The "standard" peptide bond is formed between the α-carboxy group of an amino acid and the α-amino group of a second amino acid. The β-carboxyl of Asp and the γ-carboxyl of Glu can form amide bonds as well, then the α-carboxy function remains free:
α-peptide H-Asp-Ala-OH
β-peptide H-Asp(Ala-OH)-OH = H-β-Asp-Ala-OH
γ-peptide H-Glu(Ala-OH)-OH = H-γ-Glu-Ala-OH
Bachem offers a large choice of Asp and Glu derivatives suitable for synthesizing isoaspartyl and isoglutamyl peptides e.g. Fmoc-Asp-OtBu, Boc-Glu-OBzl