Avidin derivatives and uses thereof

A compound of the formula I: ##STR1## wherein R is H or --N.dbd.N-2-carboxyphenyl; A is (CH.sub.2).sub.n or --CH.dbd.CH--, wherein n is an integer from 0 to 10, or A may also be --CH(COOH)-- when R is --N.dbd.N-2-carboxyphenyl; and X is a radical selected from the group consisting of: (i) Cl; (ii) COOR.sub.1, wherein R.sub.1 is p-nitrophenyl or N-succinimidyl; (iii) CONH--NHR.sub.2, wherein R.sub.2 is H, COO(t-butyl) or COObenzyl; (iv) CONH--[B]--NHR.sub.3, wherein R.sub.3 is H, COOR.sub.1, or CO--[B']--maleimido, wherein R.sub.1 is t-butyl, p-nitrophenyl or N-succinimidyl, and B and B', the same or different, are (CH.sub.2).sub.n wherein n is an integer from 2 to 10; (v) CONH--[B]--COOR.sub.4, wherein R.sub.4 is H, C.sub.1 -C.sub.8 alkyl, N-succinimidyl; (vi) CONH--[B]--OH; (vii) CONH--[B]--CONH--NHR.sub.2, wherein R.sub.2 is H, COO(t-butyl) or COObenzyl; and (viii) NHR.sub.2, wherein R.sub.2 is H, COO(t-butyl) or COObenzyl, when A is --CH(COOH)-- and R is --N.dbd.N-2-carboxyphenyl. The 4'-hydroxyazobenzene-2-carboxylic acid (HABA) compounds are novel reagents for labeling, isolation and detection of biological molecules.