Paul K Hansma

Publications of Paul K Hansma

• A new device for performing reference point indentation without a reference probe.

  Authors: Daniel Bridges, Connor Randall, Paul K Hansma

  The Review of scientific instruments. 04/2012; 83(4):044301.

  Here we describe a novel, hand-held reference point indentation (RPI), instrument that is designed for clinical measurements of bone material properties in living patients. This instrument differsHere we describe a novel, hand-held reference point indentation (RPI), instrument that is designed for clinical measurements of bone material properties in living patients. This instrument differs from previous RPI instruments in that it requires neither a reference probe nor removal of the periosteum that covers the bone, thus significantly simplifying its use in patient testing. After describing the instrument, we discuss five guidelines for optimal and reproducible results. These are: (1) the angle between the normal to the surface and the axis of the instrument should be less than 10°, (2) the compression of the main spring to trigger the device must be performed slowly (>1 s), (3) the probe tip should be sharper than 10 μm; however, a normalized parameter with a calibration phantom can correct for dull tips up to a 100 μm radius, (4) the ambient room temperature should be between 4 °C and 37 °C, and (5) the effective mass of the bone or material under test must exceed 1 kg, or if under 1 kg, the specimen should be securely anchored in a fixation device with sufficient mass (which is not a requirement of previous RPI instruments). Our experience is that a person can be trained with these guidelines in about 5 min and thereafter obtain accurate and reproducible results. The portability, ease of use, and minimal training make this instrument suitable to measure bone material properties in a clinical setting.

• Microindentation for in vivo measurement of bone tissue mechanical properties in humans.

  Authors: Adolfo Diez-Perez, Roberto Güerri, Xavier Nogues, Enric Cáceres, Maria Jesus Peña, Leonardo Mellibovsky, Connor Randall, Daniel Bridges, James C Weaver, Alexander Proctor, Davis Brimer, Kurt J Koester, Robert O Ritchie, Paul K Hansma

  Journal of bone and mineral research : the official journal of the American Society for Bone and Mineral Research. 02/2010; 25(8):1877-85.

  Bone tissue mechanical properties are deemed a key component of bone strength, but their assessment requires invasive procedures. Here we validate a new instrument, a reference point indentationBone tissue mechanical properties are deemed a key component of bone strength, but their assessment requires invasive procedures. Here we validate a new instrument, a reference point indentation (RPI) instrument, for measuring these tissue properties in vivo. The RPI instrument performs bone microindentation testing (BMT) by inserting a probe assembly through the skin covering the tibia and, after displacing periosteum, applying 20 indentation cycles at 2 Hz each with a maximum force of 11 N. We assessed 27 women with osteoporosis-related fractures and 8 controls of comparable ages. Measured total indentation distance (46.0 +/- 14 versus 31.7 +/- 3.3 microm, p = .008) and indentation distance increase (18.1 +/- 5.6 versus 12.3 +/- 2.9 microm, p = .008) were significantly greater in fracture patients than in controls. Areas under the receiver operating characteristic (ROC) curve for the two measurements were 93.1% (95% confidence interval [CI] 83.1-100) and 90.3% (95% CI 73.2-100), respectively. Interobserver coefficient of variation ranged from 8.7% to 15.5%, and the procedure was well tolerated. In a separate study of cadaveric human bone samples (n = 5), crack growth toughness and indentation distance increase correlated (r = -0.9036, p = .018), and scanning electron microscope images of cracks induced by indentation and by experimental fractures were similar. We conclude that BMT, by inducing microscopic fractures, directly measures bone mechanical properties at the tissue level. The technique is feasible for use in clinics with good reproducibility. It discriminates precisely between patients with and without fragility fracture and may provide clinicians and researchers with a direct in vivo measurement of bone tissue resistance to fracture.

• Effect of Ca2+ ions on the adhesion and mechanical properties of adsorbed layers of human Osteopontin.

  Authors: Bruno Zappone, Philipp Thurner, Jonathan Adams, Georg E Fantner, Paul K Hansma

  Biophysical journal. 07/2008;

  Using an Atomic Force Microscope (AFM) and a Surface Force Apparatus (SFA) we have measured the surface coverage, adhesion and mechanical properties of layers of Osteopontin (OPN), a phosphoproteinUsing an Atomic Force Microscope (AFM) and a Surface Force Apparatus (SFA) we have measured the surface coverage, adhesion and mechanical properties of layers of Osteopontin (OPN), a phosphoprotein of the human bones, adsorbed on mica. OPN is believed to connect collagen fibrils of the bone in a matrix that dissipates energy, reducing the risk of fractures. AFM normal force measurements showed large adhesion and energy dissipation upon retraction of the tip, which were due to the breaking of the many OPN-OPN and OPN-mica bonds formed during tip-sample contact. The dissipated energy increased in the presence of Ca(2+) ions due to the formation of additional OPN-OPN and OPN-mica salt bridges between negative charges. The forces measured by SFA between two macroscopic mica surfaces were mainly repulsive and became hysteretic only in the presence of Ca(2+): adsorbed layers underwent an irreversible compaction during compression due to the formation of long-lived calcium salt bridges. This provides an energy storage mechanism, which is complementary to energy dissipation and may be equally relevant to bone recovery after yield. The prevalence of one mechanism or the other appears to depend on the confinement geometry, adsorption protocol and loading-unloading rates.

• Nanoscale ion mediated networks in bone: osteopontin can repeatedly dissipate large amounts of energy.

  Authors: Georg E Fantner, Jonathan Adams, Patricia Turner, Philipp J. Thurner, Larry W Fisher, Paul K Hansma

  Nano letters. 09/2007; 7(8):2491-8.

  In the nanocomposite bone, inorganic material is combined with several types of organic molecules, and these complexes have been proposed to increase the bone strength. Here we report on a mechanismIn the nanocomposite bone, inorganic material is combined with several types of organic molecules, and these complexes have been proposed to increase the bone strength. Here we report on a mechanism of how one of these components, human osteopontin, forms large mechanical networks that can repeatedly dissipate energy through work against entropy by breaking sacrificial bonds and stretching hidden length. The behavior of these in vitro networks is similar to that of organic components in bone, acting as an adhesive layer in between mineralized fibrils.

• Hierarchical assembly of the siliceous skeletal lattice of the hexactinellid sponge Euplectella aspergillum.

  Authors: James C Weaver, Joanna Aizenberg, Georg E Fantner, David Kisailus, Alexander Woesz, Peter Allen, Kirk Fields, Michael J Porter, Frank W Zok, Paul K Hansma, Peter Fratzl, Daniel E Morse

  Journal of structural biology. 05/2007; 158(1):93-106.

  Despite its inherent mechanical fragility, silica is widely used as a skeletal material in a great diversity of organisms ranging from diatoms and radiolaria to sponges and higher plants. In additionDespite its inherent mechanical fragility, silica is widely used as a skeletal material in a great diversity of organisms ranging from diatoms and radiolaria to sponges and higher plants. In addition to their micro- and nanoscale structural regularity, many of these hard tissues form complex hierarchically ordered composites. One such example is found in the siliceous skeletal system of the Western Pacific hexactinellid sponge, Euplectella aspergillum. In this species, the skeleton comprises an elaborate cylindrical lattice-like structure with at least six hierarchical levels spanning the length scale from nanometers to centimeters. The basic building blocks are laminated skeletal elements (spicules) that consist of a central proteinaceous axial filament surrounded by alternating concentric domains of consolidated silica nanoparticles and organic interlayers. Two intersecting grids of non-planar cruciform spicules define a locally quadrate, globally cylindrical skeletal lattice that provides the framework onto which other skeletal constituents are deposited. The grids are supported by bundles of spicules that form vertical, horizontal and diagonally ordered struts. The overall cylindrical lattice is capped at its upper end by a terminal sieve plate and rooted into the sea floor at its base by a flexible cluster of barbed fibrillar anchor spicules. External diagonally oriented spiral ridges that extend perpendicular to the surface further strengthen the lattice. A secondarily deposited laminated silica matrix that cements the structure together additionally reinforces the resulting skeletal mass. The mechanical consequences of each of these various levels of structural complexity are discussed.

• The role of calcium and magnesium in the concrete tubes of the sandcastle worm.

  Authors: Chengjun Sun, Georg E Fantner, Jonathan Adams, Paul K Hansma, J Herbert Waite

  The Journal of experimental biology. 04/2007; 210(Pt 8):1481-8.

  Sandcastle worms Phragmatopoma californica build mound-like reefs by sticking together large numbers of sand grains with cement secreted from the building organ. The cement consists of protein plusSandcastle worms Phragmatopoma californica build mound-like reefs by sticking together large numbers of sand grains with cement secreted from the building organ. The cement consists of protein plus substantial amounts of calcium and magnesium, which are not invested in any mineral form. This study examined the effect of calcium and magnesium depletion on the structural and mechanical properties of the cement. Divalent ion removal by chelating with EDTA led to a partial collapse of cement architecture and cement dislodgement from silica surfaces. Mechanical properties examined were sand grain pull-out force, tube resistance to compression and cement adhesive force. EDTA treatment reduced sand grain pull-out forces by 60% and tube compressive strength by 50% relative to controls. EDTA lowered both the maximal adhesive force and energy dissipation of cement by up to an order of magnitude. The adhesiveness of calcium- and magnesium-depleted cement could not be restored by re-exposure to the ions. The results suggest that divalent ions play a complex and multifunctional role in maintaining the structure and stickiness of Phragmatopoma cement.

• Applied physics. High-speed atomic force microscopy.

  Authors: Paul K Hansma, Georg Schitter, Georg E Fantner, Craig Prater

  Science (New York, N.Y.). 11/2006; 314(5799):601-2.

• Sacrificial bonds and hidden length: unraveling molecular mesostructures in tough materials.

  Authors: Georg E Fantner, Emin Oroudjev, Georg Schitter, Laura S Golde, Philipp Thurner, Marquesa M Finch, Patricia Turner, Thomas Gutsmann, Daniel E Morse, Helen Hansma, Paul K Hansma

  Biophysical journal. 03/2006; 90(4):1411-8.

  Sacrificial bonds and hidden length in structural molecules and composites have been found to greatly increase the fracture toughness of biomaterials by providing a reversible, molecular-scaleSacrificial bonds and hidden length in structural molecules and composites have been found to greatly increase the fracture toughness of biomaterials by providing a reversible, molecular-scale energy-dissipation mechanism. This mechanism relies on the energy, of order 100 eV, needed to reduce entropy and increase enthalpy as molecular segments are stretched after being released by the breaking of weak bonds, called sacrificial bonds. This energy is relatively large compared to the energy needed to break the polymer backbone, of order a few eV. In many biological cases, the breaking of sacrificial bonds has been found to be reversible, thereby additionally providing a "self-healing" property to the material. Due to the nanoscopic nature of this mechanism, single molecule force spectroscopy using an atomic force microscope has been a useful tool to investigate this mechanism. Especially when investigating natural molecular constructs, force versus distance curves quickly become very complicated. In this work we propose various types of sacrificial bonds, their combination, and how they appear in single molecule force spectroscopy measurements. We find that by close analysis of the force spectroscopy curves, additional information can be obtained about the molecules and their bonds to the native constructs.

• Sacrificial bonds and hidden length dissipate energy as mineralized fibrils separate during bone fracture.

  Authors: Georg E Fantner, Tue Hassenkam, Johannes H Kindt, James C Weaver, Henrik Birkedal, Leonid Pechenik, Jacqueline A Cutroni, Geraldo A G Cidade, Galen D Stucky, Daniel E Morse, Paul K Hansma

  Nature materials. 09/2005; 4(8):612-6.

  Properties of the organic matrix of bone as well as its function in the microstructure could be the key to the remarkable mechanical properties of bone. Previously, it was found that on the molecularProperties of the organic matrix of bone as well as its function in the microstructure could be the key to the remarkable mechanical properties of bone. Previously, it was found that on the molecular level, calcium-mediated sacrificial bonds increased stiffness and enhanced energy dissipation in bone constituent molecules. Here we present evidence for how this sacrificial bond and hidden length mechanism contributes to the mechanical properties of the bone composite, by investigating the nanoscale arrangement of the bone constituents and their interactions. We find evidence that bone consists of mineralized collagen fibrils and a non-fibrillar organic matrix, which acts as a 'glue' that holds the mineralized fibrils together. We believe that this glue may resist the separation of mineralized collagen fibrils. As in the case of the sacrificial bonds in single molecules, the effectiveness of this mechanism increases with the presence of Ca2+ ions.

• Sacrificial bonds in polymer brushes from rat tail tendon functioning as nanoscale velcro.

  Authors: Thomas Gutsmann, Tue Hassenkam, Jacqueline A Cutroni, Paul K Hansma

  Biophysical journal. 08/2005; 89(1):536-42.

  Polymers play an important role in many biological systems, so a fundamental understanding of their cross-links is crucial not only for the development of medicines but also for the development ofPolymers play an important role in many biological systems, so a fundamental understanding of their cross-links is crucial not only for the development of medicines but also for the development of biomimetic materials. The biomechanical movements of all mammals are aided by tendon fibrils. The self-organization and biomechanical functions of tendon fibrils are determined by the properties of the cross-links between their individual molecules and the interactions among the cross-links. The cross-links of collagen and proteoglycan molecules are particularly important in tendons and, perhaps, bone. To probe cross-links between tendon molecules, we used the cantilever tip of an atomic force microscope in a pulling setup. Applying higher forces to rat tail tendon molecules with the tip led to a local disruption of the highly organized shell of tendon fibrils and to the formation or an increase of a polymer brush of molecules sticking out of the surface. The cross-linking between these molecules was influenced by divalent Ca2+ ions. Furthermore, the molecules of the polymer brush seemed to bind back to the fibrils in several minutes. We propose that sacrificial bonds significantly influence the tendon fibrils' self-organization and self-healing and therefore contribute to toughness and strength.

• Influence of the degradation of the organic matrix on the microscopic fracture behavior of trabecular bone.

  Authors: Georg E Fantner, Henrik Birkedal, Johannes H Kindt, Tue Hassenkam, James C Weaver, Jacquelin A Cutroni, Bonnie L Bosma, Lukmaan Bawazer, Marquesa M Finch, Geraldo A G Cidade, Daniel E Morse, Galen D Stucky, Paul K Hansma

  Bone. 12/2004; 35(5):1013-22.

  In recent years, the important role of the organic matrix for the mechanical properties of bone has become increasingly apparent. It is therefore of great interest to understand the interactionsIn recent years, the important role of the organic matrix for the mechanical properties of bone has become increasingly apparent. It is therefore of great interest to understand the interactions between the organic and inorganic constituents of bone and learn the mechanisms by which the organic matrix contributes to the remarkable properties of this complex biomaterial. In this paper, we present a multifaceted view of the changes of bone's properties due to heat-induced degradation of the organic matrix. We compare the microscopic fracture behavior (scanning electron microscopy; SEM), the topography of the surfaces (atomic force microscopy; AFM), the condition of bone constituents [X-ray diffraction (XRD), thermogravimetric analysis (TGA), and gel electrophoresis], and the macromechanical properties of healthy bovine trabecular bone with trabecular bone that has a heat-degraded organic matrix. We show that heat treatment changes the microfracture behavior of trabecular bone. The primary failure mode of untreated trabecular bone is fibril-guided delamination, with mineralized collagen filaments bridging the gap of the microcrack. In contrast, bone that has been baked at 200 degrees C fractures nondirectionally like a brittle material, with no fibers spanning the microcracks. Finally, bone that has been boiled for 2 h in PBS solution fractures by delamination with many small filaments spanning the microcracks, so that the edges of the microcracks become difficult to distinguish. Of the methods we used, baking most effectively weakens the mechanical strength of bone, creating the most brittle material. Boiled bone is stronger than baked bone, but weaker than untreated bone. Boiled bone is more elastic than untreated bone, which is in turn more elastic than baked bone. These studies clearly emphasize the importance of the organic matrix in affecting the fracture mechanics of bone.

• Rigid design of fast scanning probe microscopes using finite element analysis.

  Authors: Johannes H Kindt, Georg E Fantner, Jackie A Cutroni, Paul K Hansma

  Ultramicroscopy. 09/2004; 100(3-4):259-65.

  To improve the performance of atomic force microscopes regarding speed and noise sensitivity, it is important to consider the mechanical rigidity of the actuator (scanner), and the overall mechanicalTo improve the performance of atomic force microscopes regarding speed and noise sensitivity, it is important to consider the mechanical rigidity of the actuator (scanner), and the overall mechanical structure. Using finite element analysis in the design process, it was possible to increase the first resonance frequency from 950 Hz for the whole system to 23.4 kHz for the whole system. This constitutes a factor of approximately 25 in resonance frequency and a factor of 625 in stiffness and, hence, noise immunity.

• High-resolution AFM imaging of intact and fractured trabecular bone.

  Authors: Tue Hassenkam, Georg E Fantner, Jacqueline A Cutroni, James C Weaver, Daniel E Morse, Paul K Hansma

  Bone. 08/2004; 35(1):4-10.

  Nanoscale structural analyses of biomineralized materials can frequently help elucidate important structure-function relationships in these complex organic-inorganic composites. Atomic forceNanoscale structural analyses of biomineralized materials can frequently help elucidate important structure-function relationships in these complex organic-inorganic composites. Atomic force microscope (AFM) imaging of the exterior surface of trabecular bone reveals a densely woven structure of collagen fibrils, banded with a 67-nm periodicity, and densely packed mineral plates. The mineral plates on the collagen fibrils overlap and exhibit a large range of plate diameters from 30 to 200 nm. On the collagen fibrils, small nodular features, spaced 20-30 nm, run perpendicular to the fibrils. In some cases, these nodules are also seen on filaments extending between collagen fibrils. We hypothesize that these protrusions are noncollagenous proteins such as proteoglycans and may have collapsed into compact structures when the sample was dried. AFM images of pristine fractured surfaces reveal a dense array of mineral plates. In a few isolated locations, short sections of bare collagen fibrils are visible. In other regions, the existence of the underlying collagen fibrils can be inferred from the linear patterns of the mineral plates. Fractured samples, rinsed to remove mineral plates, reveal separated collagen fibrils on the fractured surfaces. These fibrils are often covered with protrusions similar to those observed on the exterior surfaces but are less organized. In addition, as on the exterior surfaces, there are sometimes small filaments extending between neighboring collagen fibrils. These studies provide important insights into the nanostructured architecture of this complex biocomposite.

• Force spectroscopy of collagen fibers to investigate their mechanical properties and structural organization.

  Authors: Thomas Gutsmann, Georg E Fantner, Johannes H Kindt, Manuela Venturoni, Signe Danielsen, Paul K Hansma

  Biophysical journal. 05/2004; 86(5):3186-93.

  Tendons are composed of collagen and other molecules in a highly organized hierarchical assembly, leading to extraordinary mechanical properties. To probe the cross-links on the lower level ofTendons are composed of collagen and other molecules in a highly organized hierarchical assembly, leading to extraordinary mechanical properties. To probe the cross-links on the lower level of organization, we used a cantilever to pull substructures out of the assembly. Advanced force probe technology, using small cantilevers (length <20 microm), improved the force resolution into the sub-10 pN range. In the force versus extension curves, we found an exponential increase in force and two different periodic rupture events, one with strong bonds (jumps in force of several hundred pN) with a periodicity of 78 nm and one with weak bonds (jumps in force of <7 pN) with a periodicity of 22 nm. We demonstrate a good correlation between the measured mechanical behavior of collagen fibers and their appearance in the micrographs taken with the atomic force microscope.

• Nanostructural features of demosponge biosilica.

  Authors: James C Weaver, Lía I Pietrasanta, Niklas Hedin, Bradley F Chmelka, Paul K Hansma, Daniel E Morse

  Journal of structural biology. 01/2004; 144(3):271-81.

  Recent interest in the optical and mechanical properties of silica structures made by living sponges, and the possibility of harnessing these mechanisms for the synthesis of advanced materials andRecent interest in the optical and mechanical properties of silica structures made by living sponges, and the possibility of harnessing these mechanisms for the synthesis of advanced materials and devices, motivate our investigation of the nanoscale structure of these remarkable biomaterials. Scanning electron and atomic force microscopic (SEM and AFM) analyses of the annular substructure of demosponge biosilica spicules reveals that the deposited material is nanoparticulate, with a mean particle diameter of 74+/-13 nm. The nanoparticles are deposited in alternating layers with characteristic etchant reactivities. Further analyses of longitudinally fractured spicules indicate that each deposited layer is approximately monoparticulate in thickness and exhibits extensive long range ordering, revealing an unanticipated level of nanoscale structural complexity. NMR data obtained from differentially heated spicule samples suggest that the etch sensitivity exhibited by these annular domains may be related to variation in the degree of silica condensation, rather than variability in the inclusion of organics. In addition, AFM phase imaging in conjunction with results obtained from HF and alkaline etching experiments suggest that at various stages in spicule biosynthesis, regions of unusually low silica condensation are deposited, indicating a possible interruption in normal spicule formation. While this discovery of nanoparticulate silica aggregation in demosponge skeletal elements is likely to reflect the intrinsic kinetic tendency of silica to form such particles during polycondensation, the heirarchical organization of these nanoparticles is biologically unique.

• Evidence that collagen fibrils in tendons are inhomogeneously structured in a tubelike manner.

  Authors: Thomas Gutsmann, Georg E Fantner, Manuela Venturoni, Axel Ekani-Nkodo, James B Thompson, Johannes H Kindt, Daniel E Morse, Deborah Kuchnir Fygenson, Paul K Hansma

  Biophysical journal. 05/2003; 84(4):2593-8.

  The standard model for the structure of collagen in tendon is an ascending hierarchy of bundling. Collagen triple helices bundle into microfibrils, microfibrils bundle into subfibrils, and subfibrilsThe standard model for the structure of collagen in tendon is an ascending hierarchy of bundling. Collagen triple helices bundle into microfibrils, microfibrils bundle into subfibrils, and subfibrils bundle into fibrils, the basic structural unit of tendon. This model, developed primarily on the basis of x-ray diffraction results, is necessarily vague about the cross-sectional organization of fibrils and has led to the widespread assumption of laterally homogeneous closepacking. This assumption is inconsistent with data presented here. Using atomic force microscopy and micromanipulation, we observe how collagen fibrils from tendons behave mechanically as tubes. We conclude that the collagen fibril is an inhomogeneous structure composed of a relatively hard shell and a softer, less dense core.

• Molecular nanosprings in spider capture-silk threads.

  Authors: Nathan Becker, Emin Oroudjev, Stephanie Mutz, Jason P Cleveland, Paul K Hansma, Cheryl Y Hayashi, Dmitrii E Makarov, Helen G Hansma

  Nature materials. 05/2003; 2(4):278-83.

  Spider capture silk is a natural material that outperforms almost any synthetic material in its combination of strength and elasticity. The structure of this remarkable material is still largelySpider capture silk is a natural material that outperforms almost any synthetic material in its combination of strength and elasticity. The structure of this remarkable material is still largely unknown, because spider-silk proteins have not been crystallized. Capture silk is the sticky spiral in the webs of orb-weaving spiders. Here we are investigating specifically the capture spiral threads from Araneus, an ecribellate orb-weaving spider. The major protein of these threads is flagelliform protein, a variety of silk fibroin. We present models for molecular and supramolecular structures of flagelliform protein, derived from amino acid sequences, force spectroscopy (molecular pulling) and stretching of bulk capture web. Pulling on molecules in capture-silk fibres from Araneus has revealed rupture peaks due to sacrificial bonds, characteristic of other self-healing biomaterials. The overall force changes are exponential for both capture-silk molecules and intact strands of capture silk.

• Investigations into the polymorphism of rat tail tendon fibrils using atomic force microscopy.

  Authors: Manuela Venturoni, Thomas Gutsmann, Georg E Fantner, Johannes H Kindt, Paul K Hansma

  Biochemical and biophysical research communications. 05/2003; 303(2):508-13.

  Collagen type I displays a typical banding periodicity of 67 nm when visualized by atomic force or transmission electron microscopy imaging. We have investigated collagen fibers extracted from ratCollagen type I displays a typical banding periodicity of 67 nm when visualized by atomic force or transmission electron microscopy imaging. We have investigated collagen fibers extracted from rat tail tendons using atomic force microscopy, under different ionic and pH conditions. The majority of the fibers reproduce the typical wavy structure with 67 nm spacing and a height difference between the peak and the grooves of at least 5 nm. However, we were also able to individuate two other banding patterns with 23+/-2 nm and 210+/-15 nm periodicities. The small pattern showed height differences of about 2 nm, whereas the large pattern seems to be a superposition of the 67 nm periodicity showing height differences of about 20 nm. Furthermore, we could show that at pH values of 3 and below the fibril structure gets dissolved whereas high concentrations of NaCl and CaCl(2) could prevent this effect.

• The backbone conformational entropy of protein folding: experimental measures from atomic force microscopy.

  Authors: James B Thompson, Helen G Hansma, Paul K Hansma, Kevin W Plaxco

  Journal of molecular biology. 10/2002; 322(3):645-52.

  The energy dissipated during the atomic force microscopy-based mechanical unfolding and extension of proteins is typically an order of magnitude greater than their folding free energy. The vastThe energy dissipated during the atomic force microscopy-based mechanical unfolding and extension of proteins is typically an order of magnitude greater than their folding free energy. The vast majority of the "excess" energy dissipated is thought to arise due to backbone conformational entropy losses as the solvated, random-coil unfolded state is stretched into an extended, low-entropy conformation. We have investigated this hypothesis in light of recent measurements of the energy dissipated during the mechanical unfolding of "polyproteins" comprised of multiple, homogeneous domains. Given the assumption that backbone conformational entropy losses account for the vast majority of the energy dissipated (an assumption supported by numerous lines of experimental evidence), we estimate that approximately 19(+/-2)J/(mol K residue) of entropy is lost during the extension of three mechanically stable beta-sheet polyproteins. If, as suggested by measured peak-to-peak extension distances, pulling proceeds to near completion, this estimate corresponds to the absolute backbone conformational entropy of the unfolded state. As such, it is exceedingly close to previous theoretical and semi-empirical estimates that place this value at approximately 20J/(mol K residue). The estimated backbone conformational entropy lost during the extension of two helical polyproteins, which, in contrast to the mechanically stable beta-sheet polyproteins, rupture at very low applied forces, is three- to sixfold less. Either previous estimates of the backbone conformational entropy are significantly in error, or the reduced mechanical strength of the helical proteins leads to the rupture of a subsequent domain before full extension (and thus complete entropy loss) is achieved.

• Imaging Powders with the Atomic Force Microscope: From Biominerals to Commercial Materials.

  Authors: Gernot Friedbacher, Paul K Hansma, Emannuel Ramli, Galen D Stucky

  Science (New York, N.Y.). 10/1991; 253(5025):1261-1263.

  Atomically resolved images of pressed powder samples have been obtained with the atomic force microscope (AFM). The technique was successful in resolving the particle, domain, and atomic structure ofAtomically resolved images of pressed powder samples have been obtained with the atomic force microscope (AFM). The technique was successful in resolving the particle, domain, and atomic structure of pismo clam (Tivela stultorum) and sea urchin (Strongylocentrotus purpuratus) shells and of commercially available calcium carbonate (CaCO(3)) and strontium carbonate (SrCO(3)) powders. Grinding and subsequent pressing of the shells did not destroy the microstructure of these materials. The atomic-resolution imaging capabilities of AFM can be applied to polycrystalline samples by means of pressing powders with a grain size as small as 50 micrometers. These results illustrate that the AFM is a promising tool for material science and the study of biomineralization.