8.25 Water Molecular Interactions

Dipolar nature of water

Hydrogen atoms have localized positive charges and the oxygen has a partial negative charge

____ arrangement of electron pairs and hydrogen atoms around oxygen atom of water

Tetrahedral

Hydrogen bonds are ___ and ___ than covalent O-H bond

longer and weaker

____ hydrogen bonds possible per water molecule

4

Liquid water average ___ hydrogen bonds

3.4

Ice has ____ hydrogen bonds

4

Entropy effect is important for

clathrate-like structures

Lifetime of a hydrogen bond

1-20 ps (10^-2 s)

Based on lifetime of a hydrogen bond, liquid water can be described as a

“flickering structure”

Hydrogen bonding is ___ when 3 atoms involved lie in a line

stronger

This amount of energy differentiates a functional folded protein from a precipitated protein

10 kJ mol^-1

Strength of hydrophobic interactions (displacement of water)

0.4 - 4 kJ/mol

Strength of pi stacking (aromatic ring stacking)

0.4 - 4 kJ/mol

Strength of van der waals interactions (weak but many)

0.4 - 4 kJ/mol

Strength of hydrogen bond interaction

4 - 40 kJ/mol

Closer hydrogen bonding is ____

stronger

Strength of an electrostatic interaction

4 - 40 kJ/mol

Strength of salt bridge interaction (hydrogen bonding plus electrostatic)

40 - 400 kJ/mol

Amphipathic definition

both hydrophilic and hydrophobic

Long-chain fatty acids have very _________ chains, each of which is surrounded by a layer of highly ordered water molecules

hydrophobic alkyl

Purpose of clustering together in micelles

Reduce hydrophobic surface area exposed to water, fewer water molecules are required to form the shell of ordered water around a hydrophobic surface

Energy gained by freeing immobilized water molecules (entropy increase) stabilizes

the micelle

Enzyme-substrate interaction is stabilized by

hydrogen bonding, ionic, and hydrophobic interactions

H+ or H3O+ found in reality

H3O+

Keq equation

[H+][OH-] / [H2O]

Keq at 25 degrees Celsius

1.8E-16

Kw equation; Ion product at 25C, at neutral pH

K_eq [H2O] = [H+][OH-]; 1E-14M²; 1E-7

pH =

log(1/[H+]) OR -log[H+]

Acid dissociation equilibrium constant… K =

([H3O+][A-]) / [HA][H2O])

In dilute aqueous solution, water concentration is ___

essentially constant… 55.5 M

K_a = K[H2O] =

[H+][A-] / [HA]

pKa =

log(1/K_a) OR -logK_a

pH = pKa when

[HA] = [A-]

Hederson-Hasselbalch Equation

pH = pKa + log [A-]/[HA]

Buffers consist of…

an aqueous solution of a weak acid and its conjugate base

Buffers tend to resist changes in ____ when small amounts of acid or base are added

pH

Pepsin

digestive enzyme secreted into gastric juice, which has a optimal pH of ~1.5

Trypsin

digestive enzyme that acts in the small intestine, optimal pH is around 7

Alkaline phosphatase

hydrolytic enzyme in bone tissue that aids bone mineralization optimizes around 8.5

Lipid bilayers form _____________ in water

spontaneously

Weak _____ and _____ dissolves in water (Referred to as what?); It can also act as a _____ in the presence of a weak acid and its salt

acids and bases, ionization behavior, buffer

Enzymes perform optimally at near ________ _____________ ___, with few exceptions

neutral physiological pH, ~7.4

For macromolecules, the most stable structure usually _______ weak interactions; Some are bound so tightly that ____ molecules are considered part of the crystal structure

Maximizes; water

Proton _______ is due to water being _______ ______, gives water _________ _______

hopping, partially ionized, electrical conductivity