Another application involves the study of branched ubiquitin chains, where the folding of the branched protein molecules could be solved through racemic protein crystallography. Similarly, Dubiquitin was prepared in unmodified form and used to facilitate crystallization of the resulting branched L-ubiquitin proteins (Figure ). A further example involved the preparation of branched ubiquitin proteins in both enantiomeric forms for racemic protein crystallography (Figure ), but the iso-peptide linkages of the D-proteins contained a non-native cysteine residue to facilitate ligation. A key challenge, as presented in the former examples, is the preparation of PTM proteins in all D-form. Glycosylated proteins possess glycans in native D-chirality, which would require complex synthesis from the corresponding L-carbohydrates for a true racemic crystal. In addition, preparation of branched ubiquitin chains requires the use of non-natural amino acids as auxiliaries for attachment of the ubiquitin, but they often suffer from poor ligation efficiency (see section 3.3.2).