prov:value
| - ester-lower alkyl, amido-lower alkyl, ether-lower alkyl, or thioether-lower alkyl groups, the sulfur analogues of those substituents, or the side-chain substituents from naturally occurring amino acids, and the closely related analogues of these sidechains, X3 is an amino acid, or a polyamide linked via its carboxy terminus,X5 and X6 are each independently, H or OR, where R is H, a protecting gro
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