basic biochemistry 301
TRANSCRIPT
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