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3.052 Nanomechanics of Materials and Biomaterials

3.052 Nanomechanics of Materials and Biomaterials. LECTURE #9 : QUANTITATIVE TREATMENT OF INTRA- AND INTERMOLECULAR FORCES. Prof. Christine Ortiz DMSE, RM 13-4022 Phone : (617) 452-3084 Email : cortiz@mit.edu WWW : http://web.mit.edu/cortiz/www. Review : Lecture #4

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3.052 Nanomechanics of Materials and Biomaterials

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  1. 3.052 Nanomechanics of Materials and Biomaterials LECTURE #9 : QUANTITATIVE TREATMENT OF INTRA- AND INTERMOLECULAR FORCES Prof. Christine Ortiz DMSE, RM 13-4022 Phone : (617) 452-3084 Email : cortiz@mit.edu WWW : http://web.mit.edu/cortiz/www

  2. Review : Lecture #4 Experimental Aspects of Force Spectroscopy III : I.Comparison of high-resolution force spectroscopy techniques : • atomic force microscopy (AFM), surface forces apparatus (SFA), optical tweezers (OT), biomembrane surface probe (BSP) II.Conversion of raw data in a high-resolution force spectroscopy experiment : • sensor output, s  transducer displacement, d force, F • z-piezo deflection, z  tip-sample separation distance, D III.Typical force spectroscopy data for a weak cantilever on stiff substrate (ksample>>kcantilever) : APPROACH : (*sample and tip come together) • A: tip and sample out of contact, no interaction, cantilever undeflected, zero force (set F=0) • B/C: attractive interaction pulls tip down to surface and tip jumps to contact, cantilever exhibits mechanical instability • D: contact, constant compliance regime, no sample indentation, tip and sample move in unison (Ds/Dz=1) RETRACT :(*sample and tip move apart) • D: repulsive contact, constant compliance Regime, tip deflected up • E: attractive force (adhesion) keep tip attached to surface, tip deflected down • F: tip pulls off from surface, cantilever instability • G: same as region A F=kd d=s/m D=zd D D D D A A B/C B/C G F G E E F Adhesive Interaction

  3. Types of Intra- and Intermolecular Interactions in Different Materials

  4. Biomolecular Adhesion • controlled by bonds between molecular “ligands” and cell surface “receptors” which exhibit the “lock-n-key principle” (e.g. biotin-streptavidin) • Grubmüller, et al, Science 1996 (*http://www.mpibpc.gwdg.de/abteilungen/ 071/strept.html) (*http://www.amber.ucsf.edu/amber/tutorial/streptavidin/index.html) • complex, multiatomic, relatively weak • formed by an assembly ofmultiple, weak non-covalent interactions (e.g. H-bonding, coulombic, van der Waals, hydrophilic / hydrophobic, electrostatic) •complementary, sterically-contrained geometric considerations • specificity

  5. r(nm) BRIDGING THE GAP BETWEEN LENGTH SCALES Force, F (nN) kc 0 Tip-Sample Separation Distance, D (nm)

  6. Characterizing an Individual Intra- and Intermolecular Interaction interaction distance (nm) interaction energy (kJ/mol) interaction force (electromagnetic in origin) (nN)

  7. Characterizing an Individual Intra- and Intermolecular Interaction interaction distance (nm) interaction energy (kJ/mol) interaction force (electromagnetic in origin) (nN)

  8. Soft Repulsion B=10-134Jm12 n=12 Steric Repulsion Interaction Potentials • Due to overlap of negatively charged electron clouds (e.g. Pauli Exclusion principle) and (+) charged nuclei, quantum mechanical in origin; “short-range”, i.e. takes place over the order of distances of bond lengths ~0.1 nm Hard-Core Repulsion n= Soft Repulsion s

  9. Attractive Interaction Potentials • longer range > ~1 nm • A is a constant determined by the polarizability or ease of distortion of electron cloud London dispersion interaction A=10-77Jm6 m=6

  10. Net or Complete Interaction Potential : The Lennard-Jones or “6-12” Potential r(nm)

  11. Interaction Strength DEB

  12. Equilibrium Interaction Distance, re re’>re re re r (nm)

  13. Force Profile for The Lennard-Jones or “6-12” Potential ro re rs Frupture r(nm)

  14. More Complicated Interaction Potentials • Grubmüller, et al, Science 1996 (*http://www.mpibpc.gwdg.de/abteilungen/071/strept.html) R. MERKEL*†, P. NASSOY*‡, A. LEUNG*, K. RITCHIE* & E. EVANS*§, Nature 397, 50 - 53 (1999)

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