English

• Cyclodextrins (CDs) are star supramolecular compounds possessing features of low toxicity and good biocompatibility, which have been extensively employed in the biomedical and biochemical areas [1, 2]. The structure characterization of CDs is that they have the hydrophilic exterior and hydrophobic interior [3, 4]. Natural CDs are typically classed into α-, β-, and γ-CD containing six, seven, and eight d-glucopyranose units, respectively [5]. Among them, β-CD has attracted most attention due to its low cost and easy modification. β-CD-based materials have been developed and reported in targeted drug delivery, bioimaging, and separation research fields [6-8]. For separation, the exterior of β-CD possesses abundant hydroxyl groups, so β-CD-based materials can be applied to the enrichment of hydrophilic compounds, many of which play a critical role in biological processes [9-11]. However, the interior of β-CD is hardly to be used for the separation of these compounds due to its hydrophobicity. Thus, it is of great significance to make full use of the interior cavity of β-CD to achieve high separation efficiency of hydrophilic compounds.

  As a well-known host molecule, the interior cavity of β-CD endows it with specific recognition capacity toward nonpolar guest molecules [12]. Adamantane (AD), one of the most common guest molecules of β-CD, has a high binding affinity with β-CD (K_a ≈ 2 × 10⁴ L/mol) [4, 13]. In addition, AD is easy to be functionalized due to its high chemical activity [14, 15]. When different AD derivatives are included in the interior cavity of β-CD, various β-CD-based materials possessing different physical and chemical properties will be obtained. For example, Pan et al. [16] utilized the hydrophilic or hydrophobic AD guest molecules to controllably tune the wettability of the host metal-phenolic networks. Inspired by this, host-guest complexes prepared by the inclusion of hydrophilic AD derivatives with β-CD can be expected for highly efficient capture of hydrophilic compounds.

  Glycoproteins are an important type of hydrophilic compounds generated from protein glycosylation, which are closely related to many kinds of diseases [17-21]. Mass spectrometry (MS) is a powerful tool for glycoprotein analysis, but it is vital to carry out an enrichment process before MS detection [22-24]. In our previous studies, we devoted great efforts to explore β-CD-based materials for the capture of glycopeptides prior to MS analysis [25-27]. As a proof-of-concept, a peanut agglutinin-β-CD functionalized polymer monolith was developed for the specific identification of immunoglobulin G galactosylation [28]. In addition, Song et al. [10] synthesized carboxymethyl-β-CD modified magnetic nanoparticles and took advantages of the hydrophilicity of carboxymethyl-β-CD to enrich glycopeptides. However, the hydrophobic interior cavity of β-CD has not found their applications in the field of glycopeptide enrichment.

  Herein, a novel β-CD-based material was prepared by one-pot self-assembly based on host-guest interaction and metal-phenolic interaction for the selective enrichment of glycopeptides. Catechol groups modified β-CD was employed as not only host molecule but also polyphenol ligand in self-assembly process. l-Cysteine (l-Cys) or glutathione (GSH) derived AD was selected as guest molecule since l-Cys and GSH have been widely used for the enrichment of glycopeptides due to their strong hydrophilicity [29-31]. And magnetic nanoparticles (MNPs) were chosen as substrates to achieve fast separation [32]. Most importantly, Fe ions originated from MNPs can directly chelate with catechol groups modified β-CD, and no other metal ions are introduced. The as-prepared magnetic hydrophilic self-assembly materials possess strong hydrophilicity, good biocompatibility, and superparamagnetism. Combining with MS detection, the developed materials were utilized for the enrichment of glycopeptides from trypsin digests of standard and real biosamples. The reported strategy not only greatly explores the application of β-CD hydrophobic interior cavity in the enrichment of hydrophilic compounds, but also provides more possibility for constructing various β-CD-based materials with attractive application potential.

  The complete synthetic route of magnetic hydrophilic β-CD-based self-assembly materials (abbreviated as MCDC and MCDG) is shown in Scheme 1a. The morphologies of MNPs, MCDC, and MCDG were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). From SEM images (Figs. S5a-c in Supporting information), it can be found that MCDC and MCDG both have a rougher surface than MNP. And the average sizes of MNP and MCDC/MCDG are about 100 and 200 nm, respectively. TEM images are demonstrated in Figs. S5d-f (Supporting information), clearly showing that MCDC/MCDG has the flower-like appearance and larger sizes compared to MNP. These results indicate the successful coverage of host-guest complexes on the surface of MNP. Energy dispersive spectroscopy (EDS) pattern was also determined with MCDC as the representative. As shown in Fig. S6a (Supporting information), the signals of C, N, Fe, O, and S elements all appear and are consistent with element mapping images (Figs. S6b-g in Supporting information). According to the synthesis procedure of MCDC, Fe and O elements are assigned to MNPs; C element corresponds to β-CD; while N and S elements can be attributed to the inclusion of Cys@AD; and Si can be assigned to the silicon wafer used as the slide. EDS images of MCDG was shown in Fig. S7 (Supporting information). Similarly, the signals of C, N, Fe, O and S elements appear and are assigned to each component, respectively. However, it is different that MCDG has extremely low content of S, which may be caused by the composition of l-Cys and GSH. Comparing to l-Cys, GSH has relatively low content of S element.

  Scheme 1

  

  Scheme 1.  Schematic representation of (a) synthetic route and (b) workflow of glycopeptide enrichment with MCDC or MCDG.

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  X-ray photoelectron spectroscopy (XPS) was employed to analyze the element compositions of MCDC and MCDG. As shown in Fig. 1a, the peaks of Fe 2p, Fe 3p, O 1s, C 1s, N 1s, and S 2p can be evidently observed in the spectra of MCDC and MCDG [33, 34]. The existence of C 1s, N 1s, and S 2p peaks indicate the inclusion of guest molecules with β-CD, which further confirms the success of the self-assembly protocol of MCDC/MCDG. In addition, XPS result of MCDG is consistent with its EDS result, and further indicates that MCDG has extremely low content of S. Fourier transform infrared (FT-IR) spectroscopy was performed toward MNP and MCDC/MCDG (Fig. 1b). In the spectrum of MNP, the typical absorption peak at 572 cm⁻¹ is ascribed to the vibration of Fe-O-Fe [35, 36], and this peak still exists in the spectra of MCDC and MCDG. In addition, the signals at 2921 and 2850 cm⁻¹ belong to the stretching vibration of C-H and the signals at 1030 and 1078 cm⁻¹ are attributed to C-O-C vibration [3, 37], both illustrating the existence of β-CD. In the spectrum of MCDC, the peaks at 1630 and 1581 cm⁻¹ can be assigned to the bending vibration of N-H, while those at 1409 and 1296 cm⁻¹ are ascribed to the vibration of C-N and C-O [38, 39], confirming the introduction of Cys@AD. In the curve of MCDG, the peak at 1724 and 1238 cm⁻¹ are assigned to C=O and C-N stretching vibrations [30, 40], which can be attributed to the inclusion of GSH@AD. All these results indicate the successful synthesis of MCDC and MCDG.

  Figure 1

  

  Figure 1.  (a) XPS patterns of MCDC and MCDG. (b) FT-IR, (c) VSM, and (d) XRD patterns and (e) water contact angles of MNPs, MCDC, and MCDG.

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  The magnetic hysteresis loops of MNP and MCDC/MCDG were investigated by vibrating sample magnetometer (VSM). Although the maximum saturation magnetization value of MCDC/MCDG (68 emu/g) is lower compared with MNP (109 emu/g), they still have strong magnetic response [41], which can be rapidly separated within 15 s using an external magnetic field (Fig. 1c). In addition, the structure and crystal phase of MNP, MCDC, and MCDG were examined by powder X-ray diffraction (XRD). Four diffraction peaks at 2θ of 30.5°, 35.6°, 53.7°, and 63.1° appear, which are attributed to (220), (311), (511), and (440) planes of MNPs (JCPDS NO. 19-0629) [42, 43], and these peaks can also be observed in the patterns of MCDC and MCDG (Fig. 1d). Such results confirm that the crystal structure of MNP is well maintained after introducing the host-gest complex. Last but most important, water contact angle test was employed to prove the hydrophilicity of MCDC/MCDG (Fig. 1e). Compared with the contact angle values of MNPs (55°), MCDC (27°) and MCDG (36°) possess the characterization of brilliant hydrophilicity due to the introduction of hydrophilic host-guest complexes. In other words, hydrophilic guest molecules included in the interior cavity of β-CD, Cys@AD and GSH@AD, endow MCDC/MCDG with a great number of hydrophilic sites.

  The enrichment performance of as-prepared MCDC/MCDG was investigated by HRP trypsin digests. A typical process is shown in Scheme 1b, including loading, washing, elution, and MS detection. Firstly, the compositions of loading buffer and eluent were investigated and optimized [44, 45]. As shown in Figs. S8 and S9 (Supporting information), when ACN/TFA (85/5, v/v) and ACN/TFA (30/0.1, v/v) were selected as the loading buffer and eluent, respectively. MCDC achieves highest enrichment efficiency (25 glycopeptide peaks with high intensity). Similarly, the optimal loading buffer and eluent were determined to be ACN/FA (90/1, v/v) and ACN/FA (40/1, v/v) for MCDG.

  Under the optimized experimental conditions, the enrichment capacity of MCDC and MCDG was evaluated. The mass spectrum obtained by direct detection is illustrated in Fig. 2a, in which a large number of nonglycopeptides greatly suppress the signal of glycopeptides. After the enrichment with MCDC and MCDG, 26 and 24 glycopeptides with high signal intensity were obtained, respectively (Figs. 2b and c). The detailed information of the enriched standard glycopeptides with MCDC and MCDG are listed in Table S1 (Supporting information).

  Figure 2

  

  Figure 2.  MALDI-TOF mass spectra of HRP trypsin digests. (a) Direct analysis; Analysis after enrichment with MCDC (b) and MCDG (c). "*" indicates glycopeptides.

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  Taking MCDC as the representative, the detection limit was investigated by loading HRP trypsin digests with different concentrations, i.e., 10, 1, 0.1, and 0.05 fmol/µL. 14 glycopeptides were captured by MCDC when HRP digest concentration is 10 fmol/µL (Fig. S10 in Supporting information), while 4 glycopeptides peaks were still detected even when the concentration is as low as 0.05 fmol/µL (Fig. S10d). These results indicate that the developed MCDC-based MALDI-TOF MS method has high sensitivity toward glycopeptides.

  The enrichment specificity of the magnetic hydrophilic self-assembly material was evaluated by the mixture of HRP and BSA trypsin digests with different mass ratios (1:10, 1:100, and 1:500). As shown in Fig. S11 (Supporting information), nonglycopeptides dominate the spectrum obtained by direct analysis, indicating that the detection of glycopeptides is almost impossible from complex samples without enrichment. However, after enrichment with MCDC, the signals of glycopeptides were significantly improved even at HRP: BSA ratios of 1:100 and 1:500 (Figs. S11d and f). Such results demonstrate the high selectivity of MCDC for enriching glycopeptides in complex samples.

  To study the reusability of the magnetic hydrophilic self-assembly material, MCDC was reused to enrich glycopeptides from HRP digests by repetitive cycles of adsorption-desorption. MCDC was thoroughly eluted with sufficient eluent and washed three times with loading buffer before each enrichment step. As shown in Fig. S12 (Supporting information), the signal intensity (m/z = 3671.9 Da as the characteristic glycopeptide peak) and number of captured glycopeptides do not change significantly after the first and eighth cycles, implying that the magnetic hydrophilic self-assembly materials possess excellent reusability for glycopeptide enrichment. In addition, the performance of MCDC for glycopeptide enrichment is compared with other reported materials. As shown in Table S2 (Supporting information) [22, 39, 46-48], the present MCDC-based MALDI-TOF MS method has characterization of low detection limit, high selectivity and reusability.

  Human serum and saliva, the most common human body fluids containing a large number of interfering substances, usually hinder the identification of low-abundance glycopeptides [49-51]. Therefore, these two samples were employed to evaluate the practical applicability of the developed materials. As shown in Fig. 3a, for human serum, the signals of glycopeptides were greatly suppressed by nonglycopeptides and hardly detected before enrichment. However, 70 peaks attributed to glycopeptides appear with high intensity after captured by MCDC (Fig. 3b). Detailed information of the identified glycopeptides is listed in Table S3. For human saliva, nearly no signals of glycopeptides were detected without the enrichment process (Fig. 3c). Whereas, 59 peaks ascribed to glycopeptides were identified after enrichment with MCDC (Fig. 3d). The detailed sequence information about glycopeptides from saliva is listed in Table S4 (Supporting information). All the above results confirm the great practicability of the magnetic hydrophilic self-assembly materials, implying that the present MCDC-based MALDI-TOF MS method has attractive potential in the field of glycopeptide researches.

  Figure 3

  

  Figure 3.  MALDI-TOF mass spectra of (a, b) human serum and (c, d) saliva. (a, c) Direct analysis; (b, d) after enrichment with MCDC. "*" indicates glycopeptides.

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  In summary, a novel magnetic hydrophilic self-assembly material was synthesized through a facile self-assembly process and applied to glycopeptide enrichment by coupling with MALDI-TOF MS analysis. The as-prepared MCDC/MCDG has brilliant hydrophilicity due to the inclusion of l-Cys/GSH modified AD with the interior cavity of β-CD, which was successfully used to capture glycopeptides from trypsin digests of standard and real samples. Results demonstrated that the magnetic hydrophilic self-assembly material possesses low detection limit (0.05 fmol/µL), high selectively (1:500), as well as excellent reusability (8 times). The material was also proved to exhibit the ability to specifically enrich glycopeptides from real human serum and saliva samples. Importantly, it can be anticipated that this study will provide a promising strategy to extend the applications of β-CD and serve as an applicable platform for glycoprotein analysis.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was financially supported by the Fundamental Research Funds for the Central Universities, Jilin University, China.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.06.042.

  
  
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  Scheme 1  Schematic representation of (a) synthetic route and (b) workflow of glycopeptide enrichment with MCDC or MCDG.

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  Figure 1  (a) XPS patterns of MCDC and MCDG. (b) FT-IR, (c) VSM, and (d) XRD patterns and (e) water contact angles of MNPs, MCDC, and MCDG.

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  Figure 2  MALDI-TOF mass spectra of HRP trypsin digests. (a) Direct analysis; Analysis after enrichment with MCDC (b) and MCDG (c). "*" indicates glycopeptides.

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  Figure 3  MALDI-TOF mass spectra of (a, b) human serum and (c, d) saliva. (a, c) Direct analysis; (b, d) after enrichment with MCDC. "*" indicates glycopeptides.

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