WATER: THE MIRACULOUS SOLVENT

Water is the most abundant compound in living systems.

It provides the solvent phase for all intracellular reactions.

Solvent interactions with water determine the three-dimensional shapes of
     biological macromolecules.

The ionization of water influences cellular activity.

PHYSICAL PROPERTIES:
 

Water has a higher melting point, boiling point, and heat of vaporization than other compounds of similar molecular weight.

            Water           Ammonia           Methane

  M.W.         18                    17                     16

  M.P.            0                 -77.7                 -182

  B.P.         100                 -33.4                 -162

  Heat
    of           40.7                 23.2                   8.3
  Vap.

WHY?

Due to attractive forces between the water molecules, known as hydrogen bonds:
These bonds arise due to the polar nature of the water molecule.

There is separation of charge across the molecule, and these charges allow water molecules to interact with each other.  
Hydrogen bonds can form between other molecules and water, and between several molecules themselves.
The attractive forces give water a liquid crystalline structure, which stabilize into a rigid crystalline lattice when water freezes. In ice the water molecules are farther away from each other than in the liquid, and thus ice has a lower density than liquid water, and IT FLOATS!

SOLVENT PROPERTIES:

Due to its polar nature, water is an excellent solvent:
 

1) Crystalline salts completely dissociate.

2) Neutral organics with polar functional groups.

3) Amphiphilic compounds form micelles,
                monolayers, and bilayers.

OSMOTIC PRESSURE:
 
The addition of solute to water creates osmotic pressure, which is proportional to the number of solute particles.
 

Water will move from areas of low osmotic pressure to areas of high osmotic pressure (the opposite of diffusion!).

You need to know what the terms hypertonic, hypotonic, and isotonic mean.


LAW OF MASS ACTION

            A + B       <====>      C + D

Given the above system at equilibrium:
 
Addition of A or B or removal of  C or D will

cause the equilibrium to shift to the Right.

Removal of A or B or addition of C or D will

cause the equilibrium to shift to the Left.


EQUILIBRIUM CONSTANT

    Changes in concentration for both spontaneous
                and nonspontaneous reactions:

        Spontaneous:
 
 


        Nonspontaneous:



                Equilibrium Constant:

                            Keq. =    [C] * [D]/[A] * [B]

                            For a spontaneous reaction, Keq. > 1.

                           For a nonspontaneous reaction, Keq. < 1.

        N.B. [ ] means molar concentration.
 

IONIZATION OF WATER

    In solution, water will ionize:

H2O   <====>        H+           +         OH- The equilibrium expression for this reaction is:
     
                Keq.[H+] * [OH-]/[H2O]                

      The concentration of pure water is 55 M,

        so multiplying both sides by 55 yields:

                Kw = 1 x 10-14 = [H+] * [OH-]
 
 
 
 
 

This is the ion product of water, and it
 is the basis for the pH scale:

            pH = -log [H+]
 

There are two important things to remember
 about the pH scale:
 

1) Low pH means high [H+].

2) A small change in pH is a  big change in [H+]

ACIDS AND BASES
 
According to the Brönsted-Lowry theory:
 
An acid is a proton donor, e.g. HA.

A base is a proton acceptor, e.g. A-.


     Chemical Reaction:

                HA  <====>  H+        +          A-

              "acid"                             "conjugate base"

        The equilibrium expression for this is:

                Keq. =  Ka = [H+] * [A-]/[HA]            
 

If we take the logarithm of both sides, substitute,
and rearrange, we get the Henderson-Hasselbalch
equation:

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


What is the pKa?

 

Acids have low pKas (less than 7).

Bases have high pKas. (more than 7).

The further from 7, the stronger.


At a pH below its pKa, a molecule will
                be protonated.

At a pH above its pKa, a molecule will
            be unprotonated.
 

N.B. The pKa is a physical constant
         for a given molecule. It cannot
                         change.

        The pH is a property of the solution,
                and it can change.

TAKE HOME MESSAGE I
 
At pH below its pKa, an acid will be
protonated and have no charge.

At pH above its pKa, an acid will be
unprotonated, and have a negative charge.

At pH below its pKa, a base will be
protonated and have a positive charge.

At pH above its pKa, a base will be
unprotonated, and have no charge.


BUFFERS

    Definition: A solution that resists a change in pH.

    Recipe: A solution of a weak acid and its
                            conjugate base.

Titration curve:
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

TAKE HOME MESSAGE II
 

1) Buffers work best at pHs near their pKa.

2) The pKa is the pH where the molecule
        shifts from giving up to accepting a
                proton, or vice versa.


PHYSIOLOGICALLY IMPORTANT BUFFERS

1) Bicarbonate Buffer

2) Phosphate Buffer