Preparation of Highly Porous Coordination Polymer Coatings on Macroporous Polymer Monoliths for Enhanced Enrichment of Phosphopeptides

Podsumowanie

A procedure for the preparation of porous hybrid separation media composed of a macroporous polymer monolith internally coated by a high surface area microporous coordination polymer is presented.

Streszczenie

We describe a protocol for the preparation of hybrid materials based on highly porous coordination polymer coatings on the internal surface of macroporous polymer monoliths. The developed approach is based on the preparation of a macroporous polymer containing carboxylic acid functional groups and the subsequent step-by-step solution-based controlled growth of a layer of a porous coordination polymer on the surface of the pores of the polymer monolith. The prepared metal-organic polymer hybrid has a high specific micropore surface area. The amount of iron(III) sites is enhanced through metal-organic coordination on the surface of the pores of the functional polymer support. The increase of metal sites is related to the number of iterations of the coating process.

The developed preparation scheme is easily adapted to a capillary column format. The functional porous polymer is prepared as a self-contained single-block porous monolith within the capillary, yielding a flow-through separation device with excellent flow permeability and modest back-pressure. The metal-organic polymer hybrid column showed excellent performance for the enrichment of phosphopeptides from digested proteins and their subsequent detection using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The presented experimental protocol is highly versatile, and can be easily implemented to different organic polymer supports and coatings with a plethora of porous coordination polymers and metal-organic frameworks for multiple purification and/or separation applications.

Wprowadzenie

Porous coordination polymers (PCPs) are coordination compounds based on metal centers linked by organic ligands with repeating coordination entities extending in 1, 2 or 3 dimensions that can be amorphous or crystalline^1-3. In recent years, this class of porous materials has attracted widespread attention due to their high porosity, wide chemical tunability, and their stability. PCPs have been explored for a range of applications including gas storage, gas separation, and catalysis^3-6, and very recently, the first analytical applications of PCPs have been described⁷.

Because of their enhanced chemical functionality and high porosity PCPs have been targeted for their huge potential for the improvement of purification processes and chromatographic separations, and a number of reports concerning this topic have been published^7-13. However, the performance of PCPs is not currently at an equivalent level with existing chromatographic materials likely due to fast diffusion through large interparticle voids in packed beds of these solids due to their typically irregularly shaped morphologies of their particles or crystals. This irregularly distributed packing leads to a lower than expected performance, as well as high column backpressures and undesirable peak shape morphologies^14,15.

In order to solve the problem of fast diffusion through the inter-particle voids and concomitantly enhance the performance of PCPs for analytical applications, the development of a hybrid material based on a macroporous polymer monolith¹⁶ that contains the PCP on the surface of the macropores would be desirable. Polymer monoliths are self-contained, single-piece materials that can sustain convective flow through their pores, which makes them one of the most efficient alternatives to bead packings and have been successfully commercialized by several companies^17,18. Porous polymer monoliths are usually based on the polymerization of a monomer and a crosslinker in the presence of porogens, which are typically binary mixtures of organic solvents. The obtained monolithic materials have a microglobular structure and a high porosity and flow permeability.

A simple approach to unify these materials to prepare a polymer monolith containing a PCP is based on the direct addition of as-synthesized PCPs in the polymerization mixture of the monolith. This approach resulted in PCPs mostly buried within a polymer scaffold, and not being active for the further application of the final material^14,15. A different synthetic approach is clearly needed in order to, for example, develop uniform films of PCPs, or crystalline metal-organic frameworks (MOFs) where the majority of the pores contained within the crystal are accessible from the macropores of the polymer monolith.

Herein we report a simple protocol for the preparation of a metal-organic polymer hybrid material (MOPH) based on a macroporous polymer support with suitable functional groups for the attachment of PCPs, which can be easily implemented as a self-contained single-piece polymer monolith in a column format with optimum properties for flow-through applications. The polymer synthesis procedure is followed by a simple room temperature solution-based method to grow a PCP coating on the internal surface of the pores of the monolith^19-20. As the first example, we describe the preparation of an iron(III) benzenetricarboxylate (FeBTC) coordination polymer film within a macroporous poly(styrene-divinylbenzene-methacrylic acid) monolith. This method is effective for the preparation of bulk powders as well as capillary columns and the described protocol is readily implementable to other PCPs. As an example of the potential of MOPHs as functional materials for flow-through applications, we applied the developed FeBTC MOPH which contains a dense coating of Fe(III) centers to enrich phosphopeptides from digested protein mixtures exploiting the binding affinity of phosphopeptides to Fe(III). The developed protocol²¹ comprises three main parts: Preparation of the macroporous organic polymer monolith support; growth of the PCP coating on the surface of the pores of the monolith; application for the enrichment of phosphopeptides.

Protokół

NOTE: Before beginning, check all relevant material data sheets (MSDSs). Several of the chemicals used in the synthetic and application procedures are toxic. Please follow all appropriate safety practices and use adequate protective equipment (lab coat, full-length pants, closed-toe shoes, safety glasses, gloves). Please use all cryogenic personal protective equipment when handling liquid nitrogen for the nitrogen adsorption measurements (insulated gloves, face shield).

1. Porous Polymer Monolith Preparation in Bulk and Capillary Column Format

1. Bulk Polymer Monolith for Characterization
   1. Purify styrene, divinylbenzene and methacrylic acid through a column of basic alumina, in order to remove the polymerization inhibitors. Place 10 g of basic alumina in a 25 ml disposable plastic syringe with a plug of glass wool fiber packed in the syringe tip. Percolate approximately 10 ml of the monomer through the column.
   2. Load the monomers (50 mg styrene, 100 mg divinylbenzene and 50 mg methacrylic acid) and the pore forming agents (300 mg toluene and 300 mg isooctane) in a 1 ml glass vial. Add the initiator of the polymerization, 4 mg of 2,2’-azobisisobutyronitrile (AIBN, 1% with respect to monomers).
   3. Homogenize by sonication for 10 min. Remove dissolved oxygen by bubbling nitrogen through the liquid for 10 min. Seal the vial cap with paraffin film and place it in a water bath at 60 °C for 6 hr to polymerize the mixture.
   4. Cool to room temperature and break the vial carefully. Transfer the polymer monolith into a cellulose extraction thimble. Place the extraction thimble into a Soxhlet extraction chamber and assemble it to a round bottom flask that contains a volume of methanol, which is at least three times the volume of the extraction chamber. Assemble a condenser to the upper part of the extraction chamber. Perform Soxhlet extraction by boiling the methanol for 16 hr, ensuring the complete removal of the unreacted monomers and pore forming agents.
   5. Dry overnight in a vacuum oven at 60 °C. Confirm the presence of carboxylic functional groups to attach the PCP by Fourier Transform Infrared Spectroscopy (FT-IR). Measure surface area by nitrogen adsorption porosimetry.
2. Functionalization of Silica Capillaries for the Preparation of Monolithic Columns
   1. Cut 2 m of a polyimide-coated 100 µm i.d. fused silica capillary. Connect it to a 0.25-0.50 ml glass syringe and wash the capillary with acetone. Remove the acetone by rinsing the capillary with water.
   2. In order to activate the internal silica coating of the capillary, use a syringe pump to flow a 0.2 M aqueous NaOH solution at 0.25 µl/min for 30 min. Rinse with water until the effluent is neutral.
   3. Use pH paper strips to check effluent pH. In order to protonate the silanol groups of the capillary, pump a 0.2 M aqueous HCl solution through the capillary at 0.25 µl/min for 30 min. Rinse with water until the effluent is neutral. Rinse with ethanol.
   4. Pump a 20% (w/w) ethanol solution of 3-(trimethoxysilyl)propyl methacrylate (pH 5 adjusted with acetic acid) at 0.25 µl/min for 1 hr. In this step, the silica capillary is functionalized with vinyl groups in order to attach the polymer monolith to the capillary inner surface.
   5. Rinse with acetone, dry in a nitrogen stream and leave at room temperature overnight before use. Cut the capillary into shorter pieces of length 20 cm.
3. Preparation of Monolithic Capillary Columns
   1. Prepare an identical polymerization mixture as for the bulk polymer monolith (section 1.1) in a 1 ml glass vial with a rubber septum. Add initiator 1% AIBN with respect to monomers. Homogenize by sonication for 10 min.
   2. Purge the polymerization mixture with nitrogen by coupling a non-functionalized silica capillary to a nitrogen stream.
      1. Insert the nitrogen stream capillary through the rubber septum of the vial and immerse it into the polymerization mixture so that the nitrogen bubbles through the liquid. Leave the vial cap slightly loose to avoid overpressure. Purge for 10 min.
      2. Lift the nitrogen stream capillary from the polymerization mixture to the headspace of the vial, and close tightly the cap. Insert a functionalized capillary through the septum into the polymerization mixture. The excess of pressure generated into the capillary through the nitrogen injected into the headspace pumps the polymerization mixture through the functionalized capillary.
      3. Collect several drops of polymerization mixture from the effluent of the capillary to ensure that it is completely filled and close it with a rubber septum. Take the capillary out of the vial very carefully and close the inlet of the capillary with a rubber septum.
   3. Polymerize the mixture contained in the capillary in a water bath at 60 °C for 6 hr. Cool at room temperature and cut a few millimeters of both ends of the capillary. Remove unreacted monomers and pore forming agents by flushing the column with acetonitrile using an HPLC pump at 3 µl/min for 30 min. Check backpressure of the capillary column.

2. Growth of the Iron-benzenetrycarboxylate (FeBTC) PCP

1. Growth of the FeBTC MOPH on a Bulk Polymer Monolith for Characterization
   1. Grind the previously dried monolith using a mortar and pestle.
   2. Immerse 100 mg of the monolith powder in 5 ml of 2 mM FeCl₃·6H₂O in ethanol for 15 min. Vacuum filter using a nylon filter (0.22 µm) and wash the powder with ethanol. Immerse the monolith powder in 5 ml of 2 mM 1,3,5-benzenetricarboxylic acid (BTC) in ethanol for 15 min. Vacuum filter using a nylon filter (0.22 µm) and wash the powder with ethanol.
   3. Repeat step number 2 as desired. The growth of the final metal-organic coating will be defined by the number of applied cycles. Typically, between 10 and 30 cycles are performed. Confirm the presence of new pores by nitrogen adsorption porosimetry. Measure the amount of additional metal sites by thermogravimetric analysis (TGA).
2. Growth of the FeBTC MOPH on a capillary monolithic column for the enrichment of phosphopeptides
   1. Using a syringe pump. Flush the capillary monolith with 2 mM FeCl₃·6H₂O in ethanol for 15 min at 2 µl/min. Wash with ethanol for 15 min at 2 µl/min. Flush the capillary monolith with a 2 mM BTC in ethanol for 15 min at 2 µl/min. Wash with ethanol for 15 min at 2 µl/min.
   2. Repeat step 1 as desired. The growth of the final metal-organic coating will be defined by the number of cycles performed.

3. Protein Digestion and Enrichment of Phosphopeptides

1. Protein Digestion
   1. Dissolve 0.5 ml of non-fat milk in 1 ml of water and divide it into 200 µl fractions.
   2. For the protein digestion add 160 µl 1 M ammonium bicarbonate and 50 µl 45 mM dithiothreitol to each fraction, in order to cleave the disulfide bonds. Incubate at 50 °C in a thermomixer for 15 min.
   3. Add gradually 50 µl of an aqueous solution of iodoacetamide 100 mM, while the solution cooled down to room temperature. Iodoacetamide will prevent the formation of new disulfide bonds.
   4. Incubate in the dark for 15 min at room temperature. Add 1 ml deionized water. Add 2 µg trypsin and digest proteins in a thermomixer at 37 °C for 14 hr.
   5. Terminate digestion by acidification with 10 µl of 1% trifluoroacetic acid, and placing it in the thermomixer for 5 min at room temperature. Store the digested proteins at –20 °C.
2. Enrichment of phosphopeptides using a capillary MOPH column.
   1. Flush the column with 100 µl of a 4:1 mixture of acetonitrile containing a 0.1% trifluoroacetic acid for 10 min at a flow rate of 1 µl/min. Pump the protein digestion through the column at 2 µl/min for 30 min.
   2. Wash out the non-phosphorylated peptides again with a 4:1 mixture of acetonitrile containing a 0.1% trifluoroacetic acid for 10 min at a flow rate of 1 µl/min. Wash with water for 10 min at a flow rate of 1 µl/min.
   3. Elute phosphopeptides using a 250 mM pH 7 phosphate buffer solution pumped at 1 µl/min for 15 min. Collect the eluent in a vial and desalt the solution using a standard protocol¹⁹. Prepare a 2 mg/ml 2,5-dihydroxybenzoic acid to use it as the matrix for the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Drawn 2 µl of the 2,5-dihydroxybenzoic acid into the tip to elute the phosphopeptides and spot them directly on to the MALDI plate.
   4. Analyze the spots by MALDI-TOF-MS and regenerate the column by flushing thoroughly with water and then methanol.

Wyniki

A schematic illustration of the PCP growth on the pore surface of the organic polymer monolith is shown in Figure 1. In this figure, we illustrate the initial Fe(III) atoms retained on the pore surface of the original polymer monolith coordinated to carboxylic functional groups. Using the protocol described herein additional organic ligand and Fe(III) ions are added to the surface, shaping a porous coordination network within the polymer monolith. Figure 1 also shows schematically the us...

Dyskusje

The original polymer monolith contains carboxylic functional groups able to bind to metals. Coordinating the initial metal sites on the original material, we are able to grow a PCP coating (Figure 1A), incorporating a number of additional metal sites shaping a microporous network. This makes the presented MOPH materials attractive for extraction or purification procedures where metallic species are involved, such as the immobilized metal-ion affinity chromatography (IMAC) technique. The general procedure...

Materiały

Name	Company	Catalog Number	Comments
Polyimide-coated capillaries	Polymicro Technologies	TSP100375	100 μm i.d.
3-(trimethoxysilyl)propyl methacrylate, 98%	Sigma-Aldrich	440159
Styrene, 99%	Sigma-Aldrich	W323306	Technical grade
Divinylbenzene, 80%	Sigma-Aldrich	414565
Methacrylic acid, 98%	Mallinckrodt	MK150659
Toluene, ≥99.5%	EMD chemicals	MTX0735-6
Isooctane, ≥99.5%	Sigma-Aldrich	650439
2,2'-azobisisobutyronitrile, 98%	Sigma-Aldrich	441090
Aluminium oxide (basic alumina)	Sigma-Aldrich	199443
Iron (III) chloride hexahydrate, 97%	Sigma-Aldrich	236489
1,3,5-benzenetrycarboxylic acid, 95%	Sigma-Aldrich	482749
Acetonitrile, ≥99.5%	Sigma-Aldrich	360457
Ammonium bicarbonate, ≥99.5%	Sigma-Aldrich	9830
Trifluoroacetic acid, ≥99%	Sigma-Aldrich	302031
Ethanol, ≥99.8%	Sigma-Aldrich	2854
Iodoacetamide, ≥99%	Sigma-Aldrich	I1149
Dithiothreitol, ≥99%	Sigma-Aldrich	43819
Monobasic sodium phosphate dihydrate, ≥99%	Sigma-Aldrich	71505
Dibasic sodium phosphate dihydrate, ≥99%	Sigma-Aldrich	71643
Phosphoric acid, ≥85%	Sigma-Aldrich	438081
2,5-dihydroxybenzoic acid, ≥99%	Sigma-Aldrich	85707
Trypsin	Sigma-Aldrich	T8003	Bovine pancreas
β-casein	Sigma-Aldrich	C6905	Bovine milk
ZipTip pipette tips	Merck Millipore	ZTC18S096	C18 resin