Introduction to Biophysics Lecture 17 Self assembly

An optical tweezer is a scientific instrument that uses a focused laser beam to provide an attractive or repulsive force (typically on the order of piconewtons), depending on the refractive index mismatch to physically hold and move microscopic dielectric objects. Optical tweezers have been particularly successful in studying a variety of biological systems in recent years. Microscope objective The force applied on the object depends linearly on displacement from the trap center. Dielectric particles are attracted along the gradient to the region of strongest electric field, which is the center of the beam. The scattering force results in the particle being displaced slightly downstream from the exact position of the beam waist.

Surfactants (amphiphilic), what are they?

Amphiphilic molecules reduce the oil-water interface tension

The spontaneous orientation of molecules in phase boundaries. hydrophobic hydrophilic State of minimal surface tension in homogeneous aqueous phase Minimum of surface tension (phenomenological description) – on molecular level – result of dynamics of water structure and hydrophobic interactions.

There is a large diversity in lipid aggregates in aqueous solutions Condition to form micelle: V(of N*Vtail)S(N*Shead) Sonication of lipid suspensions

Discuss temperature dependence of CMC How CMC will change for two-tail surfactant?

Cell Membrane Cell geometry of phospholipids limits the membrane thickness. bi-layer is fluid due to nonspecific nature of hydrophobic interactions, membrane readily accepts embedded objects (membrane proteins)

Cell membrane is a well organized structure fulfilling a broad spectrum of physiological functions: Separation from environment. A barrier of diffusion. Controls the ionic composition of the cytoplasm by highly specific transporters (molecular and ion pumps). Mechanical structure to guaranty cell integrity, shape, movement, endocytose. As a surface, forms a dynamic matrix for enzymatic reactions. Central in biological communication –membrane receptors. Immunological recognition. Place for energy conversion processes. Electrically isolating leaflet it contains a mosaic of various passive an active electrical devices, controlling membrane potential.

Biophysical aspects of protein folding The sequence of protein completely determines its folded structure Native conformation is the minimum of the free energy Delicate balance  20kBTr Hydrophobicities of the core residues are conserved Hydrophobic interactions are believed to give the dominant force driving protein folding. Thermodynamic stability of folded protein under physiological conditions is in contrast to random-walk behavior of polymers. Denaturation (unfolding): temperature (both high and low), non-polar solvents, small concnetrations of surfactants.

Reading: Nelson Chapter 8 Propose the model to fit data of McBain (Figure 8.6) use mass action rule for reaction (N monomers) (one aggregate)