Disulfide controls S1-S3 were nearly inert to TrxR1 (Fig. ); though piperazinamide sidechain 1 gave faster reaction speeds compared to less soluble methyl and morpholinamide sidechains, presumably due to hydrophobic adsorptive loss of the intact probes as well as local pH modulation. This effect was often seen later with Aand G-type probes, and supported the choice of piperazinamides as the standard sidechain for the probes. Diselenide control Se3 was completely nonresponsive to TrxR1, as it later proved also to be to all other reductants except to DTT (Fig. , Fig. ).