MACROMOLECULES I
THE IMPORTANCE OF WEAK
INTERACTIONS
BONDS OF INTEREST


Covalent bonds—NOT a weak interaction.
G= -50 to -110 kcal/mol
Weak interactions in biological systems

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Hydrogen bonds
Ionic bonds
Van der waals bonds
Hydrophobic interactions
HYDROGEN BONDS
Power in Numbers
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.
Generalized H-bonding
H-bonding between water molecules
Hydrogen bonding of water molecules
H
H
O
O
H
O
H
H
H
H-bonding of hydroxyl groups
R O H O R
H
H-bonding of amino groups
R
R
R— N
N
R
R
Keto groups
R
H
O
H O
R
H-bonds will form with various combinations
of hydroxyl, amino and keto groups that
meet the H-bonding criteria.
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 Protein
DNA Double Helix
Hydrogen bonding between bases in DNA
guanine
cytosine
Three H-bonds between G and C
IONIC BONDS
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Interaction between opposite charges
Many biologically significant molecules are
charged at physiological pH.
Proteins EX:
Glutamic acid : negative
Lysine: positive
PROTEINS

Glutamate

Lysine
Van der Waals


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Result of a temporary dipole moment in
atoms close to one another.
Non-specific
Very weak
Requires close proximity
Hydrophobic Interactions
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Why do non-polar substances not dissolve
in water? EX.
Are not bonds. What are they?
Valuable in

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Protein 3º structure
Base stacking in DNA
Selective Advantage of Weak
Interactions in Biological Systems


Allows specificity—small differences are
easily distinguished (Ex. Allows
recognition).
Dynamic—Interior of cell remains fluid,
allows responsiveness.
Summary of Chemical Interactions
Bond: Covalent
Energy
needed to ~100
break: kcal/mole
Comments: Electrons
shared
Strength
class- strong
ification:
Hydrogen
Ionic
Van der
Waals
Hydrophobic
forces
~3
~3
~5
~1
Water-water;
Organic-water;
Organicorganic
Full charge
transfer;
Can attract
H-bond;
Strong in
crystal
Fluctuating
induced
dipole;
Close range
only
Not a bond
per se;
Entropy
driven;
Only works
in water
weak
weak
weak
weak
Water cages around methane: CH4
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