Advances in enrichment of phosphorylated peptides and glycopeptides by smart polymer-based materials

Protein post-translational modification (PTM) is at the forefront of focus of proteomics research. It not only regulates protein folding, state, activity, localization, and protein interactions, but also helps scientists understand the biological processes of organisms more comprehensively, providing stronger support and basis for the prediction, diagnosis, and treatment of diseases. In living organisms, there are more than 300 types of PTMs of proteins and their modification processes are dynamic. At the same time, protein modifications do not exist in isolation. The occurrence of the same physiological or pathological process requires the joint action of various modified proteins, which affect and coordinate with each other. Owing to the low abundance of PTM products (e. g., phosphorylated peptides or glycopeptides) and the presence of strong background interference, it is difficult to analyze them directly through mass spectrometry. Therefore, the development efficient materials and techniques for the selective enrichment of PTM peptides is urgently needed. Conventional separation methods have partially solved the challenges involved in the enrichment of glycopeptides and phosphorylated peptides; however, there are some inevitable issues, such as the excessive binding force of metal ions (e. g., Fe3+and Ti4+) toward multiple phosphorylated peptides, resulting in difficulty in elution and identification through mass spectrometry. In addition, owing to the insufficient binding affinity of materials toward glycopeptides, most glycopeptides that have been identified at present are of the sialic acid type, and a large number of neutral glycans, for instance, O-link glycopeptides and high mannose-type glycans are difficult to enrich and identify.The emergence of smart polymers provides a new avenue for the development of PTM-enriched materials. Several studies have reported that smart polymers can reversibly change their structure and function through external physical, chemical, or biological stimulation, to achieve highly controllable adsorption and desorption of phosphorylated peptides and glycopeptides. Based on this strategy, a series of novel enrichment materials and methods have been developed, which have greatly attracted the interest of researchers. On the one hand, the response changes of smart polymers include the increase or decrease of hydrophobicity, the change of shape and morphology, the redistribution of surface charge, the exposure or hiding of affinity ligands, etc. Changes in these properties can be achieved by simply changing external conditions such as temperature, pH, solvent polarity, and biomolecules. These properties, in turn, enable the fine-tuning of the affinity between the target and the smart polymers. Furthermore, the affinity can provide an additional driving force, which can significantly improve biological separation.On the other hand, smart polymers provide a series of convenient and expandable platforms for integrating various functional modules, such as specific recognition components, which will facilitate the development of novel enrichment materials for protein methylation, acetylation, and ubiquitination. Smart polymer materials show great potential in the field of separation, which is promising for the analysis and research of protein PTMs. This review summarizes the research progress of smart polymer materials for the separation and enrichment of phosphorylated peptides and glycopeptides according to nearly 50 representative articles from the Web of Science in the past two decades.