A compound represented by Formula I: or a salt thereof, wherein: A 1 is a 5-6 membered heteroaryl, optionally substituted with 1, 2, or 3 substituents independently selected from the group consisting of alkyl, haloalkyl, halogen, hydroxyl, alkoxy, -CO 2 H, -CO 2, -CO 2 -(optionally substituted heterocycloalkyl), -C N(R 6)(R 7), - N(R 6)C (R 7), alkylene-(optionally substituted heterocyclyl), nitro, alkylene-O-alkylene-CO 2 H, alkylene-O-alkylene-CO 2 -, -SO 2 -N(R 6)-alkylene-CO 2 H, -SO 2 -N(R 6)-alkylene-CO 2 -, -N(R 6)-SO 2 -alkylene-CO 2 H, -N(R 6)-SO 2 -alkylene-CO 2 -, -SO 2 -N(R 6)-(optionally substituted heterocycloalkyl), -SO 2 -N(R 6) 2, -SO 2 -N(R 6)-alkylene-(optionally substituted heterocyclyl), X 1, and alkylene-X 1; and wherein: X 1 represents independently for each occurrence an ester, a succinimidyl ester, a carboxamide, propargyl, azidoalkyl, isothiocyanate, -NH 2, -OH, -SH, -SO 3 H, carboxyl, -C Cl, - O R 8, -CON NH 2, an acetoxymethyl ester, a substituted or unsubstituted N-hydroxysuccinimidyl ester, a substituted or unsubstituted N-hydroxysulfosuccinimido ester, a nitrophenyl ester, a fluorophenyl ester, alkyne, azide, hydrazide, alkoxylamine, -NCS, -CHO, - COCH 2 I, a phosphoramidite, a phthalamido, or a maleimide; R 1 and R 2 each represent independently hydrogen, alkyl, cycloalkyl, heterocycloalkyl, or -C N(R 6)(optionally substituted alkyl); or R 1 and R 2 are taken together with the nitrogen atom to which they are attached to form a monocyclic or bicyclic ring; R 3 and R 4 each represent independently hydrogen, alkyl, cycloalkyl, heterocycloalkyl, or -C N(R 6)(optionally substituted alkyl); or R 3 and R 4 are taken together with the nitrogen atom to which they are attached to form a monocyclic or bicyclic ring; R 5 represents independently for each occurrence C 1-6 alkyl; R 6 represents independently for each occurrence hydrogen or alkyl; R 7 represents independently for each occurrence hydrogen, alkyl, alkylene-CO 2 H, alkylene-C N(R 6) 2, alkylene-(optionally substituted heterocyclyl), optionally substituted heterocyclyl, alkylene-(optionally substituted heteroaryl), or hydroxyl alkylene-(optionally substituted heterocyclyl); R 8 represents independently for each occurrence hydrogen, alkyl, or aryl, and wherein each aforementioned alkyl may be a straight-chain alkyl, branched-chain alkyl, cycloalkyl, alkyl substituted cycloalkyl, or cycloalkyl substituted alkyl and may be a substituted alkyl having substituents replacing a hydrogen on one or more carbons of the hydrocarbon backbone. A 1 is a 5-6 membered heteroaryl, optionally substituted with 1, 2, or 3 substituents independently selected from the group consisting of alkyl, haloalkyl, halogen, hydroxyl, alkoxy, -CO 2 H, -CO 2, -CO 2 -(optionally substituted heterocycloalkyl), -C N(R 6)(R 7), - N(R 6)C (R 7), alkylene-(optionally substituted heterocyclyl), nitro, alkylene-O-alkylene-CO 2 H, alkylene-O-alkylene-CO 2 -, -SO 2 -N(R 6)-alkylene-CO 2 H, -SO 2 -N(R 6)-alkylene-CO 2 -, -N(R 6)-SO 2 -alkylene-CO 2 H, -N(R 6)-SO 2 -alkylene-CO 2 -, -SO 2 -N(R 6)-(optionally substituted heterocycloalkyl), -SO 2 -N(R 6) 2, -SO 2 -N(R 6)-alkylene-(optionally substituted heterocyclyl), X 1, and alkylene-X 1; and wherein: X 1 represents independently for each occurrence an ester, a succinimidyl ester, a carboxamide, propargyl, azidoalkyl, isothiocyanate, -NH 2, -OH, -SH, -SO 3 H, carboxyl, -C Cl, - O R 8, -CON NH 2, an acetoxymethyl ester, a substituted or unsubstituted N-hydroxysuccinimidyl ester, a substituted or unsubstituted N-hydroxysulfosuccinimido ester, a nitrophenyl ester, a fluorophenyl ester, alkyne, azide, hydrazide, alkoxylamine, -NCS, -CHO, - COCH 2 I, a phosphoramidite, a phthalamido, or a maleimide; R 1 and R 2 each represent independently hydrogen, alkyl, cycloalkyl, heterocycloalkyl, or -C N(R 6)(optionally substituted alkyl); or R 1 and R 2 are taken together with the nitrogen atom to which they are attached to form a monocyclic or bicyclic ring; R 3 and R 4 each represent independently hydrogen, alkyl, cycloalkyl, heterocycloalkyl, or -C N(R 6)(optionally substituted alkyl); or R 3 and R 4 are taken together with the nitrogen atom to which they are attached to form a monocyclic or bicyclic ring; R 5 represents independently for each occurrence C 1-6 alkyl; R 6 represents independently for each occurrence hydrogen or alkyl; R 7 represents independently for each occurrence hydrogen, alkyl, alkylene-CO 2 H, alkylene-C N(R 6) 2, alkylene-(optionally substituted heterocyclyl), optionally substituted heterocyclyl, alkylene-(optionally substituted heteroaryl), or hydroxyl alkylene-(optionally substituted heterocyclyl); R 8 represents independently for each occurrence hydrogen, alkyl, or aryl, and wherein each aforementioned alkyl may be a straight-chain alkyl, branched-chain alkyl, cycloalkyl, alkyl substituted cycloalkyl, or cycloalkyl substituted alkyl and may be a substituted alkyl having substituents replacing a hydrogen on one or more carbons of the hydrocarbon backbone.