[Application of smart responsive materials in phosphopeptide and glycopeptide enrichment]

Phosphorylation and glycosylation of proteins, two of the most widely studied post-translational modifications (PTMs), have shown increasing potential in the early non-invasive diagnosis, prognosis, and therapeutic evaluation of diseases. Besides regulating the function of cell membranes and intracellular signal transduction, protein phosphorylation participates in mitochondrial function and cellular and transcriptional metabolism. Protein glycosylation plays an important role in both intracellular and extracellular signal transduction and intracellular endocytosis. Aberrant phosphorylation and glycosylation of proteins are frequently observed in clinical proteomic studies and in the discovery of disease-related biomarkers. There are generally three methods for detecting protein phosphorylation/glycosylation: isotope radiolabeling, western blotting, and mass spectrometry. Mass spectrometry has become the most important and advantageous detection method due to its high throughput and time- and labor-efficiency. However, phosphopeptides and glycopeptides have low stoichiometry and ionization efficiency, and a large number of non-phosphopeptides and -glycopeptides interference. These issues make it difficult to directly detect phosphopeptides and glycopeptides by mass spectrometry. Therefore, the enrichment of phosphopeptides and glycopeptides before mass spectrometry detection is a key step. At present, a variety of materials have been developed for enrichment studies of phosphopeptides and glycopeptides. For example, immobilized metal affinity (IMAC) and metal oxide affinity chromatography (MOAC) methods are mostly used for the enrichment of phosphopeptides. The IMAC mainly uses positively charged metal ions and negatively charged phosphate groups to attract each other for the purpose of enriching phosphopeptides. MOAC materials rely on the chelation of metal atoms and phosphate oxygens to capture phosphopeptides. IMAC and MOAC materials rely on strong interactions between metals and phosphate groups, which often lead to difficult elution. The enrichment method for glycopeptides is mainly based on the difference in hydrophilicity between glycopeptides and non-glycopeptides, which are mainly enriched by hydrophilic interaction chromatography (HILIC). In addition, materials containing compounds such as boronic acid and lectin materials are also widely used for the separation and enrichment of glycopeptides. Smart responsive materials have also been successively reported for the enrichment of phosphopeptides and glycopeptides due to their unique responsiveness and reversibility. Smart responsive materials can respond to external stimuli; undergo structural and property changes; and convert signals such as optical, electrical, thermal, and mechanical into biochemical signals. Responsive molecules are a prerequisite for determining the response properties of smart responsive materials, and their reversible isomerization under different stimuli (such as temperature, pH, light, mechanical stress, and electromagnetic field) will lead to dynamic changes in the physical and chemical properties of materials. Compared with traditional materials, smart responsive materials can be reversibly "turned on" and "off" with better controllability. Exogenous stimuli, including temperature, light, ultrasound, electromagnetic field, and mechanical stress, can be implemented in a specific time and space. Exogenous responsive materials do not depend on changes in the reaction system itself and are non-invasive. Enzymes, pH, redox, solution polarity, and ionic strength are endogenous stimuli. Endogenous responsive materials depend on changes in the reaction system itself, and sometimes the regulation process requires the introduction of other chemicals into the reaction system. The identification, capture, and release of phosphopeptides or glycopeptides can be achieved by modulating the interactions between smart responsive materials and phosphopeptides or glycopeptides (such as hydrogen bonds, and electrostatic and hydrophobic interactions). This review classifies smart responsive materials according to the types of stimuli, which are specifically divided into exogenous and endogenous responsive materials. The enrichment of phosphopeptides and glycopeptides of exogenous/endogenous responsive materials and endogenous/exogenous co-responsive materials are summarized. In addition, we discuss the development prospects of smart responsive materials in the enrichment of phosphopeptides and glycopeptides, and also raised the challenges existing in the application of smart responsive materials in other protein post-translational modifications.