How Ectoine Regulates Skin Hydration: Zwitterionic Structure, Preferential Hydration, and Barrier-Support Mechanisms

1. Where Does Ectoine Come From?

1.1 Discovered in Microorganisms from Highly Saline and Alkaline Lakes in Egypt

In the 1970s, researchers isolated the extremely halophilic phototrophic bacterium Ectothiorhodospira halochloris from highly saline and alkaline soda lakes in Egypt. In 1985, Galinski et al. isolated and identified a cyclic amino acid derivative from halophilic bacteria of this genus and named it ectoine.[1][2]

The ectoine used in skincare products is generally not extracted directly from Egyptian salt-lake water. Modern production mainly relies on microbial fermentation. For example, Halomonas elongata can be used to biosynthesize ectoine, which is then isolated and purified to obtain the final raw material.[3]

1.2 Why Do Halophilic Microorganisms Need Ectoine?

High-salt environments reduce extracellular water activity, causing water to move out of cells. Once cells lose water, protein folding, enzyme activity, cell membrane integrity, and normal metabolism may all be affected.

Microorganisms can balance osmotic pressure by increasing the concentration of intracellular solutes. However, if large amounts of inorganic salts accumulate, the resulting high ionic concentrations may interfere with proteins and metabolic reactions. Ectoine, by contrast, is a compatible solute: it can accumulate to relatively high concentrations inside cells, helping maintain osmotic balance while causing comparatively little interference with cellular metabolism.[2][3]

The basic function of ectoine in microorganisms can be summarized as follows:

When environmental salinity increases, ectoine helps cells retain water and maintain an appropriate hydration environment around proteins and cell membranes.

This survival mechanism also provides the molecular basis for the use of ectoine in skincare.

2. Why Can a Zwitterionic Structure Interact with Water?

2.1 Positive and Negative Charges Within the Same Molecule

The molecular formula of ectoine is C₆H₁₀N₂O₂, and its chemical name is (S)-2-methyl-1,4,5,6-tetrahydropyrimidine-4-carboxylic acid. Its structure contains a nitrogen-containing heterocyclic ring and a carboxylic acid group.[2]

 

In aqueous solution, ectoine exists predominantly in a zwitterionic form:

① The carboxylic acid group dissociates to form a negatively charged carboxylate group, —COO⁻;

② The nitrogen-containing group in the tetrahydropyrimidine ring forms a positively charged region;

③ The molecule has an overall net charge close to zero, but its positive and negative charges are spatially separated.

Water molecules are themselves polar. The oxygen side carries a partial negative charge, while the hydrogen side carries a partial positive charge. Water molecules can therefore orient themselves according to the charge distribution of ectoine and interact with it through ion–dipole interactions and hydrogen bonding.

Molecular simulations have shown that the charge distribution of ectoine plays an important role in forming a stable and well-defined hydration shell. When the charges of the model molecule are reduced, the accumulation and orientation of surrounding water molecules are also markedly weakened.[4]

2.2 A Dynamic Hydration Layer Does Not Contain a Fixed Number of Water Molecules

The relationship between ectoine and water can be represented schematically as follows:

Ectoine±···(H₂O)n

Here, Ectoine± represents ectoine in its zwitterionic form, while n represents the water molecules involved in local hydration. The value of n is not fixed and does not indicate the formation of a compound with a defined stoichiometric ratio.

Water molecules around ectoine continuously enter, leave, and change orientation. Different studies use different definitions for the first hydration shell, directly contacting water molecules, and water molecules influenced by the solute, so the reported hydration numbers may also differ.

Spectroscopic and molecular simulation studies indicate that ectoine primarily affects water structure within the local environment surrounding the molecule. It can form relatively strong ectoine–water interactions and alter the arrangement and exchange dynamics of hydrogen bonds among nearby water molecules, but it does not immobilize large amounts of water into a static, highly ordered structure.[4][5]

3. Preferential Exclusion and Preferential Hydration: How Ectoine Protects Biological Interfaces

3.1 Ectoine Is Hydrophilic, So Why Does It Not Accumulate Extensively on Protein Surfaces?

When proteins and biological membranes are present in an aqueous environment, polar and charged groups on their surfaces interact with water molecules. As a result, a continuously exchanging layer of interfacial water already exists at these surfaces.

When ectoine is added to the solution, water and ectoine are not distributed in exactly the same proportions at all locations. Studies have found that within the first hydration shell of proteins and membranes, the relative proportion of water is higher, while the proportion of ectoine is lower than that in the bulk solution.[6]

This is because ectoine itself is a highly hydrophilic zwitterion and can establish a favorable hydration environment in the aqueous phase. At the same time, its direct interactions with protein or membrane surfaces are not strong enough to extensively replace the water molecules already present at these interfaces. Therefore, once the system reaches equilibrium, the immediate interfaces of proteins and membranes remain predominantly occupied by water, while ectoine is distributed to a greater extent in the surrounding aqueous phase.

From the perspective of ectoine, this phenomenon is known as preferential exclusion; from the perspective of water, it is referred to as preferential hydration. These two terms describe the same interfacial distribution state rather than two sequential processes.

Neutron scattering and membrane diffraction studies have observed this phenomenon in both maltose-binding protein and purple membrane models: ectoine is relatively depleted from the first hydration shell of biological macromolecules, while the interface remains predominantly hydrated by water.[6]

3.2 Why Is This Interfacial Distribution Beneficial for the Stability of Biological Structures?

Proteins require appropriate interactions with their surrounding aqueous environment to maintain their normal structure and function. When ectoine is preferentially excluded, it does not extensively replace the water molecules at the protein surface, allowing the immediate protein interface to remain predominantly hydrated by water.[6]

For proteins, preferential exclusion also affects the stability balance between folded and unfolded states. A normally folded protein has a relatively compact structure and exposes a comparatively small surface area to the solution. When a protein unfolds, more structures that were originally buried within the protein become exposed to the solvent, increasing the solvent-accessible surface area.

Because direct contact between ectoine and the protein surface is relatively unfavorable from a thermodynamic perspective, the greater the exposed surface area of a protein, the more pronounced the effect of this unfavorable interaction on the free energy of that conformation. Consequently, the thermodynamically unfavorable effect of ectoine on the unfolded state is generally greater than that on the compact folded state, making the folded state relatively more stable. This is one of the important mechanisms by which preferentially excluded compatible solutes stabilize proteins.[6]

For biological membranes, the mode of action is somewhat different. Neutron diffraction studies have shown that ectoine likewise does not preferentially bind to proteins or lipids on model membrane surfaces, but is instead distributed mainly in the aqueous phase between membranes; the immediate hydration region at the membrane surface remains occupied predominantly by water.[6]

Therefore, the protective effects of ectoine on proteins and membranes do not depend on the formation of a physical coating layer, nor do they require extensive binding to or insertion into biological membranes. Rather, in an ectoine-containing aqueous environment, ectoine helps maintain a predominantly water-based hydration state at the immediate interfaces of biological macromolecules and, through the thermodynamic effects associated with preferential exclusion, promotes the relative stability of compact protein conformations.

4. How Does Ectoine Act on the Skin Stratum Corneum?

4.1 Water, Proteins, and Lipids in the Stratum Corneum

The skin stratum corneum consists of corneocytes and intercellular lipids:

Region of the Stratum Corneum	Main Components	Relationship to Skin Hydration
Inside corneocytes	Keratin, natural moisturizing factors, etc.	Affects the water content, flexibility, and mechanical state of corneocytes
Between corneocytes	Ceramides, cholesterol, and free fatty acids	Forms lamellar lipid structures that restrict the outward diffusion of water

Ectoine is not a stratum corneum lipid and therefore does not directly replenish ceramides or rebuild the intercellular lipid layers. Its effects related to skin hydration are reflected more in the state of keratin within corneocytes and in the hydration kinetics of the stratum corneum.[7]

4.2 Improving Keratin Hydration and Drying Stress

When the stratum corneum loses water, keratin bundles tend to aggregate, tissue volume contracts, and mechanical stress within the stratum corneum increases accordingly. This may contribute to sensations of tightness, roughness, and reduced flexibility of the skin.

An ex vivo human stratum corneum study found that ectoine treatment increased the dispersion and hydration of keratin bundles within corneocytes. In a stratum corneum model in which lipids had been partially extracted, ectoine reduced the rate of mechanical stress development during drying by approximately 30% and increased the rate of stress reduction during rehydration by approximately 20%.[7]

However, these effects were mainly reflected in the rate of stress changes during dehydration and rehydration of the stratum corneum. Under extremely dry conditions (<5% RH), the study observed a higher peak drying stress in the ectoine-treated group, indicating that its effects are influenced by environmental humidity and the extent of dehydration.[7]

This study reveals an important aspect of the moisturizing effects of ectoine:

The sensation of skin dryness is associated not only with water loss, but also with the aggregation, contraction, and stress changes of keratin during dehydration.

By improving the hydration state around keratin, ectoine allows the stratum corneum to respond more gradually to water loss and subsequent rehydration, which may help maintain skin flexibility and comfort.

5. Ectoine and Transepidermal Water Loss

5.1 The Difference Between Skin Water Content and TEWL

Transepidermal water loss (TEWL) refers to the flux of water moving from within the body through the epidermis and stratum corneum and ultimately evaporating into the external environment.

Skin water content and TEWL reflect two different aspects:

① Skin water content indicates how much water is retained in the stratum corneum at a given time;

② TEWL indicates the rate at which water diffuses outward through the stratum corneum;

③ Humectant ingredients primarily increase local hydration;

④ Occlusive ingredients primarily reduce water evaporation from the skin surface;

⑤ The lipid barrier affects the resistance to water diffusion through the stratum corneum.

5.2 Hydration and TEWL in Human Studies

A randomized, double-blind, vehicle-controlled study compared a lotion containing 2% ectoine with a vehicle lotion and untreated areas. After four weeks of continuous use, the increase in stratum corneum hydration was greater in areas treated with the ectoine-containing lotion than in those treated with the vehicle lotion, indicating that formulations containing ectoine can further improve skin hydration.[8]

Regarding TEWL, an exploratory trial involving five volunteers first applied lotions containing 0%, 2%, or 5% ectoine continuously, followed by sodium dodecyl sulfate (SDS) challenge to disrupt the skin barrier. The increase in TEWL was lower in areas treated with ectoine-containing formulations, suggesting that such formulations may improve the skin's tolerance to surfactant-induced irritation.[9]

Existing studies provide relatively substantial support for the ability of ectoine to improve skin water content, whereas TEWL data more often reflect barrier-protective effects under conditions of induced irritation. Ectoine itself is not a highly occlusive film-forming agent, and its effects on TEWL may result from the combined contributions of keratin hydration, the mechanical state of the stratum corneum, and the complete lotion formulation.

6. Formulation Design with Ectoine

Different moisturizing and barrier-supporting ingredients act at different levels of the stratum corneum. Formulation strategies involving ectoine can therefore be designed around four aspects: “increasing water availability, regulating hydration, replenishing lipids, and reducing evaporation.”

Ingredient System	Main Function	Complementary Role with Ectoine
Humectants such as hyaluronic acid and glycerol	Increase water content at the skin surface and create a hydrophilic environment	Help attract and retain water, while ectoine further regulates the local hydration state
Ectoine	Forms a dynamic hydration layer and regulates the hydration of keratin and biological interfaces	Links water retention with structural stability of the stratum corneum
Ceramides, cholesterol, and free fatty acids	Support the intercellular lamellar lipid structure	Replenish barrier lipids that ectoine cannot directly provide
Oils, silicones, or other film-forming ingredients	Reduce water evaporation from the skin surface	Prolong the moisturizing effects of humectant and hydration systems

6.1 Ectoine and Hyaluronic Acid

Hyaluronic acid (HA) is a hydrophilic polysaccharide. When applied topically, it can form a hydrophilic, hydrated film on the skin surface, increasing water content and lubrication at the surface of the stratum corneum. Human studies have also observed increases in measured skin hydration following the use of hyaluronic acid-containing serums.[10]

The respective roles of hyaluronic acid and ectoine can be summarized as follows:

① Hyaluronic acid improves the ability of the skin surface to retain water;

② Ectoine regulates hydration at the molecular scale and influences changes in the state of keratin during dehydration and rehydration.

A study in children with mild-to-moderate atopic dermatitis used a combination cream containing 1% ectoine and 0.1% hyaluronic acid. After four weeks of continuous use, the cream showed better clinical symptom scores than the vehicle cream.[11]

This study demonstrates that ectoine and hyaluronic acid can be incorporated into a complete moisturizing skincare formulation. Because the test product also contained other emollient and soothing ingredients, the study results reflect the overall effect of the formulation rather than a simple additive effect of the two individual ingredients.

6.2 Ectoine and Ceramide Systems

Ceramides, cholesterol, and free fatty acids are major components of the intercellular lipids of the stratum corneum. Together, these three lipid classes form an ordered lamellar structure that restricts the passage of water and external substances through the stratum corneum. Studies using barrier-damage models have shown that complete mixtures of physiological lipids facilitate barrier recovery, and that the ratios among these lipids can also influence the recovery process.[12]

Ectoine and ceramide systems act on different structural components:

① Ectoine primarily targets keratin hydration, the dynamic hydration environment, and the regulation of drying stress;

② Ceramides, cholesterol, and free fatty acids primarily support the intercellular lipid structure;

③ Appropriate oils or film-forming ingredients further reduce water evaporation from the skin surface.

Therefore, combining ectoine with a ceramide-based system can simultaneously address protein hydration within corneocytes and the lipid barrier between corneocytes. The formulation value of this combination lies in the complementarity of their sites and modes of action.

7. Representative Chemicals Related to Ectoine-Mediated Skin Hydration and Barrier-Support Mechanisms

Table 1. Products Related to Compatible Solutes and Hydration Regulation

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Zwitterionic compatible solute	107-43-7	B105556	Betaine, anhydrous	Moligand™, ultrapure grade, ≥99%	A zwitterionic compatible solute that can be used in studies of osmoregulation, hydration layers, protein stabilization, and comparative research with ectoine
Ectoine derivative	165542-15-4	H344938	(4S,5S)-1,4,5,6-Tetrahydro-5-hydroxy-2-methyl-4-pyrimidinecarboxylic acid	Moligand™, ≥95%	A hydroxylated derivative of ectoine that can be used in studies of local hydration, preferential exclusion, membrane interfaces, and protection against osmotic stress
Core compatible solute	96702-03-3	E427171	Ectoine	10 mM in Water	Ready-to-use aqueous ectoine solution for hydration, osmoregulation, and stress-protection experiments in cell, protein, and membrane models
Saccharide protective solute	99-20-7	D110019	D-Trehalose, anhydrous	≥99%	A non-reducing disaccharide that can be used in studies of dehydration and rehydration, protein stabilization, membrane protection, and hydration-protection controls
Core compatible solute	96702-03-3	E292674	Ectoine	≥99%	High-purity ectoine for studies of zwitterionic hydration, preferential exclusion, protein and membrane interfaces, and mechanisms of skin hydration

Table 2. Products Related to Stratum Corneum Humectancy, Moisturization, and Hydration

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Natural moisturizing factor-related ingredient	28874-51-3	L303332	Sodium L-pyrrolidone-5-carboxylate	50%, oily	A hydrophilic ingredient related to natural moisturizing factors that can be used in studies of stratum corneum humectancy, water retention, and moisturizing systems
Polyol humectant	56-81-5	G755728	Glycerol	Anhydrous grade, UltraBio™, molecular biology grade, ≥99.5% (GC)	A classic polyol humectant that can be used in studies of stratum corneum hydration, water-activity regulation, and osmotic environments in cell culture
Natural moisturizing factor-related ingredient	57-13-6	U111902	Urea	UltraBio™, molecular biology grade, ≥99.5% (T)	A natural moisturizing factor-related ingredient that can be used in studies of stratum corneum hydration, protein–water interactions, and keratin status under different concentration conditions
Polymeric moisturizing ingredient	9067-32-7	S774025	Sodium hyaluronate	European Pharmacopoeia (Ph. Eur.)	A hydrophilic polymeric salt that can be used in studies of hydration networks, water retention, rheological properties, and moisturizing systems formulated with ectoine
Polymeric moisturizing ingredient	9004-61-9	H131007	Hyaluronic acid	Moligand™, from rooster comb	A hydrophilic polymer that can be used in studies of hydration networks, water retention, film formation, and formulation systems combining ectoine
Moisturizing and barrier-support ingredient	81-13-0	P107368	D-Panthenol	≥98%	A provitamin-related ingredient with humectant and skin barrier-supporting properties that can be used in studies of skin hydration, stratum corneum condition, and combination formulations

Table 3. Stratum Corneum Barrier Lipids, Related Structural Lipids, and Surface-Protective Products

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Stratum corneum barrier lipid	57-88-5	C104036	Cholesterol	For cell culture, ≥99% (GC)	An important component of stratum corneum intercellular lipids that can be used together with ceramides and free fatty acids in studies of lamellar lipid structures and skin barrier models
Free fatty acid	57-10-3	P753896	Palmitic acid	Stearic acid ≤0.5%	A saturated long-chain fatty acid that can be used in studies of stratum corneum free-fatty-acid composition, intercellular lipid organization, and barrier models
Free fatty acid	57-11-4	S298768	Stearic acid	Moligand™, C18: 65%	A long-chain saturated fatty acid that can be used in studies of stratum corneum lipid composition, lamellar lipid systems, and barrier-related models
Free fatty acid	60-33-3	L100441	Linoleic acid	Moligand™, ≥99% (GC)	An essential polyunsaturated fatty acid that can be used in studies of acylceramide formation, stratum corneum lipid metabolism, and barrier structure
Ceramide lipid	2304-81-6	N130622	N-Stearoyl-D-erythro-sphingosine	≥99%	A structurally defined ceramide lipid that can be used in studies of stratum corneum lipid models, membrane systems, ceramide quantification, and analytical methods
Ceramide-backbone-related ingredient	554-62-1	D136389	Glycolipid (D-ribo-phytosphingosine)	≥98%	A molecule related to the long-chain base backbone of ceramides that can be used in studies of ceramide metabolism, stratum corneum lipid composition, and skin barrier models
Ceramide barrier lipid	178436-06-1	C995092	Ceramide 3B	≥95%	A phytosphingosine-type ceramide that can be used in studies of stratum corneum intercellular lipids, lamellar structures, membrane models, and barrier function
Emollient lipid	111-01-3	S141335	Squalane	≥98%	A stable hydrocarbon emollient that can be used in studies of skin-surface lubrication, oil-phase systems, and control formulations for hydration and barrier research
Occlusive base	8009-03-8	V105023	White petrolatum	PharmPure™, ChP	An occlusive hydrocarbon base that can be used in studies of water evaporation from the skin surface, barrier protection, and transepidermal water loss controls

Note: The products listed above are representative Aladdin products relevant to scientific research. Specific applications should be determined according to product specifications, batch COAs, and the intended reaction or evaluation system. Additional product specifications, grades, and COA information can be searched on the Aladdin official website using the “product name/CAS/catalog number.”

References

[1] Imhoff JF, Trüper HG. Ectothiorhodospira halochloris sp. nov., a new extremely halophilic phototrophic bacterium containing bacteriochlorophyll b. Archives of Microbiology. 1977;114:115-121. doi:10.1007/BF00410772.

[2] Galinski EA, Pfeiffer HP, Trüper HG. 1,4,5,6-Tetrahydro-2-methyl-4-pyrimidinecarboxylic acid: A novel cyclic amino acid from halophilic phototrophic bacteria of the genus Ectothiorhodospira. European Journal of Biochemistry. 1985;149(1):135-139. doi:10.1111/j.1432-1033.1985.tb08903.x.

[3] Hobmeier K, Oppermann M, Stasinski N, et al. Metabolic engineering of Halomonas elongata: Ectoine secretion is increased by demand and supply driven approaches. Frontiers in Microbiology. 2022;13:968983. doi:10.3389/fmicb.2022.968983.

[4] Smiatek J, Harishchandra RK, Rubner O, Galla HJ, Heuer A. Properties of compatible solutes in aqueous solution. Biophysical Chemistry. 2012;160:62-68. doi:10.1016/j.bpc.2011.09.007.

[5] Sahle CJ, Schroer MA, Jeffries CM, Niskanen J. Hydration in aqueous solutions of ectoine and hydroxyectoine. Physical Chemistry Chemical Physics. 2018;20(44):27917-27923. doi:10.1039/C8CP05308A.

[6] Zaccai G, Bagyan I, Combet J, et al. Neutrons describe ectoine effects on water H-bonding and hydration around a soluble protein and a cell membrane. Scientific Reports. 2016;6:31434. doi:10.1038/srep31434.

[7] Bow JR, Sonoki Y, Uchiyama M, Dauskardt RH. Ectoine disperses keratin and alters hydration kinetics in stratum corneum. Biochemistry and Biophysics Reports. 2021;28:101134. doi:10.1016/j.bbrep.2021.101134.

[8] Heinrich U, Garbe B, Tronnier H. In vivo assessment of Ectoin: A randomized, vehicle-controlled clinical trial. Skin Pharmacology and Physiology. 2007;20(4):211-218. doi:10.1159/000103204.

[9] Graf R, Anzali S, Buenger J, Pfluecker F, Driller H. The multifunctional role of ectoine as a natural cell protectant. Clinics in Dermatology. 2008;26(4):326-333. doi:10.1016/j.clindermatol.2008.01.002.

[10] Draelos ZD, Diaz I, Namkoong J, Wu J, Boyd T. Efficacy evaluation of a topical hyaluronic acid serum in facial photoaging. Dermatology and Therapy. 2021;11:1385-1394. doi:10.1007/s13555-021-00566-0.

[11] Alexopoulos A, Dakoutrou M, Nasi L, et al. A randomized, observer-blind, vehicle-control, multi-center clinical investigation for assessing the efficacy and tolerability of a 1% ectoine and hyaluronic acid 0.1%-containing medical device in pediatric patients with mild-to-moderate atopic dermatitis. Pediatric Dermatology. 2023;40(1):78-83. doi:10.1111/pde.15117.

[12] Man MQ, Feingold KR, Thornfeldt CR, Elias PM. Optimization of physiological lipid mixtures for barrier repair. Journal of Investigative Dermatology. 1996;106(5):1096-1101. doi:10.1111/1523-1747.ep12340135.

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