Solutions and their problems: plan ahead!

Today we encountered the use of ammonium sulphate (or sulfate as it is now often written)for the salting out (or precipitation of proteins from complex mixtures. The first thing I want to comment on is planning and its importance in experimental work. We had a visitor from the business world who immediately understood why I had asked the group to stop, think and plan ahead before "jumping in". This is not only important in laboratory science, but is a key transferable skill, whatever you do. Our visitor's delight that this formed part of the lab classes was apparent from the big smile on his face! Please, write out a plan before you begin.

Secondly, you learnt today that ammonium sulphate is highly soluble and that in order to weigh out a few grams, it is a good idea to grind the salt before weighing. This is something I learnt in the first few weeks of my PhD and I was pleased to see everyone carrying this out without making a mess of the balance area. Really well done.

Dissolving the salt proved valuable experience, since you have probably never made up solutions from scratch. This is a basic skill and one that you have to make mistakes before you realise how best to proceed and how much you need to concentrate to ensure you get the solution to the appropriate concentration.

The use of increasing concentrations (or saturation) of the protein mixture (in this case milk (both skimmed and full),followed by high speed centrifugation was a real success and I hope it made you appreciate the power of such a simple method for concentrating proteins. Those who have yet to repeat the experiment on the full fat milk, should think about the result you might obtain compared with the skimmed milk samples.

This simple method is an industry standard and one that is less widely used in Molecular Biology Labs, but I hope you recognised the power of such a simple method today.

Key words: Ammonium sulphate, salting out, protein precipitation