Biochemistry 301

Dr. Shabb

Rm. 2744 ECJRF

Wednesday, January 9, 2002

Water and Buffers

Friday, January 11, 2002

Amino Acids

BMB 301 online

Biochemistry & Molecular Biology website

http://www.med.und.nodak.edu/bimd/biochem.html

click on “Biochemistry 301”

Life and water

Life probably began in water

If life exists on another planet, it also probably requires water

It is a ubiquitous, innocuous solvent

Water and cells

Most living cells are 60-90% water

The macromolecular components of cells assume shapes in response to water

www.smithsonianmag.si.edu

Properties of water

Polarity

Hydrogen bonding potential

Specific heat, heat of vaporization

Nucleophilic

Ionization

Water is a polar molecule

•Water has a dipole

moment

•“like dissolves like”

Fig. 2.1

Water has hydrogen

bonding potential

H-bonds are non-covalent, weak interactions

H2O is both a Hydrogen donor and acceptor

One H2O can form up to four H-bonds

Figs. 2.3, 2.4

The molecular structure

of water can

assume many forms

including…

“Ice-9”“Cat’s Cradle”

By

Kurt Vonnegut

Water and H-bonds

High specific heat

Lots of heat is needed to break H-bonds and raise H2O temperature. Therefore, H2O is a good insulator.

High heat of vaporization

Lots of heat is needed to vaporize H2O. Therefore, sweat cools.

Hydrophilicity

NaCl is an electrolyte

Anions and cations are solvated differently in H2O

Glucose is nonionic and hydrophilic

Figs. 2.6, 2.7

hydrophobicity

Hydrophobic compounds and H2O don’t mix

Amphipathic molecules are part hydrophobic and part hydrophilic

Figs 2.8, 2.9

Noncovalent

interactions in

biomolecules

Ionic

H-bond

van der Waals

Hydrophobic

Ionic>H-bond, hydrophobic>van der Waals

Fig. 2.13

Water is nucleophilic

Water participates in many chemical reactions

it is electron rich

it is a weak nucleophile

it is present in high concentration

Water weakly ionizes

Eq. 2.2

pH is a measure of [H+]

in aqueous solutions

Basic pH > 7

neutral pH = 7

acidic pH < 7

Table 2.3, Fig. 2.16

Weak acids and the acid

dissociation constant (Ka)

pKa = -log Ka

pKa is a measure of acid strength

The stronger the acid, the lower the pKa

Eq. 2.11

pKa

can be related to pH

Henderson Hasselbach equation

Eq. 2.17

pKa

and buffering capacity

One ionizable group can be titrated with one equivalent of strong base

the greatest buffering capacity of an ionizable group is at its pKa

Fig. 2.17

pKa

and buffering capacity

Each ionizable group of a cpd has a unique pKa

buffered solutions resist changes in pH Fig. 2.19