Amphiphile
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In chemistry, an amphiphile (from Greek αμφις (amphis) 'both' and φιλíα (philia) 'love, friendship'), or amphipath, is a chemical compound possessing both hydrophilic (water-loving, polar) and lipophilic (fat-loving, nonpolar) properties.[1] Such a compound is called amphiphilic or amphipathic. Amphiphilic compounds include surfactants and detergents. The phospholipid amphiphiles are the major structural component of cell membranes.
Amphiphiles are 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 the molecule are called bolaamphiphilic. The micelles they form in the aggregate are prolate.
Structure
[edit]The lipophilic 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:[citation needed]
- charged groups
- anionic. Examples, with the lipophilic part of the molecule represented by R, are:
- carboxylates: RCO2−
- sulfates: RSO4−
- sulfonates: RSO3−
- phosphates (the charged functional group in phospholipids)
- cationic. Examples:
- ammoniums: RNH3+
- anionic. Examples, with the lipophilic part of the molecule represented by R, are:
- polar, uncharged groups. Examples are alcohols with large R groups, such as diacyl glycerol (DAG), and oligo ethylene glycol with long alkyl chains.
Often, amphiphilic species have several lipophilic parts, several hydrophilic parts, or several of both. Proteins and some block copolymers are such examples.[citation needed]
Amphiphilic compounds have lipophilic (typically hydrocarbon) structures and hydrophilic polar functional groups (either ionic or uncharged).[citation needed]
As a result of having both lipophilic and hydrophilic portions, 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 organic solvents, the amphiphilic compound will partition the two phases. The extent of the hydrophobic and hydrophilic portions determines the extent of partitioning.[citation needed]
Biological role
[edit]
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 lipid bilayers, by forming a sheet composed of two layers of lipids. Each layer forms by positioning their lypophilic chains to the same side of the layer. The two layers then stack such that their lyphphilic chains touch on the inside and their polar groups are outside facing the surrounding aqueous media. Thus the inside of the bilayer sheet is a non-polar region sandwiched between the two polar sheets.[2]
Although phospholipids are the principal constituents of biological membranes,[3] there are other constituents, such as cholesterol and glycolipids, which are also included in these structures and give them different physical and biological properties.[citation needed]
Many other amphiphilic compounds, such as pepducins, strongly interact with biological membranes by insertion of the hydrophobic part into the lipid membrane, while exposing the hydrophilic part to the aqueous medium, altering their physical behavior and sometimes disrupting them.[citation needed]
Aβ proteins form antiparallel β sheets which are strongly amphiphilic,[4] and which aggregate to form toxic oxidative Aβ fibrils. Aβ fibrils themselves are composed of amphiphilic 13-mer modular β sandwiches separated by reverse turns. Hydropathic waves optimize the description of the small (40,42 aa) plaque-forming (aggregative) Aβ fragments.[5]
Antimicrobial peptides (AMPs) are another class of amphiphilic molecules, a big data analysis showed that amphipathicity best distinguished between AMPs with and without anti-gram-negative bacteria activities. The higher amphipathicity, the better chances for AMPs possessing antibacterial and antifungal dual activities.[6]
Examples
[edit]There are several examples of molecules that present amphiphilic properties:
Hydrocarbon-based 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 1-octanol (long-chain alcohol, non-ionic).[citation needed]
Many biological compounds are amphiphilic: phospholipids, cholesterol, glycolipids, fatty acids, bile acids, saponins, local anaesthetics, etc.[citation needed]
Soap is a common household amphiphilic surfactant compound. Soap mixed with water (polar, hydrophilic) is useful for cleaning oils and fats (non-polar, lipophilic) from kitchenware, dishes, skin, clothing, etc.
See also
[edit]References
[edit]- ^ Betts, J. Gordon. "3.1 The cell membrane". Anatomy & physiology. OpenStax. ISBN 978-1-947172-04-3. Retrieved 14 May 2023.
- ^ "Amphipathic". Biology-Online Dictionary. Retrieved 2016-11-17.
- ^ "Structure of a Membrane". The Lipid Chronicles. 5 November 2011. Retrieved 2020-06-02.
- ^ Schubert, D; Behl, C; Lesley, R; Brack, A; Dargusch, R; Sagara, Y; Kimura, H (14 March 1995). "Amyloid peptides are toxic via a common oxidative mechanism". Proceedings of the National Academy of Sciences of the United States of America. 92 (6): 1989–93. Bibcode:1995PNAS...92.1989S. doi:10.1073/pnas.92.6.1989. PMC 42408. PMID 7892213.
- ^ Phillips, J.C. (20 May 2015). "Thermodynamic description of Beta amyloid formation using physicochemical scales and fractal bioinformatic scales". ACS Chemical Neuroscience. 6 (5): 745–50. doi:10.1021/cn5001793. PMID 25702750.
- ^ Chien-Kuo Wang; Ling-Yi Shih; Kuan Y. Chang (2017-11-22). "Large-scale analysis of antimicrobial activities in relation to amphipathicity and charge reveals novel characterization of antimicrobial peptides". Molecules. 22 (11): 2037. doi:10.3390/molecules22112037. PMC 6150348. PMID 29165350.
External links
[edit]Amphiphile
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Definition and Classification
Definition
An amphiphile is a term derived from the Greek roots "amphi," meaning both or double, and "philos," meaning loving or affinity, thus describing a substance with dual affinity for water and oil; the word was coined by Paul Winsor in 1948.[8] At its core, an amphiphile is a molecule or chemical compound featuring both hydrophilic regions, which are polar and attract water, and hydrophobic regions, which are nonpolar and repel water, resulting in distinctive behavior at interfaces between polar and nonpolar environments.[1] This dual nature enables amphiphiles to reduce surface tension and facilitate interactions in heterogeneous systems, such as emulsions or biological membranes.[4] The terms amphiphile and amphipathic are often used synonymously to describe entities with hydrophilic and hydrophobic moieties, though amphiphile typically refers specifically to molecular compounds, whereas amphipathic can apply more broadly to any surface or structure exhibiting dual affinity properties.[9] A key concept in characterizing amphiphiles is the hydrophilic-lipophilic balance (HLB), which quantifies the relative extent of hydrophilic and lipophilic character within the molecule on a scale from 0 (highly lipophilic, favoring oil solubility) to 20 (highly hydrophilic, favoring water solubility).[10] HLB values guide the prediction of an amphiphile's behavior in formulations, such as its role in stabilizing emulsions. One common method to calculate HLB, particularly for ionic surfactants, is the Davies approach, which assigns numerical contributions to specific functional groups: the hydrophilic increments () for polar groups like sulfates (+38.7) or esters (+2.4), and the lipophilic decrements () for hydrocarbon chains like -CH (-0.475) or -CH (-0.475). The formula is given by
where the neutral value of 7 represents a balanced point, and the sums account for all relevant group contributions in the molecule.[10] This method allows for empirical estimation based on molecular structure without experimental measurement.
Types of Amphiphiles
Amphiphiles are primarily classified based on the number of hydrophobic tails attached to the hydrophilic head group, which significantly influences their packing efficiency and self-assembly tendencies. Single-chain amphiphiles, such as the surfactant sodium dodecyl sulfate (SDS), feature one hydrophobic alkyl chain, enabling them to form spherical micelles at lower concentrations due to looser packing. In contrast, double-chain amphiphiles, exemplified by phospholipids like phosphatidylcholine, possess two hydrophobic tails, promoting tighter packing and the formation of more stable bilayer structures such as vesicles. This distinction arises from the geometric constraints imposed by the tail architecture, where double chains increase the hydrophobic volume relative to the head area.[2][3] A key classification scheme divides amphiphiles according to the nature of their hydrophilic head group, particularly whether it is charged or uncharged, which affects interactions with solvents and counterions. Ionic amphiphiles bear charged head groups and are subdivided into anionic types (e.g., those with sulfate heads like SDS), cationic types (e.g., quaternary ammonium compounds such as cetyltrimethylammonium bromide), and zwitterionic types (e.g., betaines with both positive and negative charges in the head). Non-ionic amphiphiles, on the other hand, feature uncharged polar heads, such as polyether chains in compounds like polysorbate 80 (Tween 80), leading to milder interactions and better compatibility in sensitive formulations. Ionic amphiphiles often exhibit hydrophilic-lipophilic balance (HLB) values greater than 10, enhancing their water solubility compared to non-ionic counterparts.[11] Specialized amphiphiles extend these classifications with unique architectures tailored for advanced functionalities. Bolaphiles (bolaamphiphiles) consist of two hydrophilic heads connected by a long hydrophobic chain, allowing for enhanced surface activity and lower critical micelle concentrations than conventional single-chain types.[12] Peptide amphiphiles integrate bioactive peptide sequences as the hydrophilic component with alkyl chains as tails, enabling pH- or enzyme-responsive self-assembly into nanofibers. Nucleic acid amphiphiles, such as DNA-lipid conjugates, combine oligonucleotide sequences with hydrophobic lipid moieties and have emerged prominently since the 2010s for programmable nanostructures in biomedical contexts. Macrocyclic amphiphiles, including calixarenes modified with hydrophobic cavities and polar rims, form host-guest complexes due to their cyclic topology, facilitating selective encapsulation.[13][7][14] The foundational understanding of amphiphile types traces back to Irving Langmuir's 1917 studies on monolayers at the air-water interface, where he first systematically classified amphiphilic behaviors based on molecular orientation and surface pressure, laying the groundwork for modern classifications.[15]Molecular Structure and Properties
Structural Components
Amphiphiles are characterized by a molecular architecture that features distinct structural components: a hydrophilic head group, a hydrophobic tail, and often a linker region connecting them. These elements confer the dual affinity for polar and nonpolar environments, enabling the molecule's amphiphilic behavior.[1] The hydrophilic head group consists of polar or charged moieties that interact favorably with water. Common examples include uncharged groups such as hydroxyl (-OH) or thiol (-SH), as well as charged groups like carboxylate (-COOH or -COO^-), sulfate (-SO4^-), phosphate (-PO4^3- or derivatives), and ammonium (-NH3^+). In phospholipids, the phosphate group serves as a key example of a charged head that enhances water solubility through ionic interactions and hydrogen bonding.[1][16] The hydrophobic tail is typically composed of nonpolar alkyl chains, ranging from C8 to C18 hydrocarbons in length, which provide the oil-soluble portion of the molecule. These chains can be saturated, such as straight-chain alkanes, or unsaturated with cis double bonds that introduce kinks, affecting molecular packing density. Branching in the alkyl chains, such as short alkyl substituents, can disrupt tight packing and influence stability in assemblies, while fluorination of the tail—replacing hydrogen with fluorine atoms—creates a more rigid, lipophobic interior due to the strong electronegativity and low polarizability of fluorine, altering hydrophobic interactions.[17][18][19] Linker regions between the head and tail provide flexibility to the overall structure. In natural lipids, glycerol often serves as a flexible spacer, forming ester or ether bonds that connect the polar head to the lipid tails. Non-ionic synthetic amphiphiles may employ ether bonds as linkers, which offer stability and conformational freedom compared to ester linkages.[20] A general representation of amphiphile structure is given by the formula $ R-(CH_2)_n-X $, where $ R $ denotes the hydrophobic tail (often an alkyl group), $ X $ is the hydrophilic head, and $ n $ typically ranges from 8 to 18 carbons to achieve optimal amphiphilicity. Longer tails in this formula tend to lower the hydrophile-lipophile balance (HLB), increasing overall hydrophobicity.[21][18] In biological amphiphiles, such as peptide-based ones, stereochemistry plays a crucial role, with chirality at carbon centers determining molecular recognition and interactions. For instance, L-amino acids predominate in natural peptides, imparting a specific handedness that influences conformational preferences and biocompatibility.[22]Key Physical Properties
Amphiphiles display limited aqueous solubility owing to their amphipathic structure, reaching a monolayer solubility limit prior to the onset of self-assembly processes.[23] For ionic amphiphiles, the Krafft point represents the critical temperature at which solubility sharply increases and equals the critical micelle concentration (CMC), influenced by factors such as hydrocarbon chain length, with longer chains yielding higher Krafft points.[24] Below this point, solubility is governed by the crystalline energy and hydration heat of the surfactant.[25] A hallmark property of amphiphiles is their ability to adsorb at the water-air interface, significantly reducing surface tension from approximately 72 mN/m for pure water to values as low as 22 mN/m.[26] This reduction arises from the orientation of hydrophilic heads toward water and hydrophobic tails away from it, stabilizing the interface.[27] The extent of adsorption is quantified by the Gibbs adsorption isotherm:
where is the surface excess concentration, is the surface tension, is the bulk concentration, is the gas constant, and is the temperature.[28]
Non-ionic amphiphiles tend to elevate the viscosity of aqueous solutions compared to ionic counterparts, with longer hydrophobic or polyoxyethylene chain lengths contributing to higher relative viscosities through enhanced molecular interactions.[29] This viscosity increase, often amplified by electrolytes, impacts solution flow and stability.[30] Additionally, non-ionic types excel in generating and stabilizing foams by lowering surface tension and forming viscoelastic films at air-water interfaces.[31]
The thermal stability of amphiphiles, particularly their melting points, is dictated by hydrophobic tail packing efficiency; even-numbered carbon chains generally pack more uniformly in crystals, resulting in higher melting points than odd-numbered chains, akin to alkane behavior.[32]
Ionic amphiphiles' properties, including solubility and surface activity, vary with pH and salinity due to counterion specificity, as described by the Hofmeister series, where chaotropic ions (e.g., I⁻) promote greater solubility and interfacial adsorption than kosmotropic ones (e.g., SO₄²⁻).[33] These behaviors establish key thresholds for self-assembly, such as the CMC.[23]