Compositions containing, methods involving, and uses of non-natural amino acids and polypeptides

A compound having the structure of Formula (XI):

• wherein:
• A is phenylene;
• B is a bond;
• R1 is H, an amino protecting group, or at least one amino acid;
• R2 is OH, an ester protecting group, or at least one amino acid;
• R is alkyl;
• each of R3 and R4 is H;
• R5 L-X, where
• X is selected from the group consisting of a polyalkylene glycol or a detectable label, wherein the detectable label is selected from a fluorescent, phosphorescent, chemiluminescent, chelating, intercalating, radioactive, and chromophoric moiety;
• and any combination of the above;
• and
• L is optional, and when present is a linker selected from the group consisting of alkylene, substituted alkylene, alkenylene, substituted alkenylene, —O-(alkylene or substituted alkylene)-, —S-(alkylene or substituted alkylene)-, —S(O)k(alkylene or substituted alkylene)-, —C(O)—, —C(O)-(alkylene or substituted alkylene)-, —C(S)—, —C(S)-(alkylene or substituted alkylene)-, —NR′-(alkylene or substituted alkylene)-, —C (O)N(R′)—, —CON(R′)-(alkylene or substituted alkylene)-, —CSN(R′)-, —CSN(R′)-(alkylene or substituted alkylene)-, —N(R′)CO-(alkylene or substituted alkylene)-, -(alkylene or substituted alkylene)-O—N═CR′—-, -(alkylene or substituted alkylene)-C(O)NR′-(alkylene or substituted alkylene)-, -(alkylene or substituted alkylene)-S(O)k-(alkylene or substituted alkylene)-S—, -( alkylene or substituted alkylene)-S—S—, —C(R′)═N—, —C(R′)═N—N(R′)—, —C(R′)═N—N═, —C(R′)2—N═N—, and —C(R′)2—N(R′)—N(R′)—, where each R′ is independently H, alkyl, or substituted alkyl;
• or a salt thereof.