Author: Dr. Richard Easton Date: 6th May 2026
Structural analysis of proteins and glycoproteins requires a comprehensive assessment of all aspects of structure at the primary, secondary, and tertiary levels, as defined by regulatory authorities through their requirement for ICH Q6B-based structural characterization (1). Meeting this standard necessitates the application of various analytical investigations and techniques, many of which utilize the powerful tool of mass spectrometry (MS). Among these, the MS-based analytical procedure which provides the most “bang for your buck” is peptide mapping.
Peptide mapping utilizes on-line liquid chromatography mass spectrometry (LC-MS) to analyze the products of proteolytic digestion of the sample. Using state-of-the-art MS systems, such as Q-TOF type instrumentation, which provide intact mass data on the peptide and concomitant amino acid sequence information through peptide fragment ion formation, peptide mapping generates a wealth of data on protein structure.
A peptide mapping study can deliver insights into multiple structural features:
Peptide mapping involves proteolytic digestion, most frequently though not exclusively using enzymes with precise substrate specificities. The resultant peptides are separated and analyzed using on-line LC-MS.
Knowledge of the expected primary protein sequence and enzyme specificity allows a theoretical digest map to be generated, against which experimental data can be compared for peptide identification.
Anomalous masses observed in the data can be further investigated through sequence ion information, providing the ability to lock on to a portion of the peptide the mass represents. This information also supports identifying the presence of contaminant proteins in the sample, as applied in Host Cell Protein (HCP) investigations. For the product itself, the combination of mass and fragment ion information allows conclusions to be drawn on the peptide structure.
The ability to accurately assess both the mass and sequence allows peptide mapping to support in-depth structural investigations of a molecule. Post-translational modifications can be assessed through mass differences from expected values and fragment ion identification of the peptide observed. Amino acid misincorporations can be assessed using the same approach, as can terminal structure assessments. Disulfide bridges can be identified through fragment ion confirmation of bridged peptides, confirming their involvement in bridge formation. Furthermore, the same data can be probed for evidence of scrambled disulfide bridges using the same approach.
Peptide mapping is clearly a highly powerful analytical technique. However, it does not address all structural questions. Its limitations are particularly evident in the analysis of glycosylation. Glycosylation is a non-templated PTM that often results in significant structural heterogeneity. Therefore, glycopeptide data in a peptide map can be complex to interpret, with low-abundance glycans being difficult to identify in the data set.
In addition, fragmentation of the glycan can occur, resulting in a profile that biases towards the presence of smaller glycan structures and in some cases, producing fragmented structures that are not present on the intact glycoprotein. For this reason, glycans should be released from glycoprotein and analyzed in specific separate studies.
Despite these limitations, peptide mapping can provide valuable data for glycopeptides, particularly in the identification of sites of N- and O-glycosylation. Linear consensus sequence(s) in the protein chain are recognized as a marker for the attachment of N-glycans. Peptide mapping can be used to identify the sites of attachment and the degree of site occupancy.
In contrast, O-glycans do not have a recognizable linear consensus sequence for their attachment to a protein chain, making peptide mapping a valuable tool for identifying them. However, as previously noted, glycans can fragment under standard, or most commonly encountered, mass spectrometric conditions.
Recent advances in mass spectrometry, particularly those employing electron-based fragmentation techniques, allow fragmentation of the amino acid backbone without loss of the glycan, thus allowing the O-glycan location to be identified, an important part of full structural characterization of O-glycan-containing glycoproteins.
Peptide mapping is a highly powerful analytical technique with broad applicability in protein characterization. It is an essential part of a full ICH Q6B structural characterization and also plays a critical role in investigating proteins, glycoproteins, and protein conjugates during development of a manufacturing and production process to ensure that no unwanted changes are occurring to the protein structure. The data feeds back to support process control in the manufacture of the product.
Whilst the applicability of peptide mapping is wide-reaching, it should not be considered a complete solution for all structural investigations at the primary protein level. Glycans should be considered under a separate analytical study package, although glycosylation site identification can be performed using appropriate mass spectrometry equipment and peptide mapping protocols.
