Isoelectric focusing
	The charge on a protein is determined by the sum of the charges on the individual amino acid side chains.  Each side chain has a different charge at any particular pH.  For example, at pH 4, aspartate is neutral, cysteine is neutral, and lysine is positive; the tripeptide DCK is positively charged. At pH 10, R is negative, C is negative, K is almost neutral, and DCK is negatively charged. Each  polypeptide is neutral at a unique pH, its isoelectric point.