Chapter 2- Water and its Role

Water is a good solvent for:

Charged and polar subsatnces

• most carbohydrates

• proteins and some amino acids

• Some alcohols

Water is a bad solvent for:

Nonpolar Substances

• Lipids (Aliphatic chains)

• Non polar amino acids

• Non polar gases

How can water be an acid and a base

Base: Take a Hydrogen off molecules and forms H30+ (or OH- to H20)

Acid: Doantes a Hydrogen to the other molecule forming OH-

Ionization of Water

Keq= 1.8 × 10^-16

Kw= 1 × 10^-14

in pure water the concentration of H+ is eqaul to the concentration of OH- (10^-7)

What is pH?

the pH and pOH must always add to 14

in nuetral solutions [H]=[OH] and pH is 7

pH can be negative

pH = -log[H]

Dissociation of Weak Electrolytes

Weak electrolytes can only partially dissociate into water

the extent of dissociation is detemined by the acid dissociation constant Ka

Ka = ([H][OH]) / [HA]

• Can calculate the pH by using this equation to find [H] and plugging that into pH = -log[H]

pKa

pKa = -log Ka

small pKa means more acidic or low pH

Big Ka means more acidic or low pH

Weak Acids and Bases

Can act as buffers- 1 pH above and below the pKa

when pH = pKa: shows the point where [H+] = [A-]

Henderson-Hasselbalch Equation

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

Noncovalent Interations

No sharing of Electrons

Examples:

• Ionic interactions- Opposite charges attract (example Salt Bridge in protein folding)

• Dipole interactions- Uncharged but opposite polar dipoles attracting

• Hydrogen Bonds

• Van Der Waals Interactions

• Hydrophobic Effect (Induced Dipole)- When two molecules get close and it causes electrons to move creating attracting opposite partial charges

Hydrogen Bonds

Between a hydrogen attached to a O, N, or F ( We wont see much F) and a lone pair of electrons

Hydrophobic Effect

AKA induced Dipole

association or interaction of nonpolar molecules or components of molecules in the aqueous solution

Main Factors:

• Protien folding

• protein-protein interactions

• formation of lipid micelles

• binding of steroid hormones to their receptor

Does not arise becaise of some attractive direct force between two nonpolar molecules

High vs Low Entropy

Entropy is the amount of “Chaos”

High entropy means the particles are moving around a lot

Low entropy means the particles arent moving much

Low entropy is thermodynamically unfavorable thus hydrophobic solutes have low solubility

Micelles

All hydrophobic groups are protected from the water and a ordered shell of H2) molecules is minimized and entropy is increased

Form a circle

Water doesnt surround the hydrophobic groups causing them to be able to move more

Dispersion of lipids in H20

Each lipid molecule forces surrounding the H2O molecules to become highly organized and reduce the amount the water can move

Clusters of lipid Molecules (Rows)

Lipids only touch the hydrophobic ends at the edge of the cluster so some water molecues arent able to move freely