Methods in Studying Proteins
Electophoresis
Electrophoresis is a standard way of isolating and characterizing proteins. It runs a protein sample through a solid matrix (which is usually polyacrylamide gel) and uses an electric field to facilitate the movement of the protein through the polyacrylamide gel.
You can imagine the polyacrylamide gel as a sort of molecular strainer, wherein the ease by which proteins or amino acids run through them depends on the size of the sample. Like if the sample is large it will have a harder time to pass through the "holes" created by the polyacrylamide gel.
On the other hand, the direction with which the sample (protein or amino acid) travels depends on the sample's charge. If the protein/aa is positively charged it will go towards the negative electrode, whereas if the protein/aa is negatively charged it will migrate towards the positive electrode.
Hence, generally speaking, polyacralyamide gel electrophoresis (PAGE) can isolate and characterize proteins based on size and charge.
However, there are variations to this method, one of which is the sodium dodecyl sulfate PAGE (SDS-PAGE), wherein a detergent is used to linearize the protein and to coat it with a negative charge. This renders the inherent charge of the proteins useless because all of them will now have a uniform negative charge, hence the separation will now depend just on size.
For more information on SDS-PAGE, please go to this link and read up on it.