Nanomechanics: A new approach for studying the mechanical properties of materials

Mitjançant l'espectroscòpia de forces atòmiques s'ha estudiat la resposta nanomecànica a la nanoindentació de la superfície més estable d'un material trencadís FCC, com és ara el MgO (100). L'expulsió del material en forma de capes demostra que la fallida trencadissa implica, de fet, l'inici de la deformació plàstica o estrès crític, i que la deformació plàstica posterior consisteix en una sèrie d'esdeveniments discrets. Es pot determinar amb precisió el mòdul de Young, E, a partir de la regió de deformació elàstica mitjançant una mecànica senzilla, atesa l'absència de dislocacions induïdes per la nanoindentació. Amb aquesta finalitat s'ha desenvolupat un nou model fisicomatemàtic, que té en compte les interaccions laterals. El valor de l'estrès crític de fricció també s'ha calculat i comentat. Com a conseqüència d'aquesta expulsió en capes, també s'ha estudiat la resposta nanomecànica de superfícies de capes primes (gruix & 1 µm) de molècules orgàniques altament orientades, ja que es tracta de materials en capes amb interaccions de tipus Van der Waals. També en aquests materials la superfície es deforma plàsticament i presenta discontinuïtats discretes en les corbes d'indentació, associades ara a les capes moleculars expulsades per l'indentador. En el cas del metall quasiunidimensional tetratiofulvalè tetracianoquinodimetà (TTFTCNQ), el valor del mòdul de Young, E & 20 GPa, coincideix amb l'obtingut per altres mètodes. En el cas de la fase ! del radical p-nitrofenil nitronil nitròxid (p-NPNN) no es disposa d'informació per a monocristalls, i el valor obtingut per a les capes primes és de E & 2 GPa.Atomic force spectroscopy was used to study the nanomechanical response to nanoindentations on the most stable face (100) of FCC brittle materials such as MgO and alkali halides. The layered expulsion of material demonstrates that brittle failure results from the critical stress brought on by plastic deformation and that plastic deformation consists of a series of discrete events. Due to the absence of indentation- induced dislocations, Young?s modulus E can be correctly estimated from the elastic deformation region using simple mechanics. A new model is developed taking into account lateral interactions. Critical shear stress is also evaluated and discussed. As a result of the layered expulsion we also studied the nanomechanical response of surfaces of highly-oriented molecular organic thin films (ca. 1 µm thickness) because these are Van der Waals layered materials. The surfaces were again found to deform plastically and there were discrete discontinuities in the indentation curves, representing the molecular layers being expelled by the penetrating tip. Here, the Hertz model is quite good at revealing the role of lateral interactions in the indentation process. For the quasi-one-dimensional metal tetrathiafulvalene tetracyanoquinodimethane (TTF-TCNQ) the value of Young?s modulus, E & 20 GPa, coincides with that obtained by other bulk methods. For the !-phase of the p-nitrophenyl nitronyl nitroxide (p-NPNN) radical, no information is available for single crystals and the estimated value obtained for the film is E & 2 GPa

Proteoglycan mechanics studied by single-molecule force spectroscopy of allotypic cell adhesion glycans

Author Posting. © American Society for Biochemistry and Molecular Biology, 2006. This article is posted here by permission of American Society for Biochemistry and Molecular Biology for personal use, not for redistribution. The definitive version was published in Journal of Biological Chemistry 281 (2006): 5992-5999, doi:10.1074/jbc.M507878200.Early Metazoans had to evolve the first cell adhesion system addressed to maintaining stable interactions between cells constituting different individuals. As the oldest extant multicellular animals, sponges are good candidates to have remnants of the molecules responsible for that crucial innovation. Sponge cells associate in a species-specific process through multivalent calcium-dependent interactions of carbohydrate structures on an extracellular membrane-bound proteoglycan termed aggregation factor. Single-molecule force spectroscopy studies of the mechanics of aggregation factor self-binding indicate the existence of intermolecular carbohydrate adhesion domains. A 200-kDa aggregation factor glycan (g200) involved in cell adhesion exhibits interindividual differences in size and epitope content which suggest the existence of allelic variants. We have purified two of these g200 distinct forms from two individuals of the same sponge species. Comparison of allotypic versus isotypic g200 binding forces reveals significant differences. Surface plasmon resonance measurements show that g200 self-adhesion is much stronger than its binding to other unrelated glycans such as chondroitin sulfate. This adhesive specificity through multiple carbohydrate binding domains is a type of cooperative interaction that can contribute to explain some functions of modular proteoglycans in general. From our results it can be deduced that the binding strength/surface area between two aggregation factor molecules is comparable with that of focal contacts in vertebrate cells, indicating that strong carbohydrate-based cell adhesions evolved at the very start of Metazoan history.This work was supported in part by Grants BIO2002-00128 and BIO2005-01591 (both to X. F.-B.) from the Ministerio de Educacio´n y Ciencia, Spain, which included Fondo Europeo de Desarrollo Regional funds

Binding of myomesin to obscurin-like-1 to the muscle M-band provides a strategy for isoform-specific mechanical protection

The sarcomeric cytoskeleton is a network of modular proteins that integrate mechanical and signalling roles. Obscurin, or its homolog obscurin-like-1, bridges the giant ruler titin and the myosin crosslinker myomesin at the M-band. Yet, the molecular mechanisms underlying the physical obscurin(-like-1):myomesin connection, important for mechanical integrity of the M-band, remained elusive. Here, using a combination of structural, cellular, and single-molecule force spectroscopy techniques, we decode the architectural and functional determinants defining the obscurin(-like-1): myomesin complex. The crystal structure reveals a trans-complementation mechanism whereby an incomplete immunoglobulin-like domain assimilates an isoform-specific myomesin interdomain sequence. Crucially, this unconventional architecture provides mechanical stability up to forces of 135 pN. A cellular competition assay in neonatal rat cardiomyocytes validates the complex and provides the rationale for the isoform specificity of the interaction. Altogether, our results reveal a novel binding strategy in sarcomere assembly, which might have implications on muscle nanomechanics and overall M-band organization.We thank the Diamond Light Source and the European Synchrotron Radiation Laboratory for access to MX and SAXS beamlines, respectively. This work was supported by a British Heart Foundation grant (PG/10/67/28527) awarded to R.A.S. and M.G. as well as MRC grant MR/J010456/1 to M.G. and a British Heart Foundation grant (PG/13/50/30426) and EPSRC Fellowship (K00641X/1) to S.G.-M

Protein S-sulfenylation is a fleeting molecular switch that regulates non-enzymatic oxidative folding

The post-translational modification S-sulfenylation functions as a key sensor of oxidative stress. Yet the dynamics of sulfenic acid in proteins remains largely elusive due to its fleeting nature. Here we use single-molecule force-clamp spectroscopy and mass spectrometry to directly capture the reactivity of an individual sulfenic acid embedded within the core of a single Ig domain of the titin protein. Our results demonstrate that sulfenic acid is a crucial short-lived intermediate that dictates the protein's fate in a conformation-dependent manner. When exposed to the solution, sulfenic acid rapidly undergoes further chemical modification, leading to irreversible protein misfolding; when cryptic in the protein's microenvironment, it readily condenses with a neighbouring thiol to create a protective disulfide bond, which assists the functional folding of the protein. This mechanism for non-enzymatic oxidative folding provides a plausible explanation for redox-modulated stiffness of proteins that are physiologically exposed to mechanical forces, such as cardiac titin

Protein nanomechanics: The power of stretching

Protein nanomechanics is a rapidly evolving field at the intersection of physics, chemistry and biology focused on the characterisation of the conformational dynamics of proteins under force, of common occurrence in vivo

The mechanochemistry of copper reports on the directionality of unfolding in model cupredoxin proteins

International audienceUnderstanding the directionality and sequence of protein unfolding is crucial to elucidate the underlying folding free energy landscape. An extra layer of complexity is added in metalloproteins, where a metal cofactor participates in the correct, functional fold of the protein. However, the precise mechanisms by which organometallic interactions are dynamically broken and reformed on (un)folding are largely unknown. Here we use single molecule force spectroscopy AFM combined with protein engineering and MD simulations to study the individual unfolding pathways of the blue-copper proteins azurin and plastocyanin. Using the nanomechanical properties of the native copper centre as a structurally embedded molecular reporter, we demonstrate that both proteins unfold via two independent, competing pathways. Our results provide experimental evidence of a novel kinetic partitioning scenario whereby the protein can stochastically unfold through two distinct main transition states placed at the N and C termini that dictate the direction in which unfolding occurs.</p