Bacteria rejoice: study identifies safe solvents.
Wood, N, JL Ferguson, HQ Nimal Gunaratne, KR Seddon, R Goodacre and GM Stephens. 2011. Screening ionic liquids for use in biotransformations with whole microbial cells. Green Chemistry http://dx.doi.org/10.1039/c0gc00579g.
Chemists took a first step towards designing a more environmentally-friendly solvent known as ionic liquids by identifying those that won’t kill the bacteria used to transform raw materials into useable products.
For the first time, a large number of the new type of solvents were tested on the E. coli bacteria strains that are widely used in industry. Many of them did not harm the bacteria and may be hopeful candidates for industry.
Ionic liquids’ popularity is growing as researchers eye them to replace toxic, smelly and polluting organic solvents currently used to produce all manner of chemicals and consumer products.
To make chemicals, solvents are commonly required.Solvents dissolve gases, solids or liquids into a solution. They can help to bring the reacting chemicals in contact, separate the products from reactants and sometimes even speed up the chemical reaction. Unfortunately, many solvents are toxic, volatile – they evaporate quickly – and/or flammable. It is therefore important to perform reactions without solvents, if possible, or use more benign solvents.
Ionic liquids are a relatively new class of solvents. They are salts – a designation for chemicals made up of both a positively and a negatively charged component– that are liquid at low temperatures, unlike common salts such as kitchen salt. Bulky positive and negative groups that make up the ionic salts hinder their packing into a solid crystal. Thus, they stay liquid to much lower temperatures – even room temperature.
People are interested in using ionic liquids because they have extremely low volatility resulting in virtually no vapor emissions. The properties of these ionic liquids (solvating power, melting point, water solubility) can also be easily varied by changing the positive and negative groups.
Researchers wish to use ionic liquids for industrial biotransformation – a process in which bacteria produce chemical products. The most commonly known biotransformation is sugar fermenting into ethanol – the way beer and wine are produced. However, fuel from renewable resources – i.e. biofuel – and even valuable chemicals can also be made this way.
Biotransformation has the potential to have a much lower impact on the environment than pure chemical reactions.
In this work by researchers from the United Kingdom, the toxicity of 90 different ionic liquids was tested in the laboratory using the bacterium Escheria coli (E. coli).
E. coli regularly receives negative press attention because it can cause food poisoning in people. However, it is also used frequently in industrial biotransformation
The major problem in biotransformations is that the produced chemicals are sometimes toxic to the bacteria. So as the bacteria become more efficient at producing the wanted product, they become less efficient because they die. And this is where ionic liquids can play an important role. By using an ionic liquid in which the reaction product, which is toxic to the bacteria, is soluble, the product could be extracted from the aqueous broth of bacteria before it accumulates too much, becomes too toxic and kills the bacteria.
This plan requires ionic liquids that are not toxic to the bacteria. The work described here begins to identify the nontoxic varieties by looking at the effect of the different ionic liquids on cell viability and growth rates. The researchers examined in high throughput tests ionic fluids that can dissolve (miscible) or remain undissolved (immiscible) in water.
Various ionic liquids were found to be non-toxic towards E. coli. There were both water miscible and immiscible ionic liquids that did not show toxicity. Importantly, the ionic liquids displayed similar toxic behavior based on their specific negative and positive chemical groups.
With this information, it is possible to start to predict the toxicity of ionic liquids towards E. coli if they contain the same charged groups as those used in this study. Surprisingly, it was also possible to prepare a type of ionic liquid called quaternary ammonium salt ionic liquids that were non-toxic to E. coli by changing the negative ion attached to it. This is a remarkable result since quaternary ammonium salts are being widely investigated for use as antibacterial agents on hard surfaces.
Many types of ionic liquids were compatible with the bacteria E. coli and could be candidates for industrial uses that transform starting materials into products.
The results are very important because the toxicity trends prove that it is possible to start to rationally design ionic liquids.
The study shows it is possible to make a certain ionic liquid that has the toxicity effect that is required by choosing the positive and negative ions that it is made from. It is already possible to adjust physical parameters, such as melting point, freezing point and solvating power, i.e. the amount of a specific chemical that can be dissolved. This work thus adds another parameter that can be manipulated, adding another benefit to the use of ionic liquids.
This work is the first in a range of studies that will need to be performed to isolate which ionic liquids will perform the best. While 90 ionic liquids is a large number, more ionic liquids will need to be studied to help form a much better idea of their toxicity towards E. coli and other bacteria useful to industry.
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