Sulfate precipitation involves adding a sulfate salt to a solution to extract proteins, using either “salting” or cold saline solutions. Precipitation is the opposite of dissolution, where solids form from a solution. More reactive materials displace less reactive ones, causing the protein to solidify. Two sulfate precipitation methods are used, salting and cold ammonium sulfate solutions.
Sulfate precipitation occurs when a precipitation reaction is caused by adding a sulfate salt to a given solution. Most of these precipitation reactions involve extracting proteins from a solution. Two main methods are used in sulfate precipitation, one of which is referred to as “salting” and involves adding excessive amounts of salt to a solution to precipitate the protein. The other uses cold saline solutions of decreasing concentration followed by gradual heating to crystallize the proteins.
Chemical reactions take many different forms. An example of a chemical reaction is dissolution, commonly experienced when sugar is added to coffee. Precipitation can be thought of as the opposite of this, as solids form from a solution and “precipitate” into the solution. This occurs when two different soluble solutions are combined inside a container to create a non-soluble substance. No matter is lost in a chemical reaction, rather a more reactive chemical displaces a less reactive one and leaves a solid as a byproduct of that reaction.
To properly understand sulfate precipitation, it is necessary to understand what actually happens at the atomic level during the dissolution reaction. When sugar is added to water, the sugar molecules bind to those in the water, of which there are always a finite number. Eventually, if a person kept adding sugar to water, the solution would be completely saturated, meaning there would be no water molecules left to bind to the sugar molecules and cause them to dissolve. This means that any sugar added after this point will not dissolve and will simply sit at the bottom of the cup.
During the precipitation of sulfate, essentially the same thing happens, except there is another compound besides the sulfate salt jostling for position with the water molecules. More reactive materials can displace bonds formed by less reactive ones. This means that the salt, which is usually ammonium sulfate when proteins are extracted, takes the place of the protein in the solution and the protein solidifies like sugar in the bottom of a cup. Since the protein had already been dissolved in the water, it appears to form out of nowhere and precipitates into the container as a solid.
The simplest method used to extract protein in this way is referred to as “salting.” This is essentially the same as the reaction described above, in that excessive amounts of salt are added to a solution to cause the protein to solidify. A second sulfate precipitation method is also used, whereby proteins are extracted using concentrated ammonium sulfate solution, and then cold ammonium sulfate solutions are added to isolate proteins that are poorly soluble at concentrations higher. The cold solutions gradually decrease in concentration and the entire solution is slowly heated to crystallize the proteins.
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