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
Cis-trans isomers
The presence of carbon-carbon double bonds leads to the possibility of having isomers present. Double bonds are rather restrictive and limit molecular movement. Groups on the same side of the double bond tend to remain in that position (cis), whereas groups on opposite sides tend to remain across the bond from each other (trans). See Figure 3-7 for an illustration of cis and trans isomers.
FIGURE 3-7: Cis and trans isomers.
If the two groups attached to either of the carbon atoms of the double bond are the same, cis-trans isomers aren’t possible. Cis isomers are the normal form of fatty acids, whereas food processing tends to convert some of the cis isomers to the trans isomers. The trans isomers are less biologically friendly (as in trans fats, which tend to be carcinogenic) than the cis isomers.
Cis-trans isomers are also possible in cyclic systems. The cis form has similar groups on the same side of the ring, whereas the trans form has similar groups above and below the ring.
Chiral carbons
Trying to put your gloves on the wrong hands is kind of like another property of biological systems: handedness. Left-handed molecules rotate plane-polarized light to the left, and right-handed molecules rotate plane-polarized light to the right.
Identifying chiral molecules
The presence of an asymmetric, or chiral, carbon atom is sufficient to produce a handed molecule.
FIGURE 3-8: The structure of glucose, a sugar with four chiral carbon atoms.
Determining the chiral form: Enantiomer or stereoisomer?
All substances have a mirror image (okay, except vampires); however, if a chiral carbon atom is present, the mirror images are nonsuperimposable. Hold out your left and right hands, palms up — they are nonsuperimposable mirror images. These two mirror images are called enantiomers. The different chiral forms differ from each other in two aspects:
How they affect light
How
