MACROMOLECULES I THE IMPORTANCE OF WEAK INTERACTIONS

BONDS OF INTEREST Covalent bonds—NOT a weak interaction.  G= -50 to -110 kcal/mol Covalent bonds—NOT a weak interaction.  G= -50 to -110 kcal/mol Weak interactions in biological systems Weak interactions in biological systems Hydrogen bonds Hydrogen bonds Ionic bonds Ionic bonds Van der waals bonds Van der waals bonds Hydrophobic interactions Hydrophobic 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 BONDS Power in Numbers H-bonding between water moleculesGeneralized H-bonding

Hydrogen bonding of water molecules H-bonding of hydroxyl groups HOR H OR H O H H O H H O H H O H

Keto groups ORH OR H-bonds will form with various combinations of hydroxyl, amino and keto groups that meet 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-R 1 | C=O | N-H | H-C-R 2 | C=O | N-H | H-C-R 3 | R 4 -C-H | H –N | O=C | R 5 -C-H | H –N | O=C | R c - C-H |

Secondary Structure in Protein

DNA Double Helix

Hydrogen bonding between bases in DNA Three H-bonds between G and C N N NH N N-H H H O guanine cytosine N N H-N-H O H

IONIC BONDS Interaction between opposite charges Interaction between opposite charges Many biologically significant molecules are charged at physiological pH. Many biologically significant molecules are charged at physiological pH. Proteins EX: Proteins EX: Glutamic acid : negative Glutamic acid : negative Lysine: positive Lysine: positive

PROTEINS Glutamate Glutamate Lysine Lysine

Van der Waals Result of a temporary dipole moment in atoms close to one another. Result of a temporary dipole moment in atoms close to one another. Non-specific Non-specific Very weak Very weak Requires close proximity Requires close proximity

Hydrophobic Interactions Why do non-polar substances not dissolve in water? EX. Why do non-polar substances not dissolve in water? EX.EX. Are not bonds. What are they? Are not bonds. What are they? Valuable in Valuable in Protein 3 º structure Protein 3 º structure Base stacking in DNA Base stacking in DNA

Selective Advantage of Weak Interactions in Biological Systems Allows specificity—small differences are easily distinguished (Ex. Allows recognition). Allows specificity—small differences are easily distinguished (Ex. Allows recognition). Dynamic—Interior of cell remains fluid, allows responsiveness. Dynamic—Interior of cell remains fluid, allows responsiveness.

Summary of Chemical Interactions : Bond: Energy needed to break: Energy needed to break: Comments : Comments : Strength class- ification: Covalent ~100 kcal/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: CH 4 Back