UNIT 1: Functional Groups and Their Properties in Biology

Functional Groups and Their Properties

Functional groups are specific groups of atoms within molecules that have characteristic properties and chemical reactivity. In biological molecules, these groups determine the structure, function, and interactions of macromolecules such as proteins, nucleic acids, carbohydrates, and lipids.

Major Functional Groups in Biological Molecules

Functional Group	Structure	Properties
Hydroxyl	R–OH	Polar
Methyl	R–CH3	Nonpolar
Carbonyl	R–C=O       |       R'	Polar
Carboxyl	R–COOH	Charged, acidic
Amino	R–NH2	Charged, basic
Phosphate	R–PO42−	Charged, acidic
Sulfhydryl	R–SH	Polar

Polarity and Charge of Functional Groups

The chemical properties of functional groups are largely determined by their polarity and charge. These properties influence how molecules interact in aqueous environments and participate in chemical reactions.

• Polar: Functional groups with atoms of differing electronegativity (such as oxygen or nitrogen) create regions of partial positive and negative charge due to uneven electron sharing. This allows them to form hydrogen bonds and interact with water (hydrophilic).

• Nonpolar: Functional groups with atoms of similar or identical electronegativity (such as carbon and hydrogen) share electrons equally, resulting in no partial charges. These groups are hydrophobic and do not interact well with water.

• Charged (Acidic): Groups like carboxyl and phosphate can donate protons (H+) to solution, becoming negatively charged. They act as acids in biological systems.

• Charged (Basic): Amino groups can accept protons from solution, becoming positively charged. They act as bases in biological systems.

Key Functional Groups: Definitions and Examples

• Hydroxyl Group (–OH): Found in alcohols and carbohydrates. Increases solubility in water due to hydrogen bonding. Example: Ethanol (C2H5OH)

• Methyl Group (–CH3): Nonpolar, found in fatty acids and some amino acids. Reduces solubility in water. Example: Methylated DNA

• Carbonyl Group (C=O): Found in aldehydes and ketones. Polar, increases reactivity. Example: Acetone (a ketone)

• Carboxyl Group (–COOH): Acts as an acid, can donate H+. Found in amino acids and fatty acids. Example: Acetic acid (CH3COOH)

• Amino Group (–NH2): Acts as a base, can accept H+. Found in amino acids. Example: Glycine (NH2CH2COOH)

• Phosphate Group (–PO42−): Contributes negative charge, found in nucleic acids (DNA, RNA) and ATP. Example: Adenosine triphosphate (ATP)

• Sulfhydryl Group (–SH): Polar, can form disulfide bonds important for protein structure. Example: Cysteine (an amino acid)

Acidic and Basic Functional Groups

• Acidic Groups: Carboxyl and phosphate groups can release H+ ions, lowering pH and becoming negatively charged.

• Basic Groups: Amino groups can accept H+ ions, raising pH and becoming positively charged.

Summary Table: Properties of Functional Groups

Group	Polarity	Charge at pH 7	Example Molecule
Hydroxyl	Polar	Uncharged	Glucose
Methyl	Nonpolar	Uncharged	Fatty acids
Carboxyl	Polar	Negative	Amino acids
Amino	Polar	Positive	Amino acids
Phosphate	Polar	Negative	ATP
Sulfhydryl	Polar	Uncharged	Cysteine

Key Equations

• Dissociation of Carboxyl Group: $\mathrm{R-COOH} \rightleftharpoons \mathrm{R-COO}^- + \mathrm{H}^+$

• Protonation of Amino Group: $\mathrm{R-NH_2} + \mathrm{H}^+ \rightleftharpoons \mathrm{R-NH_3}^+$

Applications in Biology

• Functional groups determine the solubility, reactivity, and interactions of biomolecules.

• They play critical roles in enzyme catalysis, cell signaling, and the structure of macromolecules.

• Understanding these groups is essential for studying metabolism, genetics, and molecular biology.