AMPHIPATHIC MOLECULES: STRUCTURE, CHARACTERISTICS, EXAMPLES - CHEMISTRY - 2024

The amphipathic or amphiphilic molecules are those that can feel affinity or repulsion to the same time for a given solvent. Solvents are chemically classified as polar or apolar; hydrophilic or hydrophobic. Thus, these types of molecules can "love" water, as they can also "hate" it.

According to the previous definition, there is only one way for this to be possible: these molecules must have polar and apolar regions within their structures; whether they are more or less homogeneously distributed (as is the case with proteins, for example), or they are heterogeneously localized (in the case of surfactants)

Bubbles, a physical phenomenon caused by the reduction of the surface tension of the air-liquid interface due to the action of a surfactant, which is an amphiphilic compound. Source: Pexels.

Surfactants, also called detergents, are perhaps the best known amphipathic molecules of all since time immemorial. Ever since Man was captivated by the strange physiognomy of a bubble, concerned about the preparation of soaps and cleaning products, he has come across the phenomenon of surface tension time and again.

Observing a bubble is the same as witnessing a “trap” whose walls, formed by the alignment of amphipathic molecules, retain the gaseous content of the air. Their spherical shapes are the most mathematically and geometrically stable, since they minimize the surface tension of the air-water interface.

That said, two other characteristics of amphipathic molecules have been discussed: they tend to associate or self-assemble, and some lower surface tensions in liquids (those that can do so are called surfactants).

As a result of the high tendency to associate, these molecules open a field of morphological (and even architectural) study of their nanoaggregates and the supramolecules that compose them; with the aim of designing compounds that can be functionalized and interact in immeasurable ways with cells and their biochemical matrices.

Structure

General structure of an amphipathic molecule. Source: Gabriel Bolívar.

Amphiphilic or amphipathic molecules were said to have a polar region and an apolar region. The apolar region usually consists of a saturated or unsaturated carbon chain (with double or triple bonds), which is represented as an "apolar tail"; accompanied by a "polar head", in which the most electronegative atoms reside.

The upper general structure illustrates the comments in the previous paragraph. The polar head (purple sphere) can be functional groups or aromatic rings that have permanent dipole moments, and are also capable of forming hydrogen bonds. Therefore, the highest oxygen and nitrogen content must be located there.

In this polar head there can also be ionic, negative or positive charges (or both at the same time). This region is the one that shows a high affinity for water and other polar solvents.

On the other hand, the apolar tail, given its predominant CH bonds, interacts through London scattering forces. This region is responsible for the fact that amphipathic molecules also show affinity for fats and apolar molecules in the air (N ₂, CO ₂, Ar, etc.).

In some chemistry texts the model for the upper structure is compared to the shape of a lollipop.

Intermolecular interactions

When an amphipathic molecule comes into contact with a polar solvent, say water, its regions exert different effects on the solvent molecules.

To begin with, the water molecules seek to solvate or hydrate the polar head, staying away from the apolar tail. In this process molecular disorder is created.

Meanwhile, the water molecules around the apolar tail tend to arrange themselves as if they were small crystals, thus allowing them to minimize repulsions. In this process a molecular order is created.

Between disorders and orders, there will come a point where the amphipathic molecule will seek to interact with another, which will result in a much more stable process.

Miscellas

Both will be approached through their apolar tails or polar heads, in such a way that related regions interact first. This is the same as imagining that two "purple lollipops" in the upper image approach, intertwining their black tails, or joining their two purple heads.

And so begins an interesting association phenomenon, in which several of these molecules are joined consecutively. They are not associated arbitrarily, but according to a series of structural parameters, which end up isolating the apolar tails in a kind of “apolar nucleus”, while exposing the polar heads as a polar shell.

It is said then that a spherical miscela has been born. However, during the formation of the miscela there is a preliminary stage consisting of what is known as a lipid bilayer. These and others are some of the many macrostructures that amphiphilic molecules can adopt.

Characteristics of amphipathic molecules

Association

Spherical miscellany formed by amphipathic molecules. Source: Gabriel Bolívar.

If the apolar tails are taken as black units, and the polar heads as purple units, it will be understood why in the upper image the bark of the miscela is purple and its nucleus is black. The nucleus is apolar, and its interactions with the water or solvent molecules are null.

If, on the other hand, the solvent or medium is apolar, it is the polar heads that will suffer the repulsions, and consequently they will be located in the center of the miscella; that is, it is inverted (A, lower image).

Different types of miscellar structures or morphologies. Source: Gabriel Bolívar.

The inverted miscelain is observed to have a black apolar shell and a purple polar nucleus. But, before the miscelas are formed, the amphiphilic molecules are found individually altering the order of the solvent molecules. With increased concentration, they begin to associate in a one or two layer structure (B).

From B the laminae begin to curve to form D, a vesicle. Another possibility, depending on the shape of the apolar tail with respect to its polar head, is that they associate to give rise to a cylindrical miscella (C).

Nanoaggregates and supramolecules

Therefore, there are five main structures, which reveal a fundamental characteristic of these molecules: their high tendency to associate and self-assemble into supramolécules, which aggregate to form nanoaggregates.

Thus, amphiphilic molecules are not found alone but in association.

Physical

Amphipathic molecules can be neutral or ionically charged. Those that have negative charges have an oxygen atom with a negative formal charge in their polar head. Some of these oxygen atoms come from functional groups such as -COO ^-, -SO ₄ ^-, -SO ₃ ^- or -PO ₄ ^-.

Regarding the positive charges, they generally come from amines, RNH ₃ ⁺.

The presence or absence of these charges does not change the fact that these molecules generally form crystalline solids; or, if they are relatively light, they are found as oils.

Examples

Some examples of amphipathic or amphiphilic molecules will be mentioned below:

-Fofolipids: phosphatidylethanolamine, sphingomyelin, phosphatidylserine, phosphatidylcholine.

-Cholesterol.

-Glucolipids.

-Sodium lauryl sulfate.

-Proteins (they are amphiphilic, but not surfactants).

-Phenolic fats: cardanol, cardoles and anacardial acids.

-Cetyltrimethylammonium bromide.

-Fatty acids: palmitic, linoleic, oleic, lauric, stearic.

-Long chain alcohols: 1-dodecanol and others.

-Amphiphilic polymers: such as ethoxylated phenolic resins.

Applications

Cell membranes

One of the most important consequences of the ability of these molecules to associate is that they build a kind of wall: the lipid bilayer (B).

This bilayer extends to protect and regulate the entry and exit of compounds into cells. It is dynamic, since its apolar tails rotate helping amphipathic molecules to move.

Likewise, when this membrane is attached to two ends, in order to have it vertically, it is used to measure its permeability; and with this, valuable data are obtained for the design of biological materials and synthetic membranes from the synthesis of new amphipathic molecules with different structural parameters.

Dispersants

In the oil industry, these molecules, and the polymers synthesized from them, are used to disperse asphaltenes. The focus of this application rests on the hypothesis that asphaltenes consist of a colloidal solid, with a high tendency to flocculate and sediment as a brownish-black solid that causes serious economic problems.

Amphipathic molecules help to keep asphaltenes dispersed for a longer time in the face of physicochemical changes in the oil.

Emulsifiers

These molecules help two liquids mix that would not be miscible under ordinary conditions. In ice creams, for example, they help water and air form part of the same solid together with fat. Among the most widely used emulsifiers for this purpose are those derived from edible fatty acids.

Detergents

The amphiphilic nature of these molecules is used to trap fats or apolar impurities, to then be washed away at the same time by a polar solvent such as water.

Like the example of bubbles where air was trapped, detergents trap fat within their micelles, which, having a polar shell, efficiently interact with water to remove dirt.

Antioxidants

The polar heads are of vital importance as they define the multiple uses that these molecules can have within the body.

If they possess, for example, a set of aromatic rings (including derivatives of a phenolic ring) and polar rings capable of neutralizing free radicals, then there will be amphiphilic antioxidants; and if they also lack toxic effects, then there will be new antioxidants available on the market.

References

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