MOPS (3-(N-morpholino)propanesulfonic acid) is a buffer introduced by Good et al. in the 1960s. It is a structural analog to MES. Its chemical structure contains a morpholine ring. HEPES is a similar pH buffering compound that contains a piperazine ring. With a pKa of 7.20, MOPS is an excellent buffer for many biological systems at near-neutral pH.