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