Synthesis of 5-lodo- and 5-nitro-3-indolyl phosphates as cytochemical substrates for acid phosphatase

The synthesis of 5-iodo-3-indolyl and 5-nitro-3-indolyl phosphate for cytochemical demonstration of acid phosphatase is reported. Enzymatic hydrolysis of these compounds liberates the corresponding indoxyl which is oxidized to the substituted indigo at an acid pH. In contrast, the 5-bromo and the unsubstituted indoxyl are not oxidized readily at an acid pH. Comparison of the UV and visible, spectra of the disubstituted indigo has offered a partial explanation for this difference. E.S.R. measurements showed that 5,5′-diiodoindigo, unlike 5,5′-dibromoindigo, gives a detectable radical ion at an acid pH.