Nature is using homologues like aspartic and glutamic acid to address different positions and spacer lengths with a carbon chain of 4 and 5 resp. for Asp and Glu. With adipic, pimelic and suberic acid derivatives we can extent this principle to a distance of 6, 7 and 8 atoms.

All homologues are available with appropriate protection pattern necessary in Fmoc/tBu SPPS protocols.