Immobilization of titanium dioxide/ions on magnetic microspheres for enhanced recognition and extraction of mono- and multi-phosphopeptides

Bifunctional Ti4+-modified paper for selective extraction or removal of phospholipids and paper spray mass spectrometry for bioanalysis in urine and plasma

BACKGROUND

Phospholipids (PLs) are major constituents of cell membranes, play important roles in cell proliferation and death, as well as in signal transduction, and therefore are relevant biomarkers for different pathologies. On the other hand, when the analysis of small compounds, such as therapeutics in blood is desired, then phospholipids are part of the matrix and cause serious interference during analysis. Currently, both the analysis and removal of PLs from biological samples are limited by extensive sample preparation and instrumental separation.

RESULTS

A fast and simple quantitative Ti4+-modified paper spray tandem mass spectrometric (TiPS-MS/MS) method was established in urine, where the enrichment of phospholipids was achieved, as well as reduction of matrix effects (primarily caused by high salt content) that ultimately led to improved sensitivity and selectivity. The method could achieve a physiologically relevant limit of detection (0.01-0.03 μg mL-1). Also, the usefulness of the Ti4+-modified paper was investigated in the opposite mode, namely for the selective removal of phospholipids from matrices such as plasma. Clonidine is used as model compound, as the detection of this compound is known to suffer from ion suppression by phospholipids. Compared with blank paper spray tandem mass spectrometry, the limit of detection could be improved from 0.3 μg mL-1 to 0.03 μg mL-1 by employing a Ti4+-modified paper on top of the spray tip to capture phospholipids from the sample.

SIGNIFICANCE AND NOVELTY

A novel Ti4+-modified paper was developed to allow for rapid solid-phase extraction of phospholipids from urine or selective removal from plasma, followed by direct paper spray mass spectrometric detection as a fast and convenient sample preparation and analysis combination. The paper properties are based on the Ti4+ metal ion, which can selectively bind phosphate-containing compounds under acidic conditions, and its applicability was demonstrated in relevant biological matrices.

Applicability of core-shell SiO2 microspheres with a high TiO2 loading as stationary phase for HPLC

BACKGROUND

Due to its high chemical stability, sufficient rigidity and zwitterionic ion exchange properties, TiO₂ can be considered as an alternative stationary phase material to SiO₂ for high performance liquid chromatography. TiO₂ stationary phase is usually prepared by coating TiO₂ onto SiO₂ support by sol-gel method. However, in the traditional coating method, in order to overcome the rapid hydrolysis rate of tetrabutyl orthotitanate, only a very low concentration of tetrabutyl orthotitanate can be used, resulting in a low loading of TiO₂ on the support.

RESULTS

TiO₂ core-shell spheres with a good monodispersity were prepared using 0.25 mol L^-1 tetrabutyl orthotitanate. The specific surface area, pore volume, pore diameter and TiO₂ loading of the TiO₂ core-shell spheres were 66 m² g^-1, 0.15 cm³ g^-1, 9.8 nm and 57%, respectively. The core-shell spheres were derivatized with n-octadecyltrichlorosilane and then packed into a stainless steel column to test the separation performance for neutral, basic and acidic samples in liquid chromatography. A baseline separation of polyaromatic hydrocarbons was achieved, showing a column efficiency for fluorene of 118075 plates m^-1. The prepared stationary phase was also used to separate acidic and basic mixtures, and column efficiencies of 54500 and 25836 plates m^-1 were obtained for N,N-dinitroaniline and p-chlorophenol, respectively. The relative standard deviations of the retention times of polyaromatic hydrocarbons for run-to-run, day-to-day and column-to-column repeatability were all below 5.1%.

SIGNIFICANCE AND NOVELTY

This work demonstrated that TiO₂ can be coated in the pores of the shell of SiO₂ core-shell spheres with high TiO₂ loading using a high concentration of tetrabutyl orthotitanate as the titania source. The experimental results show that the TiO₂ coated core-shell spheres can be a good alternative stationary phase for liquid chromatography.

Epitaxial Growth of Guanidyl-Functionalized Magnetic Metal-Organic Frameworks with Multiaffinity Sites for Selective Capture of Global Phosphopeptides

The flexible and controlled synthesis of metal-organic framework (MOF)-derived hybrid nanostructures is of great significance in fine tuning of their enrichment performance in large-scale and in-depth phosphoproteome analysis. Herein, a magnetic guanidyl-functionalized MOF hybrid coating with multiaffinity sites, denoted as Fe₃O₄@G-ZIF-8, was fast fabricated via a one-pot epitaxial growth strategy for the first time and applied for selective and highly efficient enrichment of global phosphopeptides. The intrinsic unsaturated metal sites of ZIF-8 endow the surface-mounted MOF coatings with immobilized metal ion affinity chromatography interaction with multiphosphorylated peptides. The oriented anchoring of bifunctional guanidineacetic acid on the magnetic MOF nanospheres provides additional affinity sites (guanidyl groups) for specific recognition of phosphopeptides by "salt bridge" interaction, as well as active site carboxyl groups for the coordination with the metal ions. The as-prepared Fe₃O₄@G-ZIF-8 exhibits large surface area (382.5 m² g^-1), good superparamagnetic property (41.6 emu g^-1) and stability, and size-exclusion effect (1.73 nm), which can serve as a specific adsorbent for global phosphopeptide analysis with satisfactory selectivity, great detection sensitivity (1 fmol), and rapid magnetic separation. Moreover, the successful application of Fe₃O₄@G-ZIF-8 for selective capture of both multi- and mono-phosphopeptides from human saliva and serum demonstrated the great potential of magnetic surface-mounted MOF coatings in effective identification of low-abundance phosphopeptides by matrix-assisted laser desorption ionization time-of-flight mass spectrometry from complicated biological matrices.

Design of a Spiropyran-Based Smart Adsorbent with Dual Response: Focusing on Highly Efficient Enrichment of Phosphopeptides

Smart responsive materials have attractive application prospects due to their tunable behaviors. In this work, we design novel spiropyran (SP)-based magnetic nanoparticles (MNP-SP) with dual response to ultraviolet light and pH and apply them to the enrichment of phosphopeptides. SP is modified on the surface of magnetic nanoparticles through a simple esterification reaction, based on which an MNP-SP-MS phosphopeptide identification platform is established. The capture and release of phosphopeptides are facilely adjusted by changing external light and the pH of the solution. The smart responsive MNP-SP has fast magnetic response performance, high sensitivity (detection limit of 0.4 fmol), and good reusability (6 cycles). In addition, MNP-SP is used for the enrichment of phosphopeptides in skimmed milk, human saliva, and human serum samples, indicating that it is an ideal adsorbent for enriching low-abundance phosphopeptides in complex biological environments.

Facile synthesis of bifunctional polymer monolithic column for tunable and specific capture of glycoproteins and phosphoproteins

Efficiently tunable capture of the glycosylated/phosphorylated proteins is critical to meet the need of in-depth glycoproteome and phosphoproteome studies. Reported here is a new bifunctional polymer monolithic column by introducing benzeneboronic acid and phosphonic acid onto monolithic column (denoted as poly (EDMA-co-VPBA-co-VPA) monolith) for tunable and specific enrichment of glycoproteins and phosphoproteins via switching different mobile phases. Based on boronate affinity and immobilized metal affinity, the as-prepared poly (EDMA-co-VPBA-co-VPA) monolith exhibited superior performance in selective separation of small molecules and biomacromolecules containing cis-diol/phosphate groups or not. And the frontal chromatography analysis showed that the binding capacity of the poly (EDMA-co-VPBA-co-VPA) monolith towards horseradish peroxidase (HRP, glycoprotein) or β-casein (phosphoprotein) is four-fold higher than that of bovine serum albumin (BSA, non-glycosylated/phosphorylated protein). Furthermore, combined with mass spectrometry identification, the successful application in specific enrichment of glycopeptides/phosphopeptides from tryptic digests of HRP/β-casein and direct capture of low abundant endogenous phosphopeptides from human serum proved great practicability in complex samples. This study provides a novel insight for fabricating the monolithic columns with multifunctionalization to facilitate further post-translational modification (PTM)-proteomics development.

Recent trends in sorbents for bioaffinity chromatography

Isolation or enrichment of biological molecules from complex biological samples is mostly a prerequisite in proteomics, genomics, and glycomics. Different techniques have been used to advance the efficiency of the purification of biological molecules. Bioaffinity chromatography is one of the most powerful technique that plays an important role in the isolation of target biological molecules by the specific interactions with ligands that are immobilized on different support materials. This review examines the recent developments in bioaffinity chromatography particularly over the past 5 years in the literature. Also properties of supports, immobilization techniques, types of binding agents, and methods used in bioaffinity chromatography applications are summarized.

Efficient separation of phosphopeptides employing a Ti/Nb-functionalized core-shell structure solid-phase extraction nanosphere

A strategy for effectively enriching global phosphopeptides was successfully developed by using ammonia methyl phosphate (APA) as a novel chelating ligand and Ti⁴⁺ and Nb⁵⁺ as double functional ions (referred to as Fe₃O₄@mSiO₂@APA@Ti⁴⁺/Nb⁵⁺). With the advantage of large specific surface area (151.1 m²/g), preeminent immobilized ability for metal ions (about 8% of total atoms), and unbiased enrichment towards phosphopeptides, Fe₃O₄@mSiO₂@APA@Ti⁴⁺/Nb⁵⁺ displays high selectivity (maximum mass ratio β-casein to BSA is 1:1500), low limit of detection (LOD, as low as 0.05 fmol), good relative standard deviation (RSD, lower than 7%), recovery rate of 87% (¹⁸O isotope labeling method), outstanding phosphopeptide loading capacity (330 μg/mg), and at least five times re-use abilities. In the examination of the actual sample, 24 phosphopeptides were successfully detected in saliva and 4 phosphopeptides were also selectively extracted from human serum. All experiments have shown that Fe₃O₄@mSiO₂@APA@Ti⁴⁺/Nb⁵⁺ exhibits exciting potential in view of the challenge of low abundance of phosphopeptides. Graphical abstract.

In situ synthesis of a novel metal oxide affinity chromatography affinity probe for the selective enrichment of low-abundance phosphopeptides

RATIONALE

Due to the dynamic nature of phosphorylation states and the low stoichiometry of phosphopeptides, it is still a challenge to efficiently capture phosphopeptides from complex biological samples before mass spectrometry analysis. Among the enrichment strategies, metal oxide affinity chromatography (MOAC) is one of the most widely used and the one with the most potential. It is based on reversible Lewis acid-base interactions between the metal oxides and the negatively charged phosphate groups to achieve the specific selection of phosphopeptides.

METHODS

A novel MOAC affinity probe, denoted as G@PDA@ZrO₂ , was successfully synthesized by in situ grafting ZrO₂ onto the surface of graphene (G) modified with polydopamine (PDA). The novel MOAC probe thus obtained was used for phosphopeptide enrichment.

RESULTS

This novel MOAC affinity probe when used to selectively enrich phosphopeptides from standard protein digest solutions exhibited a high selectivity (β-casein:bovine serum albumin = 1:1000), a low detection limit (4 fmol), and a high loading capacity (400 mg/g). At the same time, the experimental results proved that G@PDA@ZrO₂ had great recyclability (five cycles), stability, and reproducibility. Subsequently, G@PDA@ZrO₂ was applied to enrich phosphopeptides from human saliva and human serum, in which 25 and 4 phosphopeptide peaks, respectively, were detected.

CONCLUSIONS

This novel MOAC affinity probe (G@PDA@ZrO₂ ) showed good performance in enriching phosphopeptides. Thus, G@PDA@ZrO₂ has good potential in phosphopeptidomics analysis.

Targeted immobilization of titanium (IV) on magnetic mesoporous nanomaterials derived from metal-organic frameworks for high-efficiency phosphopeptide enrichment in biological samples

A selectively modified porous metal/carbon nanocomposite was fabricated to enhance the enrichment of low-abundance phosphopeptides from biological samples. The carbon matrix derived from the metal-organic framework provides a suitable pore size to allow the diffusion of peptides, while the deliberately modified metal nanoparticles within the pores enhance their interaction with the phosphopeptides. This nanocomposite shows extremely high enrichment selectivity for phosphopeptides in the MALDI-TOF MS detection, even when the molar ratio of α-casein digests versus bovine serum albumin digests was up to about 1:20,000. By combining such nanocomposite with nano-LC-MS/MS, 4556 unique phosphopeptides were identified with high selectivity (95.2%) from HeLa cell extracts. Furthermore, phosphopeptides from prostate tissue digests were also determined. A total of 277 and 1242 phosphopeptides were identified from normal and tumor tissues of a patient with prostate cancer, respectively. This indicates that phosphorylation and prostate cancer can be related to each other.

Controlled manipulation of TiO2 nanoclusters inside mesochannels of core-shell silica particles as stationary phase for HPLC separation

Based on a detailed study of the hydrolysis process of tetrabutyl orthotitanate (TBOT), TiO₂ nanoclusters were modified inside the pores of SiO₂ core-shell particles instead of the outside. The pore size distribution of SiO₂ core-shell spheres modified with TiO₂ (SiO₂@dSiO₂@TiO₂) was analyzed by Barrett-Joyner-Halenda (BJH) method and density functional theory (DFT) method, respectively. The results of the DFT calculations demonstrate that the TiO₂ nanoclusters are always first formed in bulk solution and then enter the pores. By regulating the rate of hydrolysis of TBOT, almost all of the TiO₂ nanoclusters are modified into the pores and the structure of the original SiO₂ core-shell sphere is hardly affected. The morphology of the particles was characterized by scanning electron microscopy and transmission electron microscopy. The crystal phase of TiO₂ was measured by XRD. SiO₂@dSiO₂@TiO₂ spheres functionalized with C18 were packed into a stainless steel column. The chemical stability of SiO₂@dSiO₂@TiO₂ spheres under alkaline was tested by flushing of a mobile phase at pH 13 for 7 days. The efficiency of the column after the alkali solution treatment still reaches 98,430 plates m^-1, which is only about 1.6% lower than that before the alkali solution treatment. A series of basic and acidic analytes were also separated on the column. Graphical abstract TiO₂ nanocrystals were coated into the pore of core-shell silica spheres. The prepared particles were packed into the column and separation performance up to 98,430 plates per meter was achieved.

Modification of Luffa Sponge for Enrichment of Phosphopeptides

The enrichment technique is crucial to the comprehensive analysis of protein phosphorylation. In this work, a facile, green and efficient synthetic method was set up for quaternization of luffa sponge. The resultant luffa sponge showed strong anion-exchange characteristics and a high adsorption ability for phosphate ions. Along with the unique physical properties, e.g., tenacity and porous texture, quaternized luffa sponge was demonstrated to be a well-suited solid-phase extraction (SPE) material. The quaternized luffa sponge-based SPE method was simple, cost-effective and convenient in operation, and was successfully applied to the capture of phosphopeptides from protein digests. The enrichment approach exhibited exceptionally high selectivity, sensitivity and strong anti-interference ability. Four phosphopeptides were still detected by using the digest mixture of β-casein and bovine serum albumin with a molar ratio of 1:100. 21 phosphopeptides were identified from the tryptic digest of non-fat milk.

A polymer monolith composed of a perovskite and cucurbit[6]uril hybrid for highly selective enrichment of phosphopeptides prior to mass spectrometric analysis

A hybrid monolith was prepared from perovskite and cucurbit[6]uril [poly(hydroxyethyl methacrylate-pentaerythritol triacrylate) monolith] for the enrichment of phosphopeptides. By coupling with mass spectrometry, three goals were simultaneously realized, viz. (a) selective enrichment of phosphopeptides from non-phosphopeptides, (b) identification of mono- and multi-phosphopeptides, and (c) recognition of tyrosine phosphopeptides. The perovskite introduced into the monolith warrants high selectivity for phosphopeptides even at a high (10,000:1) ratio of non-phosphopeptides to phosphopeptides, and and enables identification of eight mono- and multi-phosphopeptides from standard β-casein tryptic digests. Tyrosine phosphopeptides were specifically detected via the recognition capability of cucurbit[6]uril integrated into the monolith. The method has remarkably specific enrichment capacity for phosphopeptides from samples including human serum, nonfat milk, and human acute myelocytic leukemia cell lysate. Graphical abstractSchematic representation of a monolith integrated with perovskite and cucurbit[6]uril. The monolithic column was coupled with mass spectrometry and applied to the enrichment of phosphopeptides. The method has remarkably specific enrichment capacity for phosphopeptides from complex biological samples.

Titanium(IV)-functionalized zirconium-organic frameworks as dual-metal affinity probe for recognition of endogenous phosphopeptides prior to mass spectrometric quantification

A zirconium-organic framework was modified with titanium(IV) ions to obtain a modified framework that is shown to be a viable sorbent for selective capture of phosphopeptides. This dual-metal affinity probe exhibits 0.1 fM limits of detection and excellent size-exclusion effect (the mass ratio of β-casein digests/BSA/intact β-casein is 1:1000:1000). This is attributed to abundant Ti(IV) and Zr(IV) coordination sites and high porosity. The performance of the sorbent for extracting endogenous phosphopeptides from human serum and saliva was investigated. Especially, 105 endogenous phosphopeptides from saliva were captured specifically. In addition, the amino acid frequency of the enriched phosphopeptides was analyzed. Conservation of sequence around the identified phosphorylated sites from saliva confirmed that phosphorylation took place in the proline-directed motifs. Graphical abstractSchematic representation of a method for the specific enrichment of phosphopeptides by a modified metal-organic framework. Following size-exclusion elution, the phosphopeptides are quantified by mass spectrometry.