Rebecca M. Broyer

Emerging synthetic approaches for protein–polymer conjugations

Protein-polymer conjugates are important in diverse fields including drug delivery, biotechnology, and nanotechnology. This feature article highlights recent advances in the synthesis and application of protein-polymer conjugates by controlled radical polymerization techniques. Special emphasis on new applications of the materials, particularly in biomedicine, is provided.

Site-specific protein immobilization through N-terminal oxime linkages

Immobilizing proteins in specific orientations is important for diagnostic protein arrays, biomaterials, and other applications where retention of bioactivity is essential. We report an approach for protein micropatterning that exploits a chemoselective reaction to conjugate proteins at the N-terminus to polymer films. A copolymer from 2-hydroxyethyl methacrylate and a Boc-protected aminooxy tetra(ethylene glycol) methacrylate was synthesized by radical polymerization. Boc groups were locally deprotected using photoacid generator-based photolithography. Micropatterns were verified by fluorescence microscopy utilizing green fluorescent aldehyde microspheres. Streptavidin that was subjected to a transamination reaction to install an α-ketoamide group at the N-terminus was conjugated to the patterns by oxime bond formation. Since the majority of proteins may be modified to contain a reactive carbonyl group, this methodology should be applicable to pattern a wide variety of proteins specifically through the N-terminus.

Protein nanopatterns by oxime bond formation.

Patterning proteins on the nanoscale is important for applications in biology and medicine. As feature sizes are reduced, it is critical that immobilization strategies provide site-specific attachment of the biomolecules. In this study, oxime chemistry was exploited to conjugate proteins onto nanometer-sized features. Poly(Boc-aminooxy tetra(ethylene glycol) methacrylate) was synthesized by free radical polymerization. The polymer was patterned onto silicon wafers using an electron beam writer. Trifluoroacetic acid removal of the Boc groups provided the desired aminooxy functionality. In this manner, patterns of concentric squares and contiguous bowtie shapes were fabricated with 150-170-nm wide features. Ubiquitin modified at the N-terminus with an α-ketoamide group and N(ε)-levulinyl lysine-modified bovine serum albumin were subsequently conjugated to the polymer nanopatterns. Protein immobilization was confirmed by fluorescence microscopy. Control studies on protected surfaces and using proteins presaturated with O-methoxyamine indicated that attachment occurred via oxime bond formation.

Dual Click reactions to micropattern proteins

In the study described in this report orthogonal Click reactions were utilized to immobilize two different proteins on surfaces side-by-side and in multilayer constructs. Alkyne- and azide-functionalized poly(ethylene glycol) hydrogel features were fabricated. Copper-catalyzed Huisgen 1,3 dipolar cycloaddition and oxime chemistry were employed to conjugate an azide-functionalized ubiquitin and oxoamide-modified myoglobin, respectively. Multicomponent patterning was verified by fluorescence imaging.