Rhamnolipid
- Also known as
- RL
A rhamnolipid is a glycolipid biosurfactant made mainly by Pseudomonas bacteria. Each molecule joins one or two units of the sugar rhamnose to one or two hydroxylated fatty acids, giving mono-rhamnolipids and di-rhamnolipids.
What is a rhamnolipid?
A rhamnolipid is a biosurfactant in the glycolipid family. It was first identified in the bacterium Pseudomonas aeruginosa, which remains the best-studied producer, and it is one of the two most commercially developed biosurfactants alongside the sophorolipid.
Structure
A rhamnolipid has a sugar head group and a fatty tail:
- The head group is one or two units of rhamnose, a sugar. Molecules with one unit are mono-rhamnolipids; those with two are di-rhamnolipids.
- The tail is usually two linked hydroxy fatty acids, most commonly each with about ten carbons.
The sugar makes the head water-attracting; the fatty acid chains are water-repelling. The tail ends in a carboxylic acid group, so rhamnolipids become negatively charged as pH rises and behave much like anionic surfactants in neutral and alkaline water. The producing bacteria make a mixture of variants, and the ratio of mono- to di-rhamnolipids and the chain lengths affect solubility, foaming and surface activity.
How rhamnolipids are produced
Rhamnolipids are made by bacterial fermentation, usually on plant oils, sugars or both. A practical complication is that Pseudomonas aeruginosa is an opportunistic human pathogen, which raises safety and regulatory questions for large-scale production. Much of the work toward commercial production has therefore focused on producing rhamnolipids in non-pathogenic host organisms that have been engineered to make them, alongside improvements in yield and recovery.
Properties and uses
Rhamnolipids lower surface tension and interfacial tension, form micelles at low concentrations, and are good emulsifiers and cleaners. They are used or being developed for household and industrial cleaning, personal care, agriculture, oil recovery, including microbial enhanced oil recovery, and the cleanup of soil contaminated by hydrocarbons and metals.
Rhamnolipids and sophorolipids
| Rhamnolipid | Sophorolipid | |
|---|---|---|
| Producing organism | Bacteria, mainly Pseudomonas species | Yeast, mainly Starmerella bombicola |
| Sugar head group | One or two rhamnose units | Sophorose (two glucose units) |
| Main forms | Mono- and di-rhamnolipids | Acidic and lactonic |
| Charge behavior | Anionic above neutral pH | Largely nonionic; the acidic form can carry a charge |
Frequently asked questions
The classic producer is the bacterium Pseudomonas aeruginosa. Other bacteria, including some Burkholderia species, make related molecules, and engineered strains of non-pathogenic bacteria are used to produce rhamnolipids without handling a pathogen.
The number of rhamnose sugar units in the head group. Mono-rhamnolipids have one; di-rhamnolipids have two. The second sugar makes the molecule more water-attracting, which changes its solubility and how it behaves at interfaces.
They are used as surfactants in cleaning and personal care products, in agricultural formulations, in oil recovery and in environmental remediation, where they help mobilize hydrocarbons and metals from soil.
Sources
- Rhamnolipids rise as a green surfactant, Chemical & Engineering News, American Chemical Society, 2020
- Rhamnolipid the glycolipid biosurfactant: emerging trends and promising strategies, Peer-reviewed review, PMC7784359
- Sophorolipids and rhamnolipids as a biosurfactant: synthesis and applications, Peer-reviewed review (2021)