MES Buffer 1M, pH 6.5, also referred to as 4-Morpholineethanesulfonic acid, is a zwitterionic buffer primarily used as a running buffer for resolving very small proteins on bis-tris gels. MES is often known as one of the original Good's buffers. MES solution has a pKa value near physiological pH, it is not absorbed through cell membranes, and it is essentially transparent to UV light. Also used in cation-exchange chromatography and as a biological buffer in plant cell cultures, MES solutions can be prepared to the desired pH by titrating the free acid with sodium hydroxide.