DTT is an unusually strong reducing agent, because once oxidized, it forms a stable six-membered ring with an internal disulfide bond. The reduction usually does not stop at the mixed-disulfide species because the second thiol of DTT has a high propensity to close the ring, forming oxidized DTT and leaving behind a reduced disulfide bond. The reducing power of DTT is limited to pH values above 7, since only the negatively charged thiolate form -S- is reactive (the protonated thiol form -SH is not), the pKa of the thiol groups is 9.2 and 10.1.