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MACROMOLECULES IMACROMOLECULES I
THE IMPORTANCE OF WEAK THE IMPORTANCE OF WEAK INTERACTIONSINTERACTIONS
BONDS OF INTERESTBONDS OF INTEREST
Covalent bonds—NOT a weak Covalent bonds—NOT a weak interaction. interaction. G= -50 to -110 kcal/molG= -50 to -110 kcal/mol
Weak interactions in biological Weak interactions in biological systemssystems Hydrogen bondsHydrogen bonds Ionic bondsIonic bonds Van der waals bondsVan der waals bonds Hydrophobic interactionsHydrophobic interactions
Formed between a covalently bound H and a covalently bound acceptor
The H is bound to an electronegative atom and interacts non-covalently with another electronegative atom (usually O or N)
Much weaker than covalent bonds.
Very important in protein and nucleic acid structure.
HYDROGEN BONDSHYDROGEN BONDSPower in NumbersPower in Numbers
H-bonding between water molecules
Generalized H-bonding
Hydrogen bonding of water molecules
H-bonding of hydroxyl groupsHOR
H
O R
H
O H
H
O H
H
O HH
O H
Keto groups
OR H O RH-bonds will form with various combinationsof hydroxyl, amino and keto groups thatmeet the H-bonding criteria.
H-bonding of amino groups
HN
R
N
R
R
R
R —
Hydrogen bonding between amino acids in a protein
| H-C-R1 | C=O | N-H | H-C-R2 | C=O | N-H | H-C-R3 |
| R4-C-H | H –N | O=C | R5-C-H | H –N | O=C | Rc- C-H |
Secondary Structure in ProteinSecondary Structure in Protein
DNA Double HelixDNA Double Helix
Hydrogen bonding between bases in DNA
Three H-bonds between G and C
N
NNH
NN-H
H
H
Oguanine cytosine
NN
H-N-H
O H
IONIC BONDSIONIC BONDS
Interaction between opposite chargesInteraction between opposite charges Many biologically significant molecules Many biologically significant molecules
are charged at physiological pH.are charged at physiological pH. Proteins EX:Proteins EX: Glutamic acid : negativeGlutamic acid : negative Lysine: positive Lysine: positive
PROTEINSPROTEINS
GlutamateGlutamate
LysineLysine
Van der WaalsVan der Waals
Result of a temporary dipole moment Result of a temporary dipole moment in atoms close to one another.in atoms close to one another.
Non-specificNon-specific Very weakVery weak Requires close proximityRequires close proximity
Hydrophobic InteractionsHydrophobic Interactions
Why do non-polar substances not Why do non-polar substances not dissolve in water? dissolve in water? EX.EX.
Are not bonds. What are they?Are not bonds. What are they? Valuable inValuable in
Protein 3Protein 3ºº structure structure Base stacking in DNABase stacking in DNA
Selective Advantage of Weak Selective Advantage of Weak Interactions in Biological Interactions in Biological
SystemsSystems Allows specificity—small differences Allows specificity—small differences
are easily distinguished (Ex. Allows are easily distinguished (Ex. Allows recognition).recognition).
Dynamic—Interior of cell remains Dynamic—Interior of cell remains fluid, allows responsiveness.fluid, allows responsiveness.
Summary of Chemical InteractionsSummary of Chemical Interactions
Bond::
Energy Energy needed needed
to break:to break:
CommentsComments::
Strength Strength class-class-
ification:ification:
Covalent
~100kcal/mole
Electrons shared
strong
Hydrogen
~3
Water-water;Organic-water;Organic-organic
weak
Ionic
~5
Full charge transfer;Can attract H-bond;Strong in crystal
weak
Van der Waals
~1
Fluctuating induced dipole;Close range only
weak
Hydro-phobic forces~3
Not a bond per se;Entropy driven;Only works in water
weak
Water cages around methane: CH4
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