Puentes de Hidrgeno

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Puentes de Hidrógeno
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Intermolecular Forces
Intermolecular forces are forces between
molecules.
Intramolecular forces hold atoms together in a
molecule.

• Intermolecular vs Intramolecular
• 41 kJ to vaporize 1 mole of water (inter)
• 930 kJ to break all O-H bonds in 1 mole of water
  (intra)

Measure of intermolecular force boiling
point melting point DHvap DHfus DHsub
Generally, intermolecular forces are much weaker
than intramolecular forces.
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Types of Intermolecular Forces
2. Ion-Dipole Forces
Attractive forces between an ion and a polar
molecule
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Types of Intermolecular Forces
3. Dipole-Dipole Forces
Attractive forces between polar molecules
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Covalent Bond and Ionic Bond Ionic bond electron
transfer Covalent bond electron sharing
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Sodium Chloride ionic bond formation
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The geometry of covalent bonds
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Types of Intermolecular Forces
4. Dispersion Forces van der Walls
forces/London forces (weakest)
Attractive forces that arise as a result of
temporary dipoles induced in atoms or molecules
ion-induced dipole interaction
dipole-induced dipole interaction
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Early evidence of hydrogen bonding
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Types of Intermolecular Forces
1. Hydrogen Bond (strongest)
The hydrogen bond is a special dipole-dipole
interaction between the hydrogen atom in a polar
N-H, O-H, or F-H bond and an electronegative O,
N, or F atom. IT IS NOT A BOND.
A B are N, O, or F
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Hydrogen Bond
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The strongest H-bonds are linear
Hydrogen bonds are directional, with the
strongest bonds formed when the DHA atoms
are arranged linearly. The histogram below shows
the distribution of hydrogen bond angles in
crystal structures of small molecules.
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H-bonds direction, lengths, angles, and strength

• H-bond possesses direction and is generally
  asymmetric
• Bond strength depends on angle and length
• Small changes from linearity (20º) have minor
  effect in strength
• H-bond strength, on the other hand,decreases
  exponentially with distance
• There is a tradeoff between H-bond and covalent
  bond strength
• stronger the HO bond, the weaker the O-H
  covalent bond,
• and shorter the OO distance
• Weakening of the covalent bond is a good
  indicator of strengthening of H-bond
  energy
• Cooperativity and anti-cooperativity in H-bonding

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Hydrogen bonding in water

• H-bond partly electrostatic (90), partly
  covalent (10)
• Approximated by the following bonds
• ?a (A-H B) ?b (A- H---B ionic) ?c (A-
  H----B covalent) ?d (A H----B ionic) ?e
  (H- A---B covalent)
• ? ?a ?b ?c ?d ?e
• X-Ray spectroscopic evidence suggests that these
  interactions shift within a femtosecond
• Nuclear quantum effects strengthen the H-bond by
  increasing the dipole moment
• Donor hydrogen stretches away from its oxygen
• Acceptor lone-pair stretches away from it oxygen
• Both oxygens being pulled towards each other
• Cute Movie
• http//www.northland.cc.mn.us/biology/Biology1111/
  animations/hydrogenbonds.html

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How would you calculate the strength of an H-bond?

• Between 3 and 9 kcal/mol
• Dissociation rate constants of 4 X 1010 to 2 X
  106 s-1 from transition state theory
• R-O-H-----ORH

ROH ROH
R-O-H-----ORH
Keq
ROH2/
If Keq1, ?G0 if Keq 10, ?G1.3 kcal/mol, if
Keq 100, then ?G 2.6 kcal/mol

• But it is more complicated than this for
  biological systems in water

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Different types of H-bonds

• Common elements that form H-bonds
• S, O, N, F
• CH H-bond
• RHO----RCOH?COR
• ?-cation interaction

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Low Barrier Hydrogen bonds (LBHB)

• H-bond strength depends on its length, linearity,
  microenvironment, and the pKa values of the the H
  sharing components
• H-bonds in water are relatively weak because of
  pKa mismatch between H3O (-1.7) and H2O (15.7)
• The proton in the structure is tightly associated
  with the OH- as a water molecule
• In gas phase, dielectric constant is low
• Hydrogen bonds between heteroatoms with matched
  pKa values can be ? 2.5 Å and very strong (25-30
  kcal/mol)

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H-bonds in Proteins

• I. H-bonds contributing to structure and folding
• II. H-bonds contributing to catalysis

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?-helix

• i4, i3, i5

• Dipole moment
• charge at N-terminii, - charge at COOH
• Negative molecules (PO42- groups) bind at
  N-terminii

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?-sheets

• Antiparallel ?-sheets
• 10-14 atoms in a ring
• H-bonds linear
• Stability?

• Parallel ?-sheets
• 12 atoms in a ring
• H-bonds not 180?
• Stability?