BioChemL2

Lecture 2

How does water have Hydrogen Bonding?

Partial positive charge on H and partial negative charge on O causes electrostatic attraction between 2 water molecules

Hydrogen donor + hydrogen acceptor = Hydrogen bonding

Solubility of polar and charged molecules in water

• Good solvent for polar and charged substances (Amino acids, peptides, carbohydrates)

• Poor solvent for nonpolar substances (nonpolar gases)

Noncovalent interaction - Ionic interactions

Electrostatic interaction between PERMANENTLY charged species or between ion and permanent dipole

Noncovalent interaction - Van der Waals interactions

• Weak individually and easily broken (reversible)

• Universal (Occurs between ANY 2 atoms near each other)

• Attractive (dispersion) and repulsive (steric) components

• Helps binding + stability

Noncovalent interaction - Hydrophobic Effect

Ordering of water molecules around nonpolar substances & clustering of hydrophobic molecules

NOT due to any attractive force between 2 nonpolar molecules

Osmosis

Water tends to move from low concentration to high concentration of solutes (Osmotic pressure)

Isotonic Osmosis

Solution has the SAME concentration as cell —> No water movement

Hypertonic Osmosis

Solution has HIGHER concentration of solute than cell —> Cell LOSES water and shrinks

Hypotonic Osmosis

Solution has LOWER concentration than cell —> Cell GAINS water and swells

Mannitol

Osmotic diuretic; Used when body fluid pressure is too HIGH; Attracts water + salt in kidneys to excrete through urine; SEVERE DEHYDRATION is a side effect

Explain pH

Always adds up to 14; 0 is most acidic; 14 is most basic; 7 is neutral; pOH has the opposite scale (0 most basic, 14 most acidic)

Define K_a

The extent of dissociation is determined by the acid dissociation constant: K_a

Define Henderson Hasselbalch Equation

Relationship between pH and K_a

pH = K_a + log [A^-]/[HA]

pH’s affect on biological systems

Enzymes exhibit pH optima; Cells and organisms maintain specific & constant pH ~7; pKa of drug determines absorption and distribution

Drug ionization & absorption and absorbing organs

Drugs have a pKa that determines how well the body can absorb them

pKa affects absorption, distribution, metabolism, and excretion (ADME)

Solubility depends on pH of target tissue or organ

Ionized drugs are more soluble and are excreted (More hydrophillic)

Non-ionized drugs are absorbed via cell membrane (More hydrophobic)

Dissociation of Aspirin (pH and organ)

Dissociate 50:50 at pH 3.5; Absorbed in STOMACH (Acid pH and ionization)

Dissociation of Amphetamine (pH and organ)

Dissociate 50:50 at pH 9.8; Absorbed in the INTESTINES (Alkaline pH and less ionization at higher pH)