# Biochem.physio

Introduction

The earlier definition said that an acid or a base is a substance that can donate proton or hydroxyl group (NH3 is base but does not have a hydroxyl group) The definition that is used is that given by Bronsted and Lowry: An acid is a substance that can donate protons or H+ into solution while a base is a substance that can accept protons Many biological macromolecules bear numerous functional groups that determine whether they are acids or bases as well as how they will behave in certain media Under this definition in every acid base reaction, an acid HA reacts with a base H2O to form a conjugate base of an acid and a conjugate acid of the base HA + H2O -- H3O+ + A- this can also be written as HA  H+ + A- CH3COOH + H2O - CH3COO- + H3O+ NH3 + H2O  NH4+ + OH- Another definition says that an acid is a substance that can donate an electron pair while a base is a substance that can accept an electron pair. This definition is too broad especially for our biological systems and applies to non-aqueous situations.

Acids and Bases Acids and bases can be classified either as strong or weak The number of protons released determines the strength of the acid Strong acids release all their protons while weak acids don’t release all their protons ie do not ionize completely Strength of an acid is specified by its dissociation constant K K is a measure of the relative proton affinities of the HA/A- and H3O/H2O conjugate acid base pairs

K = [H3O+][A-]/[HA][H2O]


Since in aqueous solution the concentration of water is essentially constant the H2O is combined with the dissociation constant giving K = [H+][A-]/[HA] Accordingly acids with K less than 1 (K < 1) i.e. K less than that of H3O+ are weak acids Strong acids have dissociation constants larger than that of H3O+, almost completely ionize

pH Ionization of Water Proton hopping in water Sometimes the hydrogen of one water molecule will "jump" to another water molecule: H2O + H2O H3O+ + OH- This proton hopping is called the ionization of water (an ion is a positively or negatively charged atom or molecule). This ionization creates a H3O+ and a OH- molecule. The H3O+ is often written as simply H+. This is because a H3O+ is just a H+ that jumps from one water molecule to another: H2O H+ + OH- So remember, H3O+ = H

pH How much H+ and OH- exist in water? Very, very little! The ratio of either H+ or OH- to H2O in neutral water is 1:1,000,000,000! Since this is such a small amount of either H+ or OH-, they rarely meet and neutralize each other. The equilibrium constant, Keq describes the ionization equilibrium of water: Keq = [H+][OH-] Because of this relationship it is important to note that if the [H+] goes up then the [OH-] must go down, and vice-versa, for the value for the Keq of water must remain constant. For neutral water, the Keq is 1 x 10-14 M and the concentrations of [H+] and [OH-] are each 1 x 10-7 M . Let's look at that last number without the exponent: 0.0000001 M This is obviously a very small number. A more manageable way to discuss small numbers such as this is to take the negative logarithm. For the concentration of [H+], this is called the pH. In this case: -log(0.0000001 M) = 7

pH The pH of a solution is simply the negative logarithm of [H+]. The pH of a solution describes the acidity of a solution. Acidic solutions are those with a pH of less than 7 and basic solutions have a pH greater than 7. A solution, like H2O, with a pH = 7 is neutral. Similarly, the pOH could be used to describe a solution in terms of its OH- concentration. pOH is the negative logarithm of the OH- concentration. One useful thing to remember is: pH + pOH = 14. In the body, the pH of blood is 7.4. This corresponds to a [H+] of about 40 nM. This value can only vary from 37 nM to 43 nM without serious metabolic consequences.

pKa In living systems, much of the chemistry involves interactions between acids and bases. Acids are H+ donors and bases are H+ acceptors. HA H+ + A- Acid and base reactions are made up of conjugate acid-base pairs. Strong acids are those that readily give up a H+. Strong bases readily accept a H+. Weak acids do not readily give up a H+, but will under the right conditions. A weak base is one that does not easily take up a H+. The conjugate base for a strong acid is a weak base. In contrast, a strong base has a weak acid as its conjugate. However, what about strong base-weak acid conjugate pairs? A- + H+ HA

pH and pKa Think of it in similar terms: a person in dire need of a wig will grab just about anything. This is our strong base. Now on the right side of the equation we have someone with a wig who really doesn't want to give it up, just as a weak acid does not want to part with its H+.

• We can think of a strong acid or base as having a "flip side", which is its weak base or weak acid conjugate, respectively.

How readily an acid gives up its H+ is expressed by the acid dissociation constant, or Ka: Ka = [H+][A-] /[HA] The pKa is the negative logarithm of the Ka. Strong acids have small pKas. Looking at the above equation, it can be seen that when [A-] = [HA], then Ka = [H+]. Then the pKa = pH. This is the basis for the Henderson-Hasselbalch equation: pH =pKa + log[H+][A-] /[HA]

 The Henderson-Hasselbalch�      Equation