The micelles are stable spherical structures formed by hundreds of amphipathic molecules, ie, molecules that are characterized by a polar (hydrophilic) and nonpolar region (hydrophobic). Like the molecules that compose them, micelles have a strongly hydrophobic center and their surface is "lined" with hydrophilic polar groups.
They result, in most cases, from the mixing of a group of amphipathic molecules with water, so it is a way of “stabilizing” the hydrophobic regions of many molecules together, a fact that is driven by the effect hydrophobic and organized by van der Waals forces.
The structural scheme of a micelle (Source: Original English: SuperManu. Spanish: AngelHerraez / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0) via Wikimedia Commons)
Both detergents and soaps, as well as certain cellular lipids, can form micelles, which have functional relevance, at least in animals, from the point of view of fat absorption and transport of fat-soluble substances.
Phospholipids, one of the most abundant and important classes of lipids for living cells, under certain conditions can form, in addition to liposomes and bilayers, micellar structures.
Micelles can also be formed in an apolar medium and in that case they are called "reverse micelles", since the polar regions of the amphipathic molecules that form them are "hidden" in the hydrophilic center while the apolar portions are in direct contact with the medium. that contains them.
Structure
Micelles are made up of amphipathic molecules or, in other words, molecules that have a hydrophilic region (water-like, polar) and another hydrophobic region (water-repellent, apolar).
Among these molecules can be mentioned fatty acids, molecules of any detergent and phospholipids of cell membranes, for example.
In the cellular context, a micelle is commonly composed of fatty acids (of variable length), whose polar carboxyl groups are exposed towards the surface of the aggregate, while the hydrocarbon chains are “hidden” in a hydrophobic center, thus adopting a more or less spherical structure.
Phospholipids, which are other amphipathic molecules of great importance for cells, are generally incapable of forming micelles, since the two fatty acid chains that make up their “hydrophobic tails” occupy large size and make any form packing difficult. spherical.
The formation of a micelle mediated by the aqueous environment (Source: Jwleung / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0) via Wikimedia Commons)
Instead, when these molecules are in an aqueous medium, they "nestle" into bilayers (similar to a sandwich); that is, in flatter structures, where each of the "surfaces" exposed towards the medium is composed of the polar heads of the groups attached to glycerol and the "filling" of the sandwich consists of the hydrophobic tails (the fatty acids esterified to the other two carbons of the glycerol skeleton).
The only way in which it is possible for a phospholipid to participate in the formation of a micelle is when one of its two fatty acid chains is removed by hydrolysis.
Organization
In a micelle, as mentioned, the "center" sequesters the nonpolar portions of the molecules that compose them and isolates them from the water.
The central region of a micelle consists, then, of a highly disordered environment, with fluid-like characteristics, in which the radius measure is between 10 and 30% smaller than that of the fully extended chains of non-amphipathic molecules. associated with the molecular complex.
Likewise, the surface of a micelle is not homogeneous but rather "rough" and heterogeneous, of which some nuclear magnetic resonance studies indicate that only one third is covered by the polar portions of the constituent monomers.
Function
Micelles have very significant functions, both in nature and in industry and in research.
Regarding their functions in nature, these molecular aggregates are particularly important for the intestinal absorption of fats (monoglycerides and fatty acids), since micelles of different sizes and compositions can be formed from the fatty molecules ingested with food and transport them to the inside the cells of the intestinal lining, making their absorption possible.
Micelles also function in the transport of cholesterol (another class of cellular lipid) acquired with the diet and of some so-called “fat-soluble” vitamins, which is why they are also exploited pharmacologically for the transport and administration of drugs with apolar characteristics.
The detergents and soaps used daily for personal hygiene or for cleaning different types of surfaces are composed of lipid molecules capable of forming micelles when they are in an aqueous solution.
These micelles behave like tiny balls in a bearing, giving soapy solutions their slippery consistency and lubricating properties. The action of most detergents is highly dependent on their ability to produce micelles.
In the research and study of membrane proteins, for example, detergents are used to "cleanse" the cell lysates of the lipids that form the characteristic bilayers of membranes, as well as to separate the integral membrane proteins from the hydrophobic components. of this.
Training
To understand the formation of micellar structures, especially in detergents, it is necessary to take into account a somewhat abstract concept: the critical micellar concentration or CMC.
The critical micellar concentration is that concentration of amphipathic molecules at which micelles begin to form. It is a reference value above which an increase in the concentration of these molecules will only end with an increase in the number of micelles, and below which these are preferentially organized in layers on the surface of the aqueous medium that contains them..
Differences and similarities between a micelle and a bilayer formed by phospholipids (Source: March 31, 2003 in: User: Stephen Gilbert, March 31, 2003 in: User: Stephen Gilbert, December 27, 2004 in: User: Quadell, translation User: imartin6 / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/) via Wikimedia Commons)
Thus, the formation of micelles is a direct consequence of the "amphiphilicity" of surfactants and is highly dependent on their structural characteristics, especially on the shape and size relationship between the polar and apolar groups.
In this sense, the formation of micelles is favored when the cross-sectional area of the polar group is much larger than that of the apolar group, as occurs with free fatty acids, with lysophospholipids and with detergents such as sodium dodecyl sulfate (SDS).
Two other parameters on which micelle formation depends are:
- Temperature: the critical micellar temperature (CMT) has also been defined, which is the temperature above which the formation of micelles is favored
- Ionic strength: which is relevant, above all, for ionic-type detergents or surfactants (whose polar group has a charge)
References
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- Luckey, M. (2014). Membrane structural biology: with biochemical and biophysical foundations. Cambridge University Press.
- Nelson, DL, & Cox, MM (2009). Lehninger principles of biochemistry (pp. 71-85). New York: WH Freeman.
- Tanford, C. (1972). Micelle shape and size. The Journal of Physical Chemistry, 76 (21), 3020-3024.
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