NHS-PEG„-maleimide

A common choice of crosslinker for this type of reaction is sulfo-SMCC, which has been used extensively for antibody conjugation (Chapter 20, Section 1.1). A better option for dendrimer conjugation is to use a similar crosslinker design, but one that contains a hydrophilic PEG spacer arm to promote dendrimer hydrophilicity after modification. Derivatization of an amine-dendrimer with a NHS-PEG-maleimide can create an intermediate that is coated with water-soluble PEG spacers. This modification helps to mask any potential for nonspecific interactions that the PAMAM surface may have, while providing terminal thiol-reactive maleimides for coupling ligands (Figure 7.10). 

If a NHS-PEG-maleimide compound is used for this type of activation and coupling, the intermediate maleimide-activated dendrimer should be quickly purified of excess crosslinker and reaction by-products and immediately used to couple ligand. This is due to the fact that the maleimide hydrolyzes in aqueous solution at a higher rate than an SMCC-type crosslinker, because of the extreme hydrophilicity of the PEG spacer arm compared to the cyclohexane spacer of SMCC. 

Figure 7.10 An NHS-PEG-maleimide compound can be used to functionalize dendrimers to provide a hydrophilic spacer terminating in thiol-reactive groups. Thiol-containing proteins then can be conjugated to this reactive intermediate to form covalent thioether bonds.

Coupling of affinity molecules to surfaces also can be enhanced by the use of discrete PEG linkers. Nishimura et al. (2005) modified an amino surface with a NHS-PEG -maleimide crosslinker to create a hydrophilic self-assembled monolayer (SAM) surface that was thiol reactive for the conjugation of sulfhydryl-modified RNAs. This array then was used to investigate the binding specificity of synthetic kanamycins with selected RNA sequences to prove the specific interaction of ribosomal RNA with this molecule. The PEG linkers on surfaces provide lower nonspecific binding character than alkyl linkers, when preparing SAM surfaces for affinity interactions. 

In a fume hood, dissolve the NHS-PEG -maleimide compound of choice in a dry, water-miscible organic solvent to make a concentrated stock solution. For instance, to prepare a lOOmM solution of the NHS-PEGg-maleimide reagent (MW 601.6), dissolve an entire 100 mg vial of the crosslinker in 1.66 ml of DMAC (dry DMSO or DMF work well, too). 

Dissolve the amine-containing protein to be activated in 0.1 M sodium phosphate, pH 7.2 (coupling buffer). A protein concentration of l-10mg/ml in buffer will work well in this protocol. For more dilute protein solutions, greater quantities of the NHS-PEG,-maleimide compound may have to be added to obtain equivalent levels of modification. 

Dissolve NHS-PEGg-maleimide (MW 601.6) into DMSO at a concentration of 20mM. Short, PEG-type crosslinkers often exist as a thick oily mass, and preparing the solution typically involves dissolving an entire vial of the compound into DMSO to determine accurately the required concentration. Use only dry DMSO to avoid hydrolysis of the NHS ester. 

Figure 14.11 The modification of amine-containing particles with NHS-PEG4-maleimide produces hydrophilic PEG spacers containing terminal thiol-reactive groups. Coupling of thiol-containing proteins then results in the formation of thioether linkages.

The heterobifunctional PEGs are very useful in linking two entities in cases where a hydrophilic, flexible, and biocompatible spacer is needed. Preferred end groups for heterobifunctional PEGs are maleimides, vinyl sulfones, pyridyl disulfides, amines, carboxylic acids, and /V-hydroxysuccinimide (NHS) esters. 

In addition, the PEG-based heterobifunctional crosslinkers described in Chapter 18, Section 2, provide enhanced water-solubility for antibody conjugation applications. Conjugation of antibody molecules using a maleimide-PEG -NHS ester compound actually increases the solubility of the antibody and may help to maintain stability for certain sensitive monoclonals better than the traditional aliphatic crosslinkers. The methods described below for SMCC may be used with success for PEG-based reagents or other maleimide-NHS ester heterobifunctionals. 

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