Compounds for affinity chromatography and for extending the half-life of a therapeutic agent

A compound according to formula Ia as shown below,

• or a salt thereof, wherein:
• R1 is H, —Cl, —F, —Br, —I, —OH, —CN, —NO2, substituted or unsubstituted C1-C6 alkyl, substituted or unsubstituted C2-C6 alkenyl, substituted or unsubstituted cycloalkyl, aryl or heteroaryl;
• R3 is —[(CH2)2E]n(CH2)2NR′R″ wherein n is any integer from 0 through 30, R′ and R″ are each independently H, substituted or unsubstituted C1-C6 alkyl, substituted or unsubstituted C2-C6 alkenyl, or substituted or unsubstituted cycloalkyl, and E is CH2, O, NH or S, or E is absent;
• R4 is H, —Cl, —F, —Br, —I, —OH, —CN, —NO2, substituted or unsubstituted C1-C6 alkyl, substituted or unsubstituted C2-C6 alkenyl, substituted or unsubstituted cycloalkyl, aryl or heteroaryl;
• Y1 and Y3 are independently C, O, N, NR2 or S, wherein R2 is H, substituted or unsubstituted C1-C6 alkyl, substituted or unsubstituted C2-C6 alkenyl, substituted or unsubstituted cycloalkyl, aryl or heteroaryl;
• y2 is C or N;
• A and D are independently
• amidine, thioamide, or A and/or D is absent;
• B is substituted or unsubstituted C4-C9-cycloalkyl, substituted or unsubstituted C4-C9-heterocycloalkyl comprising N, substituted or unsubstituted heteroaryl comprising N, S or O, or B is absent;
• and
• Z is substituted or unsubstituted aryl or substituted or unsubstituted heteroaryl comprising N, O or S, or Z is absent.