Papers

AVS1996 Session BI+NS-TuA: Biology at the Nanoscale

Tuesday, October 15, 1996 2:00 PM in Room 203A

Tuesday Afternoon

Start	Invited?	Item
2:00 PM		BI+NS-TuA-1 Dynamic Force Spectroscopy of Molecular Adhesion Bonds E. Evans (University of British Columbia, Canada) In biology, molecular linkages at - within - and beneath soft cell interfaces arise mainly from weak-noncovalent interactions. Even though equilibrium binding affinities can be ranked for different types of bonds, there are no unitary strengths for weak bonds and the rupture forces may not follow the scale of affinities. Cohesive material strength and strength of adhesion at interfaces are governed by far from equilibrium kinetics. Under force, off rates can be modified in subtle and unexpected ways that depend on how external force is coupled through complex molecular structure to the atomic scale kinematics of bond separation. Brownian diffusion theory for kinetics illuminates the physical factors that influence dissociation under force. To examine these factors, ultrasensitive probes are now available, which can test single bond strength. However, new thinking is needed in the design of physical experiments; conventional concepts of force/action determinancy are far too naive. Peaks that appear in force distributions measured for rupture of single bonds arise because the applied force increases with time - not as the sole consequence of discrete bonds. Thus, with both computer simulations of bond rupture and prototype tests of force-driven detachment, we demonstrate how impingement to - and retraction from - surfaces must be controlled so that force probes can be used to reveal molecular scale determinants of bond strength and force-driven kinetics.
2:40 PM		BI+NS-TuA-3 An In Situ AFM Investigation of Protein Crystallization T. Land, J. De Yoreo (Lawrence Livermore National Laboratory); A. Malkin, Y. Kuznetsov (University of California, Riverside); J. Lee (Lawrence Livermore National Laboratory); A. McPherson (University of California, Riverside) In situ atomic force microscopy (AFM) has been used to investigate the crystallization of the storage protein canavalin as a function of supersaturation (0.6\<=\s\<=\3.1) and pH (7.3-8.0). The results show that growth occurs on complex vicinal hillocks formed by screw dislocation sources whose activity varies as a function of supersaturation. At high supersaturation, 2D nucleation is observed on the 1 \mu\m terraces generated by the dislocations. At lower supersaturations, 2D nucleation only occurs on large (10 \mu\m) terraces. At these supersaturations, no 2D nucleation occurs on terraces generated by the dislocations and homogenization of bunched steps is observed, showing that the terrace width is less than the diffusion length and indicating that surface diffusion dominates. From the speed of the steps which vary linearly in concentration, we find that the kinetic coefficient for step motion varies from 3 x 10\super -4\cm/sec at a pH of 7.3 to 0.6 x 10\super -4\cm/sec at a pH of 8.0. A simple Fick's law analysis of the data gives a diffusivity of 10\super -8\cm\super 2\s\super -1\. From the critical radius for 2D nucleation and the dependence of terrace width on s, we calculate a step edge free energy of 1erg cm\super -2\. The AFM images also reveal the mechanisms of defect incorporation during growth and provide insight into the processes that limit the growth rate and uniformity of these crystals. We find that at the typical conditions used to nucleate these crystals, the continual sedimentation and incorporation of micro-crystals results in extreme distortion of the lattice through formation of many stacking faults and micro-grain boundaries producing what is commonly referred to as "mosaic structure".
3:00 PM		BI+NS-TuA-4 Construction and Characterization of Photosystem I Reaction Centers on Modified Gold Surfaces I. Lee, J. Lee, E. Greenbaum (Oak Ridge National Laboratory) We have constructed and characterized Photosystem I (PSI) reaction centers on gold surfaces. The epitaxially grown gold-films were treated with 2-mercaptoethanol, mercaptoacetic acid, 2-dimethylaminoethanethiol, thioacetic acid, and mercaptoethane separately. Tapping Mode Atomic Force Microscopy was used to evaluate the coverage of PSI reaction centers adsorbed on various derivatized gold surfaces. Laser induced fluorescence was used to confirm the existence of PSI. We have found that substrate treated with 2-mercaptoethanol or mercaptoacetic acid strongly adsorbs PSI reaction centers while surface treated with mercaptoethane resists adhesion. Scanning tunneling spectroscopy was used to characterize the PSI particles constructed on the gold surface. By using this technique we were able to investigate their electronic properties and possible orientation. We have found PSI particle has a preferential orientation during adsorption process when the substrate was treated with 2-mercaptoethanol. A different preferential orientation was found for substrate treated with mercaptoacetic acid. No preferential orientation was found for substrate treated with 2-dimethylaminoethanethiol.
3:20 PM		BI+NS-TuA-5 Molecular Electronics using an Artificial Flavocytochrome C S. Nishikawa, S. Ueyama (Mitsubishi Electric Corporation, Japan) We designed and developed an artificial flavocytochrome (FC) by modifying thermophilic bacterial cytochrome c552 from Hydrogenobactor, in order to utilize the molecular function just like the molecular scale electron transfer in the photosynthetic reaction center. A FC contains two functional groups of flavin and heme. Intramolecular electron transfer of FC adsorbed on graphite and Au (111) substrates has been investigated by cyclic voltammetry and STM/STS measurements, respectively. We used NanoScope III for STM/STS measurements. Pt/Ir tips were used. STM images were recorded under the constant current of 70pA. At positive bias at substrate, many globule particles with diameter of 3-4nm were observed. These are considered to correspond to FC. At -900 mV bias, FC was not observed, and the structure of Au substrate was similar to the positive bias images. This bias dependence of the FC images can be attributed to the one-way electron transfer from flavin to heme in FC. In STS measurement, tunneling current in adsorbed FC area was higher than that in naked Au area at positive bias, while very little difference was detected at the negative bias. This rectifying property corresponds with the STM images of FC. This work was performed under the management of FED as a part of the MITI R&D program (Bioelectronic Devices project) sponsored by NEDO. The synthesis of FC was done by Dr. Y.Shibano and Ms. H.Hatanaka, Suntry Ltd.
3:40 PM		BI+NS-TuA-6 Novel All-Silicon Based Sensor Structures L. Montelius, T. Ling (Lund University, Sweden) The interest in miniaturization of modern chemical and medical sensors is steadily increasing. However, when employing methods and fabrication procedures especially developed for microelectronics, one may encounter new and unpredictable problems for applications in aqueous media. For instance, in microelectronics often a dual metal layer system is employed for making electrical contacts. In a solution such a contact may result in galvanic reactions and/or may dissolve in the cleaning agents used for sterilization of medical devices. To address such issues we have employed an all-silicon based process technology for fabrication of sturdy and reliable devices with integrated poly-Silicon electrodes that tolerate most chemical environments (1). The resistivity of poly-Si can be changed by doping, and values around 10 \super -4\ Ohmcm can easily be obtained. These values compare well with electrode carbon, with a resistivity value in the range of 10\super -2\ Ohmcm. The electrode structure has been fabricated using standard photo-lithography for pattern definition onto a photo sensitive resist and a subsequent wet etching procedure for the pattern transfer. The patterns have been either a three-electrode or an interdigitated system. In order to compare resistivity and surface capacitance of the poly-Si structure, similar electrode structures made of Aluminium have also been fabricated. The poly-Si surfaces have also been chemically modified with amino- or carboxyl groups in order to address the usefulness such electrodes would have for enabling selective chemical reactions to take place on the electrodes. This presentation will clearly show the extreme usefulness of all-silicon electrodes for chemical sensor applications. Not only is the fabrication of electronic sensors greatly simplified, but also added benefits such as the possibility to use well-documented methods for surface modifications utilizing silane coupling chemistry, is extremely valuable for the development of accurate sensor structures. One of the major improvements using poly-Si electrodes is also the sturdiness for applications requiring a tough cleaning cycle since the all-silicon structure will resist most cleaning agents.
4:00 PM		BI+NS-TuA-7 Imaging of \beta\-Amyloid Fibrils in Liquid by Scanning Force Microscopy A. Shivji, M. Davies, C. Roberts, S. Tendler, P. Williams (University of Nottingham, United Kingdom); M. Wilkinson (Smithkline Beecham Pharmaceuticals, United Kingdom) \beta\-Amyloid and its subsequent self-assembly into fibrils is a recognized pathological marker of Alzheimer's disease and has been implicated as a potential causative agent. Here, we present contact and non-contact scanning force microscopy images recorded in propanol of \beta\-amyloid fibrils grown under a range of conditions. These results are compared with previous scanning probe microscopy (SPM) images of metallic coated samples, which enabled the study of gross molecular structure and self- assembly of \beta\-amyloid peptide (1). The implicatio ns of this new data in relation to the present molecular models of \beta\-amyloid and its self-assembly are discussed. We believe that this data represents an initial step in using SPM to interrogate molecular structure in a variety of environments. This should facilitate a significant contribution to the understanding of the mechanism of \beta\-amyloid self-assembly and its relation to Alzheimer's disease. (1) A.P. Shivji, F. Brown, M.C. Davies, K.H. Jennings, C.J. Roberts, S.J.B. Tendler, M.J. Wilkonson and P.M. Williams (1995). FEBS Letters, 371, 25-28.