 3 × 10-10 cm2 sec-1) becomes apparent in the autocorrelation function (ACF) at the position of the cell membrane whereas another component (diffusion constant 2 × 10-8 cm2 sec-1) typical for a ligand diffusing in the cytosol becomes predominant in the ACF curves inside the cell. After pre-incubation with unlabeled dexamethasone in 1000-fold excess the “receptor-typical” component on the cell membrane was absent. This result is the first direct evidence for the existence of a mGR on a living cell.
The level of understanding of polymer adsorption has been somewhat limited by the lack of experimental techniques capable of testing theoretical concepts. With the development of scanning probe microscopy (SPM) techniques by Binnig et al., in particular, the atomic force microscope (AFM) being the technique discussed in this work, polymer molecule interactions and conformations can be studied on the nanometre scale. After structural data of polymers by imaging with the AFM have been obtained (Elings et al.) [2]. the AFM evolved into a novel tool for the investigation of forces on individual polymers involved (Lee et al. and Gaub et al. [3,4]). For this force spectroscopy, the immobilization of one single group, for example, the polymer end group, is preferred. For that reason, in this work end-grafting has been chosen to immobilize polyacrylamide onto gold as it has been employed, for example, for the chemisorption of (mono)thiol- functionalized poly(methacrylic acid) (PMAA) onto gold surfaces by Hadziioannou et al. [1]. Polyacrylamide attracted the attention of researchers because of its applicability to the oil recovery. Polyacrylamide itself and the adsorption of polyacrylamide have, therefore, been studied extensively. We have used polyacrylamide specifically manufactured for the end-grafting to gold surfaces by Ciba Specialty Chemicals. The attached termination group allows the formation of the gold-sulphur bond and, therefore, the polyacrylamide molecule can be covalently attached to gold surfaces. The work presented here focuses on the covalent immobilization of polyacrylamide (molecular weight 571000 Dalton) onto cantilever tips, which have been coated beforehand with chromium (thickness 4 nm) and gold (thickness 15 nm). The chemisorption of one individual molecule to the gold-coated tips has been ensured by reducing the concentration of the polyacrylamide solution. The distinction between single molecule attachments and polymer brush formation can be achieved by varying the incubation time. All force curve data were taken in an AFM (Digital Instruments Multimode) with a sealed reservoir for the liquid which was introduced or exchanged by pumping liquid through the cell. Each set of data was acquired with a freshly-cleaved mica surface and was taken with exactly the same molecule at precisely the same sample position. During the approach of the cantilever to the surface, the force exerted on the tip was limited to 200 pN to avoid damaging the molecule. When the cantilever retracts from the surface, the force exerted on the cantilever is measured. If a polyacrylamide molecule adsorbs onto the mica surface, the polymer chain is stretched and at a certain force it desorbs. An example of the desorption of a single molecule attached only at one point to the surface is shown in Figure 1. Only very clean force events, i.e., obvious stretching of a polymer chain with a clean desorption jump going back to the zero force line with identical pull off rates, were included. The force value is obtained by measuring the force at the desorption jump in the force curve. After the experiment, the cantilever spring constants were investigated with the reference cantilever method in a Digital Instruments Dimension 3100. In this way, a significant improvement in force spectroscopy has been achieved and vital information about the adsorption of polyacrylamide molecules onto surfaces in different liquid environments has been obtained. The main cause of the polyacrylamide adsorption onto mica has been demonstrated to be hydrogen bonding by performing the force spectroscopy experiments in urea and guanidine hydrochloride solutions. The number of molecule detachments from the surface seen in the force curve drops significantly after introducing these solutions and becomes zero at a concentration of 8 M in the case of urea and at 4 M in the case of guanidine hydrochloride. After exchanging with water again, the original situation can be restored, i.e., urea and guanidine hydrochloride simply block the hydrogen bonding sites along the molecule without degrading it or altering the mica surface. The value of the adsorption force can be influenced by the ionic strength of the solution in which the adsorption of polyacrylamide takes place. By increasing the concentration of magnesium chloride, the force decreases significantly. However, by increasing the ionic strength with nickel chloride, the force increases. These effects are due to screening of local charges along the molecule and changes in the adsorption site behaviour on the mica surface. By changing the pH of the solution, the conformation of the molecule in solution can be altered. For example, at pH 5 the polyacrylamide molecule possesses a higher persistence length, i.e., the molecule is more extended reducing the number of force events in the force-distance curve. This is due to the dissociation of hydrogen ions along the polymer chain causing a repulsion of the segments. Another interesting aspect is the adsorption of polyacrylamide onto surfaces in poor solvents. This experiment was performed in propanol. The force curves obtained do not show proper stretching events anymore and all the forces exerted on the tip occur at short distances from the mica surface. This is caused by the molecule coming partly out of solution and, therefore, being more confined to the tip. We have demonstrated that by covalently end-grafting a single molecule to an AFM tip, force spectroscopy can be used to reliably investigate the adsorption behaviour of an individual molecule onto surfaces in different liquid environments. Due to the covalent attachment of the molecule many parameters can be studied on the same molecule, and even a change of the surface or use of porous media is possible.
An improved shear force microscope (ShFM) is presented, where the oscillation of the probe can be excited and quantitatively detected in two orthogonal directions in the sample plane. This set-up allows a complete control of the dynamic behaviour of the probe that is necessary in order to obtain reproducible results. Increasing evidence has been collected confirming that the shear force mechanism is due to the water layer confined between the tip and the sample and that, therefore, ShFM is a true non-contact technique. To confirm the reduced tip-sample interaction with respect to conventional scanning probe techniques, the height and the width of more than 100 double stranded DNA fragments (4.3 kbp) were measured in air using different ShFM probes. The measurements were then compared with tapping mode atomic force microscopy (TMAFM) on a similar number of DNA fragments. From the statistical analysis of the data consistently higher profiles were obtained using ShFM: (1.1+/-0.2) nm instead of (0.6+/-0.1) nm that is the expected DNA height when using TMAFM in air. When acquiring high resolution images of DNA, the lower force of interaction of the shear force technique allowed the observation of a higher order periodic structure, which may reflect a conformational aspect of DNA peculiar to its interaction with mica. By improving the shear force technique, we have shown that important intrinsic advantages, such as non-contact interaction, can be fully exploited and the ShFM can become a very useful tool in the study of biomolecules.
Dual-color fluorescence cross-correlation analysis has proven to be a powerful tool to probe interactions of different molecular species in solution and living cells on a single molecule level. Probing the coordinated motion of molecules through the measurement volume, it is a much more selective and data-compressing alternative to co-localization analysis by dual-color imaging, and provides additional access to fast internal dynamics of the co-migrating molecules. However, cellular FCS applications often suffer from extremely low molecular mobility, introducing bleaching artifacts or entirely impeding fluctuation analyses of any kind. Thus, to meet the increasing demand for interaction measurements of nearly stationary molecules, such as receptor-ligand complexes on cell membranes, these limitations of conventional fluorescence correlation and cross-correlation analysis need to be overcome. This can be achieved by combining a piezo-driven stage scanning unit with the confocal FCS setup, minimizing the photodynamic strain imposed on immobile single molecules without compromising the relevant cross-correlation information. Different scanning patterns were chosen and the corresponding auto- and cross-correlation curves recorded for both in vitro and in vivo systems. Expectedly, the shape of the correlation curves depends crucially on the different modes of stage motion. Nevertheless, cross-correlation amplitudes clearly reflect on the presence or absence of linkages between the different molecular species. Marked differences between bound and unbound single molecules could be observed on immobilized proteins in PAA gels and on cell membranes.
The atomic force microscope (AFM) has been used to determine the elastic properties of living cells. Changes in the mechanical properties of COS-7 exposed to actin and microtubule destabilizing chemicals and proteins have been analyzed. Nocodazole was used to chemically destabilize microtubules (MT) and Cytochalazin to depolymerize actin filaments. “Non-chemical” destabilization of MT was obtained by over-expressing SCG-10, a protein known to depolymerize MT, whereas actin cytoskeleton destabilization was achieved by over-expressing RhoA, Rac1 and Cdc42 proteins known to affect actin filaments. Depolymerization of the different cytoskeleton components by both chemicals and proteins was confirmed optically by fluorescently labeled actin and tubulin. The study of the mechanical properties was accomplished by analyzing the indentation of the AFM tip into the living cells (1). The indentation curves had a different shape depending on the cytoskeleton component, which has been depolymerized. In the case of actin destabilization by chemicals, a softening was detected to occur close to the cell surface. In contrast, microtubule depolymerization by chemicals caused an increase in softness in the deeper parts of the cell. The chemicals targeted against the already depolimerized cytoskeleton components in transfected cells did not induce modifications in the elastic properties. Therefore, we conclude that AFM allows to identify which component of the cytoskeleton are affected by the presence of chemicals or proteins.
The view of the plasma membrane of biological cells was dramatically changed due to the discovery of lipid microdomains. Initially found as structurally distinct areas characterized by a specific protein content, the concept of lipid microdomains was rapidly taken over as a new scheme for explaining membrane targeted cellular processes. Despite its impact on the current image of cell function, there is still a lack of knowledge on the structural origin, size, distribution and mobility of such domains. We addressed these questions through high-resolution imaging of microdomains enriched in flourescence labeled lipids with a two-photon microscope. Our findings showed that domains are of heterogeneous size close to the optical resolution limit, and are dispersed randomly over the cell surface. For studying the functional role of microdomains, we investigated the interplay of lipids and certain membrane proteins utilizing simultaneous two color imaging. While in some cases we observed colocalization between lipids and proteins, on occasion proteins were confined to domains which were clearly separated from lipid domains. This is in perfect agreement with biochemical studies showing structurally distinct sets of domains confining different types of proteins. For maintaining functional flexibility of the cell, microdomains are expected to be linked to the cytoskeleton. We studied the mobility of individual microdomains using repeated imaging of the same area of the cell surface at a rate of one image per 10s. Interestingly, most domains were almost immobile within the plasma membrane. On occasion, however, domains were transported actively over distances up to several micrometers, which is strong evidence for interactions between domains and the cytoskeleton. These results for the first time demonstrate that ultra-sensitive optical microscopy is perfectly suited to resolve the dynamical processes of the organization of the plasma membrane, which allows for much deeper insights into the details of cell response.
In this work we present a framework for the calculation of the conduction properties of a metal-molecule-metal junction which is in contact with its thermal environment. The effects of thermal relaxation and dephasing on the transmission properties of the junction were studied using a simple tight binding model for the molecular conductor. The interaction between the molecular system and the thermal environment is described on the level of the Redfield theory, which is a weak electron-phonon coupling scheme, modified for the description of steady-state situations. We show that the transmitted flux consists of two (generally non-separable) components: a flux associated with the elastic tunneling and a thermally activated flux component. The coherent (tunneling) component dominates the transport at low temperatures, large energy gaps and short molecular chains. The incoherent (activated) component is important in the opposite limits. The integrated transmission provides a generalization of the Landauer conduction formula in the presence of thermal relaxation. [1] Using the same formalism, we investigate the issue of heat release on a current carrying molecule: the total amount of heat that is generated on the wire during electrical conduction. Local aspects of the heat release were investigated as well [2]. We compare quantum to classical calculations in the resonance regime, and far from it. For the quantum case we calculate the fraction of the available energy, i.e. of the potential drop, that is converted to heat on the molecular barrier and its dependence on the system parameters: the potential bias, molecule length, dephasing rate and temperature. We find that in the localization limit, where the electron is fully thermalized at each molecular site, all the available energy is dissipated on the bridge, while for short systems and weak system-bath coupling, only a small fraction of the available energy is deposited as heat on the bridge. In our simulations we got this fraction to be of the order 0.1-0.3 using a reasonable range of parameters. We also present a scheme for the analysis of local aspects of heat release, and use it to study the position dependence of the power dissipation for a specific molecular structure. Finally, using classical heat conduction theory we estimate the temperature rise on the molecule due to the heating effects. We find that, within a reasonable range of voltage and molecular parameters, it is in the few degrees range, and therefore it should not affect the molecular junction functionality. It should be emphasized that classical heat transfer theory overestimates heat conduction, so a quantum treatment of vibrational energy transmission in molecular junctions is needed in order to better estimate this temperature rise of the molecular junction. Such study is currently underway.
First results of a new dynamic approach to calculate the field enhancement at a metal ellipsoid close to a metal surface are presented. The theoretical approach is based on the solution of Maxwells equations. The numerical calculation is influenced by the dielectric constants of both metals, the angle of incidence of the monochromatic electromagnetic wave, the distance of the ellipsoid to the metal surface and the semi axis ratio of the ellipsoid.
Single molecule fluorescence experiments yield a stream of photocounts, whose statistical properties contain valuable information about processes within the molecule and in its microscopic environment. The photon statistics display features specific of single quantum systems and are therefore best discussed in the frame of a quantum mechanical theory of radiation. The present summary of a lecture given in April 2002 at the Hofgeismar Spring School on single molecules presents the main concepts used in quantum theory of light, together with a few useful references, and discusses some illustrations and applications to single molecule measurements drawn from the recent literature.
