3.052 Nanomechanics of Materials and Biomaterials Tuesday 03/06/07 Prof. C. Ortiz, MIT-DMSE 1 I LECTURE 8: INTRODUCTION TO INTRA- AND INTERMOLECULAR FORCES Outline : LAST TIME : SINGLE CELL MECHANICS .............................................................................................. 2 CLASSIFICATION OF INTRA- AND INTERMOLECULAR FORCES ...................................................... 3 SPECIFIC TYPES OF INTRA- AND INTERMOLECULAR FORCES ....................................................... 4 HYDROPHOBIC / HYDROPHILIC INTERACTIONS ................................................................................ 5 EXAMPLES .......................................................................................................................................... 6-7 Biological : Noncovalent Interactions in Folded Protein Structures .............................................. 6 Synthetic : Self-Assembling Peptide Amphiphiles for Regenerative Medicine ............................. 7 Objectives: To explore the qualitative origins of intra- and intermolecular forces Readings: Course Reader documents 16-19, Links on "Supplementary Resources" section of Stellar (Optional) Multimedia : Protein folding demo posted on stellar; Lecture 8 : Bonding and protein structure (California Lutheran University)3.052 Nanomechanics of Materials and Biomaterials Tuesday 03/06/07 Prof. C. Ortiz, MIT-DMSE 2 SINGLE CELL MECHANICS -single cell AFM imaging -motivation (musculoskeletal tissue, circulatory system, brain) -experimental methods 1) localized area of the cell is deformed AFM, magnetic bead, 2) mechanical loading of an entire cell micropipette aspiration, optical trap, 3) simultaneous mechanical loading of a population of cells (shear flow, cell force monitor) -cell modeling (Dao, et al 2003 J. Mech. Phys. Solids.) • The composite is modeled as an isotropic, elastic, continuum, incompressible (constant volume), constant surface area Constitutive Law : stress vs. strain relationship that describes a particular material 1 (applied)2=0 (by choice)3=0 (always)=F/Ao(Lo )1(Lo )2h Single macromolecule Gaussian linear elastic Hookean spring F=kr → summing over a network of random coil molecules "Triangulated Network" [1] Mohandas, et al. Mechanical properties of the red cell membrane in relation to molecular structures and genetic defects. Annu. Rec. Biophys. Struct. 1994. 23:787-818              32 3 2 2 3 2o1 2 3 3 1 2 3Strain energy of a 3D rubber elastic networknonGaussian Nonlinear Strain Hardening TermNeo-Hookean Rubber ElasticitryGU = - 3 C - 32= extension or stretch ra      ,,    fff3121 2 3o o o1 2 3o2n11 2 3LLLtio, = = =L L LG shear modulus by definitionUuniaxial normal stress, (N/m ) =constant volume constraint = from definition of extension ratio & geometry3.052 Nanomechanics of Materials and Biomaterials Tuesday 03/06/07 Prof. C. Ortiz, MIT-DMSE 3 CLASSIFICATION OF INTRA- AND INTERMOLECULAR FORCES (within individual molecules) (between individual molecules) -Definitions : Interaction (more general), force (push or pull), bond (the attraction between atoms in a molecule or crystalline structure)→ all intra- and intermolecular forces are electrostatic in origin → key to life on earth (e.g. water, cell membranes, protein folding, etc.) -strength measured relative to the thermal energy (room temperature) : kBT= 4.1 ● 10-21 J : "ruler" noncovalent  I. Primary or Chemical ● individually strong > kBT ● outer orbital e- shared that the discrete nature of the atom is lost ● quantum mechanical in origin ● covalent → possess specificity, directionality, stoichiometry ● metallic → delocalized electrons  covalent, metallic, ionic (in air) II. Secondary or Physical ● individually weak ≤ kBT ● no e- sharing; between two or more atoms so discrete nature of atoms preserved ● more subtle attraction in origin between (+) and (-) charges typically lack specificity, directionality, stoichiometry ionic (in water), polar, polarization, dispersion III. "Special" ● broad range of strength ● controversial ● not a "true" bond hydrophobic, hydrophilic, polymer effects (e.g. excluded volume, entropic elasticity), electrostatic double layer) - Biological systems and bottom-up self-assembly is based on the balance and interplay of intra- and intermolecular forces. -Noncovalent interactions allow for dynamic systems, i.e. breaking reversible reforming bonds doesn't require much energy)/individually weak, forces are cumulative → stable in parallel.3.052 Nanomechanics of Materials and Biomaterials Tuesday 03/06/07 Prof. C. Ortiz, MIT-DMSE 4 SPECIFIC TYPES OF INTRA- AND INTERMOLECULAR FORCES +- Ionic -Coulombic attraction between oppositely charged species -individually strong, however greatly weakened in the presence of water (e.g. center of proteins→strong, DNA-proteins) Polar Interactions polar = asymmetric distribution of charge ● attractive force between an ion and a permanent dipole or two permanent dipoles where the (+) charge attracts (-) (e.g. hydrogen bonds) (-all 3 atoms in water can H-bond, up to 4 per molecule, important in biology because it is able to form weak interactions with so many different chemical species) ++-+-+- Polarization Interactions ● an ion or dipole in the vicinity of a nonpolar atom or molecule causes an instantaneous polarization and electrostatic attraction +-+- larger the e- cloud easier to distort (> polarizability) London Dispersion ● nonpolar-nonpolar and induced dipole - induced dipole ● charge fluctuation, the (+) nucleus of a nonpolar atom attracts the (-) charged electron cloud of another nonpolar atom → an instantaneous induced, short lived fluctuating dipole, -takes place in all atoms / molecules, 3.052 Nanomechanics of Materials and Biomaterials Tuesday 03/06/07 Prof. C. Ortiz, MIT-DMSE 5 HYDROPHOBIC ("WATER FEARING") INTERACTIONS ● attraction and association between