Principles of Biochemistry, Volume 1 - A. Lehninger 1985

Biomolecules
Water
The properties of acids and bases are closely related to the properties of water.

The molecules of hydrochloric, sulfuric, and nitric acids, commonly referred to as strong acids, are completely ionized in dilute aqueous solutions. Similarly, molecules of strong bases such as NaOH and KOH are also fully ionized.

In biology, we more frequently encounter weak acids and weak bases, which do not ionize completely when dissolved in Water. An example of a weak acid is acetic acid (СН₃СООН), which gives vinegar its sour taste; an example of a weak base is ammonia (NH₃), an aqueous solution of which is used as a household cleaner. Weak acids and bases are common components of biological systems, playing a vital role in METABOLISM and its regulation. The behavior of aqueous solutions of weak acids and bases is much easier to understand if we first establish precise Structure/97.html">Definitions for a few terms.

Acids can be defined as proton Donors, and bases as proton acceptors. A proton donor and its corresponding proton acceptor form a conjugate acid-base pair (Table 4–3). An example of a conjugate acid-base pair is acetic acid (СН₃СООН), which acts as a proton donor, and the acetate anion (СН₃СОО^-), which Functions as a proton acceptor; they are interconverted by the following reversible reaction:

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Table 4-3. Some conjugate acid-Base Pairs. Each pair consists of a proton donor and a proton acceptor

A characteristic feature of any acid is its tendency in aqueous solution to dissociate a proton. The stronger the acid, the greater this tendency. The ability of any acid HA to release a proton and form its conjugate base A^- is characterized by the Equilibrium Constant of the reversible reaction

equal to

The equilibrium constants for such reactions are more commonly referred to as ionization constants or dissociation constants. Numerical values of the dissociation constants for several acids, frequently designated by the symbol K'_a (where the subscript "a" stands for acid), are listed in Table 4–4. Note that these acids differ widely in their proton-donating ability. Stronger acids, such as formic or lactic acid, have higher dissociation constants, whereas weaker acids, such as the Н₂РO^-₄ ion, exhibit lower values. One of the weakest acids listed in Table 4–4 is the NH⁺₄ ion, which has a very weak tendency to release a proton, as reflected by its exceptionally low dissociation constant. Conversely, the conjugate base of this acid, ammonia (NH₃), readily accepts a proton.

Table 4-4. Dissociation constants and pK' values of some common acids at 25°C

Table 4-4 lists the pK' values determined by the equation

The symbol p, just as in the case of pH, denotes the negative logarithm. The more readily an acid dissociates, the lower its pK' value. As we will soon see, determining the pK' of any weak acid is a straightforward task.