Molecular Biophysics

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Molecular Biophysics

• Molecular Forces (II)

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Recap
Water molecules have special properties They
have an electric dipole moment, and water as a
solvent has a high dielectric constant (relative
permittivity e). There are max 4 hydrogen bonds
between water molecules
Electrostatic interactions are shielded in a
solvent with high dielectric constant
The Henderson-Hasselbach eq. can be used to
calculate the ratio between protonated and
deprotonated groups HA/A- for a given pH value
Amino acids have amino and acid groups. Both
groups have pK values indicating at which pH
value they are protonated/ deprotonated.
Distinguish between hydrophilic/hydrophobic
AS polar/unpolar AS
with/without charged side groups.
Electric potential U(r) can be used to calculate
forces between charged groups Image forces arise
for charged particles at regions with big
differences of e (e.g. membrane/water interface)
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Electrostatic interactions
2 point charges
point charges, fixed dipole
Increasingly short-ranged
2 fixed dipoles
point charge rotating dipole
2 rotating dipoles
4

• Today
• Induced dipoles
• Cation p interactions
• Dispersion forces (London forces)
• Hydrophobic forces
• Hydrogen bonds
• Steric repulsion
• Lennard Jones potential

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Electron distribution in aromatic rings (p-
bonding)
Benzene (C6H6)
3 amino acids have aromatic rings as side
groups Phenylalanine, Tyrosine, Tryptophane
wikipwdia
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Lennard-Jones Potential
r0 distance parameter
e energy parameter