1,5-I-AEDANS [ 1,5-IAEDANS,5-(2-iodoacetamidoethyl)aminonaphthalene-1-sulfonate,N-iodoacetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine ] fluorescent probes which are sulfhydryl reagents; combine reactivity of iodoacetamide toward sulfhydryl groups with spectral properties of naphthalenesulfonic acids; structure