You hate the smell of mercaptoethanol. since there are no disulfide bonds in intracellular proteins, you have convinced yourself that it is not necessary to treat a cytoplasmic homogenate with mercaptoethanol prior to sds-page. you heat a sample of your homogenate in sds and subject it to electrophoresis. much to your surprise, your gel looks horrible; it is an ugly smear! you show your results to your friend who is a chemistry major, and she suggests that you treat your sample with n-ethylmaleimide (nem), which reacts with free sulfhydryls. you run another sample of your cytoplasmic homogenate after treatment with nem and sds. now the gel looks perfect! if intracellular proteins don't have disulfide bonds (and they don't) why didn't your original scheme work? and how does treatment with nem correct the problem?