PIPES (pKa = 6.80) is commonly used as a buffer for retention of actin filaments during fixation. The StockOptions buffer stock reagents are supplied as 1.0 M stock solutions in 10 milliliter volumes. The pKa of a buffer is commonly perceived as the pH of the said buffer when the concentrations of the two buffering species are equal, and where the maximum buffering capacity is achieved. It is a conjugate acid of a 3-(N-morpholino)propanesulfonate. Each StockOptions MOPS buffer … StockOptions MOPS buffer kit is a preformulated, sterile filtered set of titrated buffer stocks. MES has a lower pKa than MOPS, making the MES SDS Running Buffer faster than MOPS SDS running buffers. However, it is often forgotten, that when defined as above, pKa depends on buffer concentration and temperature. Use of MOPS buffer allows proteins to run slower than when using MES buffer. It is a member of morpholines, a MOPS and an organosulfonic acid. The difference in ion migration affects stacking and results in a difference in protein separation range between these buffers. MOPS pKa 7.2; Description. It is a tautomer of a 3-(N-morpholiniumyl)propanesulfonate. 3-(N-morpholino)propanesulfonic acid is a Good's buffer substance, pKa = 7.2 at 20 ℃. Other useful biological buffers include HEPES (pKa = 7.55), MES (pKa = 6.15), and MOPS (pKa = 7.20) (Good, et al., 1966; Perrin and Dempsey, 1974).