macromolecules i
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Transcript macromolecules i
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
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
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
Result of a temporary dipole moment in
atoms close to one another.
Non-specific
Very weak
Requires close proximity
Hydrophobic Interactions
Why do non-polar substances not dissolve
in water? EX.
Are not bonds. What are they?
Valuable in
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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