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Introduction to Statistical Thermodynamics of Soft and Biological Matter Lecture 3PowerPoint Presentation

Introduction to Statistical Thermodynamics of Soft and Biological Matter Lecture 3

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Introduction to Statistical Thermodynamics

of Soft and Biological Matter

Lecture 3

Statistical thermodynamics III

- Kinetic interpretation of the Boltzmann distribution.
- Barrier crossing.
- Unfolding of single RNA molecule.
- Diffusion.
- Random walks and conformations of polymer molecules.
- Depletion force.

- System with many possible states (M possible states)
- (different conformations of protein molecule)
- Each statehas probability
- Each state has energy

Partition function:

Probability distribution for velocities:

Maxwell-Boltzmann distribution

- velocity of a molecule

Gas of N molecules:

Example: fluctuations of polymer molecule

- energy of polymer molecule

Probability distribution:

Equipartition theorem:

Kinetic interpretation of the

Boltzmann distribution

Detailed balance (at equilibrium):

Number of molecules in state 2 and in state 1

Verify!

Unfolding of single RNA molecule

J. Liphardt et al., Science 292, 733 (2001)

Optical tweezers apparatus:

Open state:

Close state (force applied):

extension

force

Two-state system and unfolding

of single RNA molecule

J. Liphardt et al., Science 292, 733 (2001)

N-th step of random walk:

(N-1)-th step of random walk:

Verify!

Universal properties of random walk

One-dimensional random walk:

L

(step-size of random walk)

0

- random number (determines direction of i-th step)

Diffusion coefficient and dissipation

Einstein relation:

Friction coefficient:

Viscosity

Particle size

Diffusion in two and three dimensions

One-dimensional (1D) random walk:

Two-dimensional (2D) random walk:

Three-dimensional (3D) random walk:

Conformations of polymer molecules

L – length of elementary segment

- Universal properties of random walk describe conformations
- of polymer molecules.

* Excluded volume effects and interactions may change law!

c(x,t)

x

Solution of diffusion equation

The concentration profile spreads out with time

Pressure:

Osmotic forces:

Concentration difference induces

osmotic pressure

Protein solution

density:

Semi-permeable membrane

(only solvent can penetrate)

Pressure of ideal gas

N – number of particles

V - volume

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