Design of O-GlcNAcylated peptides. (a) O-GlcNAcylation is an endogenous modification of serine and threonine residues of intracellular proteins. (b) Schematic of the GLP-1 sequence and peptides G1–G4, synthesized here, which contain either an O-GlcNAc modification at serine 18 or this modification in combination with AIB substitutions at residues 8 and/or 16. (c) Schematic of the PTH(1–34) sequence and peptides P1–P4 containing O-GlcNAcylated serines at residues 14, 18, 22, or 26.