Biochemistry For Dummies. John T. Moore

test use copper compounds, and a reducing sugar produces a red precipitate with either test. These simple organic qualitative tests find some use in the biochemical tests we describe in Chapters 14 and 15, later in this book.

The carbonyl group of an aldehyde or ketone may interact with an alcohol to form acetals and hemiacetals. Modern terminology only uses the terms acetals and hemiacetals, but you may sometimes see the older terms hemiketal, which is a type of hemiacetal, and ketal, a type of acetal. See Figure 3-6 for an illustration of these compounds.

      FIGURE 3-6: Acetals, hemiacetals, hemiketals, and ketals.

The difference between a hemiacetal and a hemiketal is that in a hemiacetal R” = H, but in a hemiketal R” ≠ H. Similarly, the difference between an acetal and a ketal is that in an acetal R” = H, but in a ketal R” ≠ H.

      Carboxylic acids

      Carboxylic acids, along with phosphoric acid, are the most important biological acids. Carboxylic acids react with bases such as the amines to produce salts. The salts contain an ammonium ion from the amine and a carboxylate ion from the acid.

      Carboxylic acids combine with alcohols to form esters and can indirectly combine with amines to form amides. Hydrolysis of an ester or an amide breaks the bond and inserts water. An acid, base, or enzyme is needed to catalyze hydrolysis. Under acidic conditions, you can isolate the acid and either the alcohol or the ammonium ion from the amine. Under basic conditions, you can isolate the carboxylate ion and either the alcohol or the amine.

      Thiols and amines

      Under mild oxidation, two thiols join to form a disulfide. Mild reducing conditions, catalyzed by enzymes or through the use of certain reducing agents, reverse this process. Such formation of disulfide linkages is important in the chemistry of many proteins, such as insulin.

      

Amines are the most important biological bases. As bases, they can react with acids. The behavior is related to the behavior of ammonia.

       

       

       

       

      

Many medications have amine groups. Converting many of these amines to ammonium ions makes them more readily soluble in biological systems (mostly water). For example, the reaction of a medication with hydrochloric acid forms the chloride, which often appears on the label as the hydrochloride.

      It’s possible to replace all the hydrogen atoms from an ammonium ion, , to produce a quaternary ammonium ion, .

      Phosphoric acid

Phosphoric acid, H₃PO₄, may behave like a carboxylic acid and form esters. The esters have an organic group, R, replacing one, two, or three of the hydrogen atoms. The resultant compounds are monoesters, diesters, and triesters. The hydrogen atoms remaining in the monoesters and diesters are acidic.

      pH and functional groups

Many of the biological functions of substances are pH-dependent. For this reason, knowing which functional groups are acidic, basic, or neutral is important. Neutral functional groups behave the same no matter what the pH is. Table 3-2 lists the functional groups; whether they’re acidic, neutral, or basic; and, for acidic and basic groups, their weakness level (medium, weak, or very weak). The weaker a substance is in terms of pH, the less likely it’s affected by its solution pH (except under extreme pH conditions, which normally don’t occur in biological systems).

TABLE 3-2 Acid-Base Properties of Biologically Important Functional Groups

Functional Group	Acid, Base, or Neutral	Weakness Level
Monophosphate esters and diphosphate esters	Acid	Medium
Carboxylic acids	Acid	Weak
Phenols	Acid	Very weak
Thiols	Acid	Very weak
Amine salts	Acid	Very weak
Amines	Base	Weak
Carboxylate ions	Base	Very weak
Alcohols	Neutral
Carboxylate esters	Neutral
Ethers	Neutral
Triphosphate esters	Neutral
Thioethers	Neutral
Disulfides	Neutral
Amides	Neutral
Ketones	Neutral
Aldehydes	Neutral

Same Content, Different Structure: Isomerism

      

Isomers are compounds that have the same molecular formula but different structural formulas. (It’s all in how things are put together.) Some organic and biochemical compounds may exist in different isomeric forms. Many times, especially in biological systems, these different isomers have different properties. The two most common types of isomers in biological systems are cis-trans isomers and isomerism due