Irreversible enzyme inhibitors. CXXXIX. p-(4,6-Diamino-1,2-dihydro-2,2-dimethyl-s-triazin-1-yl)phenylpropionylsulfanilyl fluoride, an active-site-directed irreversible inhibitor of dihydrofolic reductase. 6. Effects of substitution on the propionamide bridge on isozyme specificity