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An amphipathic molecule, also called amphiphilic, is a chemical compound whose structure shows two regions of opposite polarities, one of which is polar and therefore hydrophilic while the other is nonpolar, making it hydrophobic or lipophilic. This is a very important class of chemical compounds that can interact simultaneously with an aqueous phase and an apolar organic phase, which facilitates the formation of stable mixtures between these phases, such as suspensions and colloids. On the other hand, they are also a type of compound that makes it possible to make the presence of apolar organic substances in aqueous media compatible, which is essential for the existence of life, as we know it.
Etymology of the term amphipathic
Etymologically, the term amphipathic is formed by the union of two ancient Greek words:
amphis + pathikos
Amphis means “both” or “on both sides” and pathikos , which in turn comes from the ancient Greek pathos , refers to “experience” or “feeling.” In this way, we can say that the term amphipathic refers to a chemical substance that experiences different interactions on opposite sides of its structure or that feels different attractions on both sides of the molecule.
On the other hand, a common synonym for amphipathic is amphiphilic, a term that is used in both biology and chemistry to refer to the same class of compounds. The term amphiphilic also comes from two Greek terms:
amphis + philia
Philia is an ancient Greek term that means love, so the term amphiphilic molecule refers to a molecule that is at the same time a lover of both water (hydrophilic molecule) and nonpolar compounds (lipophilic molecule). Lipophilic molecules are also called hydrophobic, since being attracted to a nonpolar substance necessarily implies repelling water.
Structure of amphipathic molecules
As mentioned above, an amphipathic molecule has two sides with different polar characteristics. This is because one end of the molecule is polar, while the other end is nonpolar.
The polar part usually makes up only a small portion of the molecule, while the nonpolar part usually consists of a long hydrocarbon chain, either fully saturated or with some unsaturation. Due to this difference in the sizes and in the number of atoms that make up each part of the molecule, the polar part is usually called the head, while the nonpolar part is called the tail.
This structural description allows us to define amphipathic or amphiphilic molecules as those chemical compounds that have a polar head and a nonpolar tail in their structure.
The polar head or hydrophilic end
The polar end of amphipathic molecules is characterized by having highly polar or even ionic functional groups. In some particularly important cases in biology, they may even possess zwitterionic domains, that is, parts of the molecule that carry opposite electrical charges, but whose net charge is zero.
Another important characteristic of the functional groups present in the polar head of amphipathic or amphiphilic molecules is that they have the ability to form one or more hydrogen bonds with water molecules. That is, they are groups that have either atoms with a net negative or positive charge, or groups with highly electronegative atoms that are polarized and have free pairs of electrons that they can share with the water molecule.
Although it is not strictly necessary, the functional groups of the polar heads are also usually protic, that is, they have the ability to act as donors of the hydrogen atom in the formation of the hydrogen bond with water.
Some examples of functional groups commonly found on the polar heads of many amphipathic molecules are:
| Functional group | Description |
| Hydroxyl groups (–OH) | The hydroxyl groups present in the functional groups of alcohols, phenols and others are protic polar groups that have the capacity to form up to three hydrogen bonds with water, two as an acceptor of the hydrogen atom and one as a donor. |
| Carboxyl group (–COOH) | They correspond to the functional group of carboxylic acids, the most common class of organic acids. They are highly polar protic groups that can form multiple hydrogen bonds with water. |
| Amino groups (–NH 2 , –NHR or –NR 2 ) | Primary, secondary, and tertiary amines all possess polar bonds and a trigonal pyramidal geometry that makes them polar. In all cases, nitrogen possesses a lone pair of electrons that it can share to form hydrogen bonds. The primaries and secondaries can also act as hydrogen donors with water. |
| Salts of carboxylic acid or carboxylate ions (–COO – ) | They are very common groups in soaps and other amphipathic molecules. Salts completely dissociate in solution, producing a net negatively charged group and many lone pairs (5 total) to form hydrogen bonds with water. |
| Ammonium salts (–NH 3 + , –NRH 2 + or –NR 2 H + ) | The protonation of amines by an acid produces positively charged ammonium ions that show ion-dipole interactions with water molecules, attracting the water oxygens, which have a partial negative charge. |
| Quaternary ammoniums (–NR 4 + ) | They are cationic functional groups in which nitrogen is directly bonded to four alkyl groups, giving nitrogen a formal positive charge. Like ammonium salts, these groups attach to oxygen in water through ion-dipole interactions. |
| Other acid groups and their conjugate bases | Many organic molecules can be functionalized by linking to them inorganic acid groups which, depending on the pH, may or may not be protonated or as their corresponding conjugate bases. These include phosphate (–OPO 3 2- ), sulfate (–OSO 3 – ), and sulfonate (–SO 3 – ) groups , to name a few. |
| esters | In addition to the functional groups mentioned above, there is a wide variety of esters formed by condensation between the hydroxyl group of an alcohol and an acid. This acid can be a short carboxylic acid, but in many cases they are strong oxacids such as sulfuric, nitric, and phosphoric acids. |
In addition to the functional groups mentioned in the table above, there are many other functional groups that are part of the polar heads of different amphipathic molecules. However, these are some of the most common. On the other hand, a polar head can have more than one functional group like those mentioned above, leading to a wide variety of different polar heads with different properties.
The apolar tail, lipophilic end or hydrophobic end
Linked to the polar head of an amphipathic molecule we will always find one or more nonpolar tails. They are called tails because they are always long chains of carbon atoms, containing in most cases more than 10 carbons, and in many cases more than 20.
Carbon-carbon bonds are completely nonpolar because they are bonds between like atoms. Furthermore, carbon-hydrogen bonds are also nonpolar because both elements have very similar electronegativities. This makes the alkyl, alkenyl, and alkynyl chains completely nonpolar. The same can be said of aryl groups (those with aromatic rings) and other cyclic hydrocarbons .
Why are the lines long?
The reason why the tails must be long for the molecule to be amphipathic is that if they are too short, even when nonpolar, the polarity of the head can overwhelm the hydrophobicity of the nonpolar chain, making the molecule hydrophilic as a whole. . This is the case, for example, with short-chain alcohols such as methanol, ethanol, and the isomers of propanol, which are all completely miscible with water and insoluble in oils, despite having alkyl groups in their structure.
On the other hand, the predominant interactions between nonpolar molecules are Van der Waals forces such as London dispersion forces. Compared to the polar and hydrogen bonding interactions of polar and ionic groups, these forces are very weak. However, they increase with the contact area and, therefore, with the length of the carbon chain.
Based on the above, for a molecule that has a polar head to exhibit observable hydrophobic behavior at the same time, and thus be considered a true amphipathic molecule, the polar tail must be long enough for the interactions of van der Waals between these chains, and between them and other nonpolar substances is strong enough to repel water.
Examples of amphipathic molecules
Amphipathic molecules in chemistry
Amphipathic molecules in chemistry include the entire family of soap and detergent compounds, surfactants or surfactant compounds, whether they are neutral, anionic or cationic. Some specific examples of these amphipathic molecules are:
- sodium palmitate
- Potassium Dodecyl Sulfate
- 1-decanol
- nonadecylammonium chloride
- cocamidopropyl betaine
- Dimethyldioctadecylammonium chloride
- benzalkonium chloride
Amphipathic molecules in biology
A great variety of compounds and chemical substances of great biological origin are amphipathic molecules. Perhaps the most common are triglycerides and fatty acids, which are the main components of cell membranes and walls that separate the interior of the cell from the environment, and that make up the membranes of the different intracellular compartments and other organelles of the cell. eukaryotic cells.
On the other hand, many proteins are themselves gigantic amphipathic molecules whose amino acids possess hydrophilic and hydrophobic residues that are ordered and oriented to give proteins their characteristic secondary and tertiary structure. Furthermore, hydrophobic tails and hydrophilic heads also play important roles in the location and function of proteins.
Some specific examples of important biological amphipathic molecules are:
- Triglycerides that are part of fats, such as triolein (ester between glycerol and 3 molecules of oleic acid), tripalmitin (ester between glycerol and 3 molecules of palmitic acid) and tristearin (ester between glycerol and 3 stearic acid molecules).
- Monoglycerides such as monolaurin and glyceryl monostearate.
Uses and importance of amphipathic molecules
It has always been said that water is the basis of life, but this would not be possible without amphipathic molecules, since cells could not form without them. This is due to the natural tendency of amphipathic or amphiphilic molecules to form liposomes and micelles, as well as different types of membranes.
If a mixture of water, oil, and an amphipathic compound is prepared, the amphipathic molecules will be distributed along the interface between the water and the oil. They will tend to be arranged in such a way that the polar head remains dissolved in the aqueous phase, while the hydrophobic or lipophilic tails remain in the oily phase.
If the mixture is shaken to break this membrane, structures can be formed in which small oil droplets are encapsulated by the amphipathic molecules and covered by the polar heads that are easily dispersed in the aqueous matrix. These structures are called micelles. This is the principle of the operation of soaps and detergents, since they encapsulate and dissolve the different fats and other apolar impurities that may be on a surface or on a fabric.
On the other hand, if we add amphipathic molecules to pure water and shake, the amphipathic molecules will tend to form a double layer with the nonpolar chains inside and the polar heads exposed to the aqueous matrix. If shaken, structures can be formed in which a part of the aqueous matrix is encapsulated by this double membrane, thus forming a liposome. These liposomes are the basis of the structure of cells.
References
Biology Online. (2022, March 18). Amphipathic – Definition and Examples – Biology Online Dictionary . Biology Articles, Tutorials & Dictionary Online. https://www.biologyonline.com/dictionary/amphipathic
Bolívar, G. (2019, July 13). Amphipathic Molecules: Structure, Characteristics, Examples . lifer. https://www.lifeder.com/moleculas-anfipaticas/
DBpedia in Spanish. (nd). About: Amphiphilic Molecule . https://en.dbpedia.org/page/Mol%C3%A9cula_anfif%C3%ADlica
Merriam-Webster.com Dictionary. (nd). amphipathic . Merriam-Webster. https://www.merriam-webster.com/dictionary/amphipathic
Trilonet. (nd). lipids Classification. saponifiable lipids. Amphipathic lipids . http://www.ehu.eus/biomoleculas/lipidos/lipid34.htm
