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Adjective

amphipathic

describing a molecule, such as a detergent, which has both hydrophobic and hydrophilic groups.
often refers to the surface(s) on a protein, particularly an alpha helix, where one surface of the alpha helix has hydrophilic amino acids and the opposite face has hydrophobic (or lipophilic) amino acids.

Extensive Definition

Amphiphile (from the Greek αμφις, amphis: both and φιλíα, philia: love, friendship) is a term describing a chemical compound possessing both hydrophilic and hydrophobic properties. Such a compound is called amphiphilic or amphipathic. This forms the basis for a number of areas of research in chemistry and biochemistry, notably that of lipid polymorphism. Organic compounds containing hydrophilic groups at both ends of a prolate molecule are called bolaamphiphilic. Common amphiphilic substances are soap and detergent.

Structure and Properties

The hydrophobic group is typically a large hydrocarbon moiety, such as a long chain of the form CH3(CH2)n, with n > 4. The hydrophilic group falls into one of the following categories:

Charged groups
- Anionic. Examples, with the hydrophobic part of the molecule represented by an R, are:
  - carboxylates: RCO2-;
  - sulfates: RSO4-;
  - sulfonates: RSO3-.
  - phosphates: The charged functionality in phospholipids.
- Cationic. Examples:
  - amines: RNH3+.
Polar, uncharged groups. Examples are alcohols with large R groups, such as diacyl glycerol (DAG), and oligoethyleneglycols with long alkyl chains.

Often, amphiphilic species have several hydrophobic parts, several hydrophilic parts, or several of both. Proteins and some block copolymers are such examples.

Molecules of amphiphilic compounds have hydrophobic (usually of hydrocarbon nature) and hydrophilic (represented by either ionic or uncharged polar functional groups) structural regions.

As a result of having both hydrophobic and hydrophilic structural regions, some amphiphilic compounds may dissolve in water and to some extent in non-polar organic solvents.

When placed in an immiscible biphasic system consisting of aqueous and hydrophobic solvent the amphiphilic compound will partition the two phases. The balance between hydrophobic and hydrophilic natures defines the extent of partitioning.

Biological role

Phospholipids, a class of amphiphilic molecules, are the main components of biological membranes. The amphiphilic nature of these molecules defines the way in which they form membranes. They arrange themselves into bilayers, by positioning their polar groups towards the surrounding aqueous medium, and their hydrophobic chains towards the inside of the bilayer, defining a non-polar region between two polar ones.

Although phospholipids are principal constituents of biological membranes, there are other amphiphilic molecules, such as cholesterol and glycolipids, which are also included in these structures and give them different physical and biological properties.

Many other amphiphilic compounds strongly interact with biological membranes by insertion of hydrophobic part into the lipid membrane, while exposing the hydrophilic part to the aqueous medium, altering their physical behaviour and sometimes disrupting them.

Examples of amphiphiles

There are several examples of molecules that present amphiphilic properties:

Surfactants are an example group of amphiphilic compounds. Their polar region can be either ionic, or non-ionic. Some typical members of this group are: sodium dodecyl sulfate (anionic), Benzalkonium chloride (cationic), Cocamidopropyl betaine (zwitterionic) and octanol (long chain alcohol, non-ionic).

Many biological compounds are amphiphilic: phospholipids, cholesterol, glycolipids, fatty acids, bile acids, saponins, etc.

External links

Estimating intestinal permeability by surface activity profiling

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