Читать книгу Biochemistry For Dummies - John Moore T., Richard Langley H., John T. Moore - Страница 24

Acids increase the hydrogen ion concentration of a solution (they lower the pH, in other words). Some acids, known as strong acids, are very efficient at changing hydrogen ion concentration; they essentially completely ionize in water. Most acids — particularly biologically important acids — aren’t very efficient at generating hydrogen ions; they only partially ionize in water. These acids are known as weak acids.

Bases accept (react with), rather than donate, hydrogen ions in solutions. Bases decrease the hydrogen ion concentration in solutions because they react with these ions. Strong bases, although they can accept hydrogen ions very well, aren’t too important in biological systems. The majority of biologically important bases are weak bases.

The Brønsted-Lowry theory helps to explain the behavior of acids and bases with respect to equilibrium. A Brønsted-Lowry acid is a hydrogen ion donor, and a Brønsted-Lowry base is a hydrogen ion acceptor. Acetic acid, a weak acid found in vinegar, partially ionizes in solution, evidenced by the following equation:

The double arrow indicates that the acetic acid doesn’t completely ionize. (For a strong acid, complete ionization would occur, indicated by a single arrow.) The equilibrium arrow indicates that all three chemical species are present in the solution: the acetic acid, the acetate ion, and the hydrogen ion, along with the water solvent.

In the Brønsted-Lowry theory, you consider the acetate ion to be a base because it can accept a hydrogen ion to become acetic acid. According to this theory, two substances differing by only one hydrogen ion — such as acetic acid and the acetate ion — are members of a conjugate acid-base pair. The species with one additional hydrogen ion is the conjugate acid (CA), and the species with one less hydrogen ion is the conjugate base (CB).

You can express the equilibrium from the acetate example, like all equilibria, by using a mass-action expression — as long as a balance among the species is present. This expression is also known as a reaction quotient or an equilibrium constant. For acetic acid, this expression is as follows:

The a subscript means that this expression represents an acid. The square brackets refer to the molar equilibrium concentrations of the species present. You can express the K_a as a pK_a. The calculation of pK_a is similar to the calculation of pH:

In terms of conjugate acids and bases, every K_a expression appears as

No variations are allowed in this equation other than the actual formulas of the conjugate acid and base.

Like an acid, a base has a K_b value (the subscript b meaning base). A weak base, like ammonia, is part of the following equilibrium:

The equilibrium constant expression for this equilibrium is

The generic form of a K_b expression is

As with a K_a expression, a K_b expression has no variations other than the actual formulas of the conjugate acid and base.

Every acid has a K_a, and its corresponding conjugate base has a K_b. The K_a and the K_b of a conjugate acid-base pair are related by the K_w — the ionization constant for water. For a conjugate acid-base pair, . In addition, you can use the following shortcut: . (Are you enjoying this little bit of math? Biochemistry has a little more math than organic chemistry, but not nearly as much as general chemistry, so hang in there!)

The K_b for the acetate ion, the conjugate base of acetic acid, would be associated with the following equilibrium expression:

The K_a for the ammonium ion, the conjugate acid of ammonia, would be associated with the following equilibrium expression:

An acid may be capable of donating more than one hydrogen ion. A biologically important example of this type of acid is phosphoric acid (H₃PO₄), which is a triprotic acid (meaning it can donate three hydrogen ions, one at a time). The equilibria for this acid are

The subscripts are modified to indicate the loss of hydrogen 1, hydrogen 2, or hydrogen 3. The associated K_a expressions are all of the form

Here’s the breakdown for each K_a:

 

 

 

The value for each successive equilibrium constant often is significantly lower than the preceding value. Table 2-2 illustrates some biologically important acids. You can refer to this table when working buffer problems or determining which acid is stronger.

TABLE 2-2 The K_a Values for Biologically Important Acids

Acid	Ka1	Ka2	Ka3
Acetic acid (CH3COOH)
Pyruvic acid (CH3COCOOH)
Lactic acid (CH3CHOHCOOH)
Succinic acid (HOOCCH2CH2COOH)
Carbonic acid (H2CO3)
Phosphoric acid (H3PO4)