Alpha hydroxy acid - AbsoluteAstronomy.com

α-Hydroxy acids, or alpha hydroxy acids (AHAs), are a class of chemical compounds that consist of a carboxylic acid

Carboxylic acid

Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...

substituted with a hydroxyl group

Hydroxyl

A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

on the adjacent carbon. They may be either naturally occurring or synthetic. AHAs are well-known for their use in the cosmetics industry. They are often found in products claiming to reduce wrinkles or the signs of aging, and improve the overall look and feel of the skin. They are also used as chemical peel

Chemical peel

A chemical peel is a body treatment technique used to improve and smooth the texture of the facial skin using a chemical solution that causes the dead skin to slough off and eventually peel off. The regenerated skin is usually smoother and less wrinkled than the old skin. Thus the term chemical...

s available in a dermatologist's office, beauty and health spas and home kits, which usually contain a lower concentration. Although their effectiveness is documented numerous cosmetic products have appeared on the market with unfounded claims of performance.
Many well-known α-hydroxy acids are useful building blocks in organic synthesis: the most common and simple are glycolic acid

Glycolic acid

Glycolic acid is the smallest α-hydroxy acid . This colorless, odorless, and hygroscopic crystalline solid is highly soluble in water. It is used in various skin-care products. Glycolic acid is found in some sugar-crops...

, lactic acid

Lactic acid

Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...

, citric acid

Citric acid

Citric acid is a weak organic acid. It is a natural preservative/conservative and is also used to add an acidic, or sour, taste to foods and soft drinks...

, mandelic acid

Mandelic acid

Mandelic acid is an aromatic alpha hydroxy acid with the molecular formula C6H5CHCO2H. It is a white crystalline solid that is soluble in water and polar organic solvents. It is a useful precursor to various drugs...

Cosmetic applications

Understanding skin structure and cutaneous aging is helpful to a discussion of the topical action of AHAs. Human skin

Skin

-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...

has two principal components, the avascular epidermis and the underlying vascular

Vascular

Vascular in zoology and medicine means "related to blood vessels", which are part of the circulatory system. An organ or tissue that is vascularized is heavily endowed with blood vessels and thus richly supplied with blood....

dermis

Dermis

The dermis is a layer of skin between the epidermis and subcutaneous tissues, and is composed of two layers, the papillary and reticular dermis...

. Cutaneous aging, while having epidermal concomitants, seems to involve primarily the dermis and is caused by intrinsic and extrinsic aging

Intrinsic and extrinsic aging

Intrinsic aging and Extrinsic aging are terms used to describe cutaneous aging of the skin and other parts of the integumentary system, which while having epidermal concomitants, seems to primarily involve the dermis. Intrinsic aging is influenced by internal physiological factors alone, and...

factors.

AHAs are a group of organic carboxylic compounds. AHAs most commonly used in cosmetic applications are typically derived from food products including glycolic acid

Glycolic acid

(from sugar cane), lactic acid

Lactic acid

(from sour milk), malic acid

Malic acid

Malic acid is an organic compound with the formula HO2CCH2CHOHCO2H. It is a dicarboxylic acid which is made by all living organisms, contributes to the pleasantly sour taste of fruits, and is used as a food additive. Malic acid has two stereoisomeric forms , though only the L-isomer exists...

(from apples), citric acid

Citric acid

Citric acid is a weak organic acid. It is a natural preservative/conservative and is also used to add an acidic, or sour, taste to foods and soft drinks...

(from citrus fruits) and tartaric acid

Tartaric acid

Tartaric acid is a white crystalline diprotic organic acid. It occurs naturally in many plants, particularly grapes, bananas, and tamarinds; is commonly combined with baking soda to function as a leavening agent in recipes, and is one of the main acids found in wine. It is added to other foods to...

(from grape wine). For any topical compound, including AHA, it must penetrate into the skin where it can act on living cells. Bioavailability

Bioavailability

In pharmacology, bioavailability is a subcategory of absorption and is used to describe the fraction of an administered dose of unchanged drug that reaches the systemic circulation, one of the principal pharmacokinetic properties of drugs. By definition, when a medication is administered...

(influenced primarily by small molecular size) is one characteristic that is important in determining compound's ability to penetrate the top layer of the skin. Glycolic acid

Glycolic acid

having the smallest molecular size is the AHA with greatest bioavailability

Bioavailability

and penetrates the skin most easily; this largely accounts for the popularity of this product in cosmetic applications.

Epidermal effect

AHAs have a profound effect on keratinization; which is clinically detectable by the formation of a new stratum corneum

Stratum corneum

The stratum corneum is the outermost layer of the epidermis, consisting of dead cells that lack nuclei and organelles. The purpose of the stratum corneum is to form a barrier to protect underlying tissue from infection, dehydration, chemicals and mechanical stress...

. It appears that AHAs modulate this formation through diminished cellular cohesion between corneocytes at the lowest levels of the stratum corneum.

Dermal effects

AHAs with greater bioavailability appear to have deeper dermal effects. Glycolic acid, lactic acid and citric acid, on topical application to photodamaged skin, have been shown to produce increased amounts of mucopolysaccharides and collagen and increased skin thickness without detectable inflammation, as monitored by skin biopsies

Alpha hydroxy acids at different concentrations

In low concentrations, 5 - 10%, as is found in many over-the-counter products, glycolic acid reduces cell adhesion in the top layer of the skin. This action promotes exfoliation of the outermost layer of the skin accounting for smoother texture following regular use of topical glycolic acid (GA). This relatively low concentration of GA lends itself to daily use as a monotherapy or a part of a broader skin care management for such conditions as acne, photo-damage, wrinkling as well as melasma. Care needs to be taken to avoid irritation as this may result in worsening of melasma

Melasma

Melasma is a tan or dark skin discoloration. Although it can affect anyone, melasma is particularly common in women, especially pregnant women and those who are taking oral or patch contraceptives or hormone replacement therapy medications...

or other pigmentary problems. Newer formulations combine glycolic acid with an amino acid such as arginine

Arginine

Arginine is an α-amino acid. The L-form is one of the 20 most common natural amino acids. At the level of molecular genetics, in the structure of the messenger ribonucleic acid mRNA, CGU, CGC, CGA, CGG, AGA, and AGG, are the triplets of nucleotide bases or codons that codify for arginine during...

and form a time-release system that reduces the risk of irritation without affecting glycolic acid efficacy. The use of an anti-irritant like allantoin

Allantoin

Allantoin is a chemical compound with formula C4H6N4O3. It is also called 5-ureidohydantoin or glyoxyldiureide. It is a diureide of glyoxylic acid....

is also helpful. Because of its safety, glycolic acid at the concentrations below 10% can be used daily by most people except those with very sensitive skin

Sensitive skin type

Sensitive skin is a skin condition in which skin which is prone to itching and irritation experienced as a subjective sensation when using cosmetics and toiletries. When questioned, over 50% of women in the UK and US report that they have sensitive skin....

.

In higher concentrations, between 10 and 50%, its benefits are more pronounced but are limited to temporary skin smoothing without much long lasting results. This is still a useful concentration to use as it can prepare the skin for more efficacious glycolic acid concentrations (50 - 70%) as well as prime the skin for deeper chemical peels such as TCA peel (trichloroacetic acid

Trichloroacetic acid

Trichloroacetic acid is an analogue of acetic acid in which the three hydrogen atoms of the methyl group have all been replaced by chlorine atoms....

).

At higher concentrations, 50-70% applied for 3 to 8 minutes under the supervision of a physician, glycolic acid promotes slitting between the cells and can be used to treat acne or photo-damage (such as mottled dyspigmentation, melasma or fine wrinkles). The benefits from such short contact application (chemical peels) depend on the pH of the solution (the more acidic the product, or the lower the pH, the more pronounced the results), the concentration of GA (higher concentrations produce more vigorous response), the length of application and prior skin conditioning such as prior use of topical vitamin A products. Although single application of 50 - 70% GA will produce beneficial results, multiple treatments every 2 to 4 weeks are required for optimal results. It is important to understand that glycolic acid peels are chemical peels with similar risks and side effects as other peels. Some of the side effects of AHAs chemical peeling can include hyper-pigmentation, persistent redness, scarring, as well as flare up of facial herpes infections ("cold sores")

Herpes simplex

Herpes simplex is a viral disease caused by both Herpes simplex virus type 1 and type 2 . Infection with the herpes virus is categorized into one of several distinct disorders based on the site of infection. Oral herpes, the visible symptoms of which are colloquially called cold sores or fever...

Chemical acidity

Although these compounds are related to the ordinary carboxylic acid

Carboxylic acid

Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...

s, and therefore are weak acids, their chemical structure allows for the formation of an internal hydrogen bond between the hydrogen at the hydroxyl group and one of the oxygen atoms of the carboxylic group. Two effects emerge from this situation:

Due to the "occupation" of electrons of the carboxylic oxygens in the hydrogen bonding, the acidic proton is held less strongly, as the same electrons are used in bonding that hydrogen too. So the pK_a of 2-hydroxypropanoic acid (lactic acid) is a full unit lower compared to that of propionic acid
Propionic acid
Propanoic acid is a naturally occurring carboxylic acid with chemical formula CH3CH2COOH. It is a clear liquid with a pungent odor...

itself (3.86 versus 4.87)
The internal bridging hydrogen is locked in its place on the NMR timescale: in mandelic acid
Mandelic acid
Mandelic acid is an aromatic alpha hydroxy acid with the molecular formula C6H5CHCO2H. It is a white crystalline solid that is soluble in water and polar organic solvents. It is a useful precursor to various drugs...

(2-hydroxy-2-phenylacetic acid) this proton couples to the one on carbon in the same way and magnitude as hydrogens on geminal
Geminal
In chemistry, the term geminal refers to the relationship between two functional groups that are attached to the same atom...

carbon atoms.

Safety

AHAs are generally safe when used on the skin as a cosmetic agent using the recommended dosage. The most common side-effects are mild skin irritations, redness and flaking. The severity usually depends on the pH

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

and the concentration of the acid used. Chemical peels tend to have more severe side-effects including blistering, burning and skin discoloration, although they are usually mild and go away a day or two after treatment.

The FDA has also warned consumers that care should be taken when using AHAs after an industry-sponsored study found that they can increase photosensitivity

Photosensitivity

Photosensitivity is the amount to which an object reacts upon receiving photons, especially visible light.- Human medicine :Sensitivity of the skin to a light source can take various forms. People with particular skin types are more sensitive to sunburn...

to the sun.

External links

U.S. Food and Drug Administration: Alpha Hydroxy Acids in Cosmetics

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Cosmetic applications

Epidermal effect

Dermal effects

Alpha hydroxy acids at different concentrations

Chemical acidity

Safety

See also

External links