Rb₁ is selected from hydrogen, —(CH₂)_1-3—C(O) OH, —(CH₂)_1-3—C(O)O(C₁-C₃ alkyl), —C(O)—[CH(R^A)]_1-2—NH—R^B, and —C(O)—[CH(R^A)]_1-2—NH—C(O)—[CH(R^A)]_1-2—NH—R^B;

each R₇ and each R₈ is independently selected from —H, —NH₂, —NR_aR_b—C(O)NH₂, —C(O)NR_aR_b, —(C(R_x)(R_y))_nNH₂, —(C(R_x)(R_y))_n NR_aR_b, —(C(R_a)(R_b))_nC(O)NH₂, —(C(R_a)(R_b))_nC(O)NR_aR_b, —OH, —(C(R_a)(R_b))_nOH, —CO₂H, —(C(R_a)(R_b))nCO₂H, —SO₃H, —(C(R_a)(R_b))_nSO₃H, deuterium, halogen, alkyl, alkoxy, alkenyl, alkynyl, cyano, cycloalkyl, heterocycloalkyl, aryl, and heteroaryl;

each R_a is hydrogen or optionally substituted alkyl;

each R_b is selected from hydrogen, alkyl substituted with a carboxyl or a carboxylate, and an amino acid or a dipeptide, wherein the amino acid or the dipeptide is bound to the nitrogen atom in R₃ through a carboxy group; or R_a and R_b are taken together to form an optionally substituted heterocyclyl;

each R_x and R_y are independently a hydrogen or a C₁-C₆ alkyl group

n is 1 or 2;

each R^A is independently selected from hydrogen or a side group of a natural or unnatural amino acid; and

R^B is selected from hydrogen and a protecting group.