Biotherapeutic
proteins are an indispensable class of
pharmaceuticals that present a high degree of structural complexity and are prone to chemical modifications during production, processing, and storage, which have to be tightly controlled.
Pyroglutamate (pGlu), a cyclization product of N-terminal Gln or Glu residues, is a widespread post-translational modification in
proteins, including
monoclonal antibodies (mAbs). The unambiguous identification and quantification of this modification in
proteins is challenging, since the mass difference of -17 Da or -18 Da, when formed from Gln or Glu, respectively, is not unique. Moreover, deamidation and
dehydration occur not only during cyclization to pGlu, but also during other reactions leading to different types of modifications, like
succinimide or isopeptide bond moieties due to cross-linking between Asn or Gln and Lys side chains. Here we report the unambiguous identification and quantification of pGlu in intact mAbs with natural
isotope distribution by NMR spectroscopy. The assignment of all 1H, 13C and 15N random coil chemical shifts of pGlu in short reference
peptides led to the identification of unique chemical shift pairs that are distinct from the random coil chemical shifts of the natural
amino-acid residues. These characteristic correlations are suited for the detection of pGlu in denatured
proteins. We achieved complete denaturation of mAbs using a straightforward protocol, and could detect and quantify pGlu, in agreement with available mass spectrometric data. The application to the mAbs
rituximab and
adalimumab illustrates the potential of our approach for the characterization of biotherapeutics containing
isotopes at natural abundance.