Modification of aniline containing proteins using an oxidative coupling strategy

Viral nanoparticles: Current advances in design and development

Viral nanoparticles (VNPs) are self-assembling, adaptable delivery systems for vaccines and other therapeutic agents used in a variety of biomedical applications. The potential of viruses to invade and infect various hosts and cells renders them suitable as potential nanocarriers, possessing distinct functional characteristics, immunogenic properties, and improved biocompatibility and biodegradability. VNPs are frequently produced through precise genetic or chemical engineering, which involves adding diverse sequences or functional payloads to the capsid protein (CP). Several spherical and helical plant viruses, bacteriophages, and animal viruses are currently being used as VNPs, or non-infectious virus-like particles (VLPs). In addition to their broad use in cancer therapy, vaccine technology, diagnostics, and molecular imaging, VNPs have made important strides in the realms of tissue engineering, biosensing, and antimicrobial prophylaxis. They are also being used in energy storage cells due to their binding and piezoelectric properties. The large-scale production of VNPs for research, preclinical testing, and clinical use is fraught with difficulties, such as those relating to cost-effectiveness, scalability, and purity. Consequently, many plants- and microorganism-based platforms are being developed, and newer viruses are being explored. The goal of the current review is to provide an overview of these advances.

Multivalent glycocyclopeptides: conjugation methods and biological applications

Click chemistry was extensively used to decorate synthetic multivalent scaffolds with glycans to mimic the cell surface glycocalyx and to develop applications in glycosciences. Conjugation methods such as oxime ligation, copper(I)-catalyzed alkyne-azide cycloaddition, thiol-ene coupling, squaramide coupling or Lansbury aspartylation proved particularly suitable to achieve this purpose. This review summarizes the synthetic strategies that can be used either in a stepwise manner or in an orthogonal one-pot approach, to conjugate multiple copies of identical or different glycans to cyclopeptide scaffolds (namely multivalent glycocyclopeptides) having different size, valency, geometry and molecular composition. The second part of this review will describe the potential of these structures to interact with various carbohydrate binding proteins or to stimulate immunity against tumor cells.

Putting precision and elegance in enzyme immobilisation with bio-orthogonal chemistry

The covalent immobilisation of enzymes generally involves the use of highly reactive crosslinkers, such as glutaraldehyde, to couple enzyme molecules to each other or to carriers through, for example, the free amino groups of lysine residues, on the enzyme surface. Unfortunately, such methods suffer from a lack of precision. Random formation of covalent linkages with reactive functional groups in the enzyme leads to disruption of the three dimensional structure and accompanying activity losses. This review focuses on recent advances in the use of bio-orthogonal chemistry in conjunction with rec-DNA to affect highly precise immobilisation of enzymes. In this way, cost-effective combination of production, purification and immobilisation of an enzyme is achieved, in a single unit operation with a high degree of precision. Various bio-orthogonal techniques for putting this precision and elegance into enzyme immobilisation are elaborated. These include, for example, fusing (grafting) peptide or protein tags to the target enzyme that enable its immobilisation in cell lysate or incorporating non-standard amino acids that enable the application of bio-orthogonal chemistry.

Enzyme Activated Gold Nanoparticles for Versatile Site-Selective Bioconjugation

A new enzymatic method is reported for constructing protein- and DNA-AuNP conjugates. The strategy relies on the initial functionalization of AuNPs with phenols, followed by activation with the enzyme tyrosinase. Using an oxidative coupling reaction, the activated phenols are coupled to proteins bearing proline, thiol, or aniline functional groups. Activated phenol-AuNPs are also conjugated to a small molecule biotin and commercially available thiol-DNA. Advantages of this approach for AuNP bioconjugation include: (1) initial formation of highly stable AuNPs that can be selectively activated with an enzyme, (2) the ability to conjugate either proteins or DNA through a diverse set of functional handles, (3) site-specific immobilization, and (4) facile conjugation that is complete within 2 h at room temperature under aqueous conditions. The enzymatic oxidative coupling on AuNPs is applied to the construction of tobacco mosaic virus (TMV)-AuNP conjugates, and energy transfer between the AuNPs and fluorophores on TMV is demonstrated.

Carbonic anhydrase modification for carbon management

Carbonic anhydrase modification (chemical and biological) is an attractive strategy for its diverse application to accelerate the absorption of CO₂ from a flue gas with improved activity and stability. This article reports various possibilities of CA modification using metal-ligand homologous chemistry, cross-linking agents, and residue- and group-specific and genetic modifications, and assesses their role in carbon management. Chemically modified carbonic anhydrase is able to improve the absorption of carbon dioxide from a gas stream into mediation compounds with enhanced sequestration and mineral formation. Genetically modified CA polypeptide can also increase carbon dioxide conversion. Chemical modification of CA can be categorized in terms of (i) residue-specific modification (involves protein-ligand interaction in terms of substitution/addition) and group-specific modifications (based on the functional groups of the target CA). For every sustainable change, there should be no/limited toxic or immunological response. In this review, several CA modification pathways and biocompatibility rules are proposed as a theoretical support for emerging research in this area.

Aptamers Chemistry: Chemical Modifications and Conjugation Strategies

Soon after they were first described in 1990, aptamers were largely recognized as a new class of biological ligands that can rival antibodies in various analytical, diagnostic, and therapeutic applications. Aptamers are short single-stranded RNA or DNA oligonucleotides capable of folding into complex 3D structures, enabling them to bind to a large variety of targets ranging from small ions to an entire organism. Their high binding specificity and affinity make them comparable to antibodies, but they are superior regarding a longer shelf life, simple production and chemical modification, in addition to low toxicity and immunogenicity. In the past three decades, aptamers have been used in a plethora of therapeutics and drug delivery systems that involve innovative delivery mechanisms and carrying various types of drug cargos. However, the successful translation of aptamer research from bench to bedside has been challenged by several limitations that slow down the realization of promising aptamer applications as therapeutics at the clinical level. The main limitations include the susceptibility to degradation by nucleases, fast renal clearance, low thermal stability, and the limited functional group diversity. The solution to overcome such limitations lies in the chemistry of aptamers. The current review will focus on the recent arts of aptamer chemistry that have been evolved to refine the pharmacological properties of aptamers. Moreover, this review will analyze the advantages and disadvantages of such chemical modifications and how they impact the pharmacological properties of aptamers. Finally, this review will summarize the conjugation strategies of aptamers to nanocarriers for developing targeted drug delivery systems.

An Azo Coupling Strategy for Protein 3-Nitrotyrosine Derivatization

Herein, a strategy for the selective derivatization of 3-nitrotyrosine-containing proteins using the classic azo coupling reaction as the key step is described. This novel approach featured multiple advantages and was successfully applied to detect picomole levels of protein tyrosine nitration in biological samples.

A sulfonium tethered peptide ligand rapidly and selectively modifies protein cysteine in vicinity

Significant efforts have been invested to develop site-specific protein modification methodologies in the past two decades. In most cases, a reactive moiety was installed onto ligands with the sole purpose of reacting with specific residues in proteins. Herein, we report a unique peptide macrocyclization method via the bis-alkylation between methionine and cysteine to generate cyclic peptides with significantly enhanced stability and cellular uptake. Notably, when the cyclized peptide ligand selectively recognizes its protein target with a proximate cysteine, a rapid nucleophilic substitution could occur between the protein Cys and the sulfonium center on the peptide to form a conjugate. The conjugation reaction is rapid, facile and selective, triggered solely by proximity. The high target specificity is further proved in cell lysate and hints at its further application in activity based protein profiling. This method enhances the peptide's biophysical properties and generates a selective ligand-directed reactive site for protein modification and fulfills multiple purposes by one modification. This proof-of-concept study reveals its potential for further broad biological applications.

An Oxidative Bioconjugation Strategy Targeted to a Genetically Encoded 5-Hydroxytryptophan

Approaches that enable the chemoselective, covalent modification of proteins in a site-specific manner have emerged as a powerful technology for a wide range of applications. The electron-rich unnatural amino acid 5-hydroxytryptophan was recently genetically encoded in both Escherichia coli and eukaryotes, thereby allowing its site-specific incorporation into virtually any recombinant protein. Herein, we report the chemoselective conjugation of various aromatic amines to full-length proteins under mild, oxidative conditions that target this site-specifically incorporated 5-hydroxytryptophan residue.

Encapsulation of Negatively Charged Cargo in MS2 Viral Capsids

Encapsulation into virus-like particles is an efficient way of loading cargo of interest for delivery applications. Here, we describe the encapsulation of proteins with tags comprising anionic amino acids or DNA and gold nanoparticles with negative surface charges inside MS2 bacteriophage capsids to obtain homogeneous nanoparticles with a diameter of 27 nm.

Dual Surface Modification of Genome-Free MS2 Capsids for Delivery Applications

One of the hallmarks of virus-like particles (VLPs) is the fact that they possess distinguishable interior and exterior surfaces. Taking advantage of our knowledge of the amino acid location from X-ray crystal structures, we have developed a series of synthetic modifications of the MS2 bacteriophage viral capsid, including small molecule and polymer attachment, as well as conjugation with peptides, DNA and other proteins. These constructs have found applications in nanomaterial fabrication and as delivery vehicles with therapeutic potential. Importantly, the dual-modification strategies described herein could be extended to other VLP systems.

Engineering peptide ligase specificity by proteomic identification of ligation sites

Enzyme-catalyzed peptide ligation is a powerful tool for site-specific protein bioconjugation, but stringent enzyme–substrate specificity limits its utility. Here, we present an approach for comprehensive characterization of peptide ligase specificity for N termini using proteome-derived peptide libraries. We used this strategy to characterize the ligation efficiency for >25,000 enzyme–substrate pairs in the context of the engineered peptide ligase subtiligase and identified a family of 72 mutant subtiligases with activity toward N-terminal sequences that were previously recalcitrant to modification. We applied these mutants individually for site-specific bioconjugation of purified proteins including antibodies, and in algorithmically selected combinations for sequencing of the cellular N terminome with reduced sequence bias. We also developed a web application to enable algorithmic selection of the most efficient subtiligase variant(s) for bioconjugation to user-defined sequences. These studies provide a new toolbox of enzymes for site-specific protein modification and a general approach for rapidly defining and engineering peptide ligase specificity.

Biodistribution of Antibody-MS2 Viral Capsid Conjugates in Breast Cancer Models

A variety of nanoscale scaffolds, including virus-like particles (VLPs), are being developed for biomedical applications; however, little information is available about their in vivo behavior. Targeted nanoparticles are particularly valuable as diagnostic and therapeutic carriers because they can increase the signal-to-background ratio of imaging agents, improve the efficacy of drugs, and reduce adverse effects by concentrating the therapeutic molecule in the region of interest. The genome-free capsid of bacteriophage MS2 has several features that make it well-suited for use in delivery applications, such as facile production and modification, the ability to display multiple copies of targeting ligands, and the capacity to deliver large payloads. Anti-EGFR antibodies were conjugated to MS2 capsids to construct nanoparticles targeted toward receptors overexpressed on breast cancer cells. The MS2 agents showed good stability in physiological conditions up to 2 days and specific binding to the targeted receptors in in vitro experiments. Capsids radiolabeled with ⁶⁴Cu isotopes were injected into mice possessing tumor xenografts, and both positron emission tomography-computed tomography (PET/CT) and scintillation counting of the organs ex vivo were used to determine the localization of the agents. The capsids exhibit surprisingly long circulation times (10-15% ID/g in blood at 24 h) and moderate tumor uptake (2-5% ID/g). However, the targeting antibodies did not lead to increased uptake in vivo despite in vitro enhancements, suggesting that extravasation is a limiting factor for delivery to tumors by these particles.

Synthesis, characterization, and electrospinning of novel polyaniline-peptide polymers

Aniline-peptide (FLDQV, FLDQVC, Dansyl-FLDQV, Dansyl-FLDQVC, and FLDQV-AMC) mixtures underwent oxidative chemical and electrochemical polymerization in excess of aniline. The products of the chemical polymerization were low molecular weight polymers containing more than 70% peptide. Electrochemically polymerized species polyaniline-FLDQV (PANI-FLDQV) consisted mainly of polyaniline units containing about 10% peptide. The solubility of the latter in 1,1,1,3,3,3-hexafluoro-2-propanol (HFP) was similar to the camphorsulfonic acid (CSA) doped emeraldine base (PANI-CSA) solubility, however the weight composition of the electrospun fibers produced from the two polymers was significantly different. 2D 1H-13C HSQC analyses were employed to analyze the binding between the aniline and peptide moieties. Binding of peptide to polyaniline is reflected by the appearance of extra cross-peaks which display line broadening between the free polyaniline and the free pentapeptide. Peptides may be chemically bonded to the polymer molecules, but they may also act as doping agents to the nitrogen atoms via hydrogen bonding.

Direct detection of nitrotyrosine-containing proteins using an aniline-based oxidative coupling strategy

A convenient two-step method is described for the detection of nitrotyrosine-containing proteins. First, nitrotyrosines are reduced to aminophenols using sodium dithionite. Following this, an oxidative coupling reaction is used to attach anilines bearing fluorescence reporters or affinity probes. Features of this approach include fast reaction times, pmol-level sensitivity, and excellent chemoselectivity.

An efficient synthesis of iminoquinones by a chemoselective domino ortho-hydroxylation/oxidation/imidation sequence of 2-aminoaryl ketones

An efficient chemoselective domino oxidative homocoupling of 2-aminoaryl ketones in the presence of 2-iodoxybenzoic acid (IBX) for the synthesis of iminoquinone has been developed. The domino reaction proceeds via three consecutive steps, such as domino ortho-hydroxylation of 2-aminoaryl ketones, oxidation of a phenol derivative to benzoquinone and dimerization through imine formation to yield iminoquinone. Importantly, this reaction allows the recycling of the oxidant IBX by recovering the by-product iodosobenzoic acid (IBA) and oxidizing it back to IBX. A four step domino strategy for the synthesis of iminoquinone through in situ generation of 2-amino benzophenone from (2-amino phenyl)(phenyl)methanol was also developed.

Development of oxidative coupling strategies for site-selective protein modification

As the need to prepare ever more complex but well-defined materials has increased, a similar need for reliable synthetic strategies to access them has arisen. Accordingly, recent years have seen a steep increase in the development of reactions that can proceed under mild conditions, in aqueous environments, and with low concentrations of reactants. To enable the preparation of well-defined biomolecular materials with novel functional properties, our laboratory has a continuing interest in developing new bioconjugation reactions. A particular area of focus has been the development of oxidative reactions to perform rapid site- and chemoselective couplings of electron rich aromatic species with both unnatural and canonical amino acid residues. This Account details the evolution of oxidative coupling reactions in our laboratory, from initial concepts to highly efficient reactions, focusing on the practical aspects of performing and developing reactions of this type. We begin by discussing our rationale for choosing an oxidative coupling approach to bioconjugation, highlighting many of the benefits that such strategies provide. In addition, we discuss the general workflow we have adopted to discover protein modification reactions directly in aqueous media with biologically relevant substrates. We then review our early explorations of periodate-mediated oxidative couplings between primary anilines and p-phenylenediamine substrates, highlighting the most important lessons that were garnered from these studies. Key mechanistic insights allowed us to develop second-generation reactions between anilines and anisidine derivatives. In addition, we summarize the methods we have used for the introduction of aniline groups onto protein substrates for modification. The development of an efficient and chemoselective coupling of anisidine derivatives with tyrosine residues in the presence of ceric ammonium nitrate is next described. Here, our logic and workflow are used to highlight the challenges and opportunities associated with the optimization of site-selective chemistries that target native amino acids. We close by discussing the most recent reports from our laboratory that have capitalized on the unique reactivity of o-iminoquinone derivatives. We discuss the various oxidants and conditions that can be used to generate these reactive intermediates from appropriate precursors, as well as the product distributions that result. We also describe our work to determine the nature of iminoquinone reactivity with proteins and peptides bearing free N-terminal amino groups. Through this discussion, we hope to facilitate the use of oxidative approaches to protein bioconjugation, as well as inspire the discovery of new reactions for the site-selective modification of biomolecular targets.

Synthetically Modified Viral Capsids as Versatile Carriers for Use in Antibody-Based Cell Targeting

The present study describes an efficient and reliable method for the preparation of MS2 viral capsids that are synthetically modified with antibodies using a rapid oxidative coupling strategy. The overall protocol delivers conjugates in high yields and recoveries, requires a minimal excess of antibody to achieve modification of more than 95% of capsids, and can be completed in a short period of time. Antibody-capsid conjugates targeting extracellular receptors on human breast cancer cell lines were prepared and characterized. Notably, conjugation to the capsid did not significantly perturb the binding of the antibodies, as indicated by binding affinities similar to those obtained for the parent antibodies. An array of conjugates was synthesized with various reporters on the interior surface of the capsids to be used in cell studies, including fluorescence-based flow cytometry, confocal microscopy, and mass cytometry. The results of these studies lay the foundation for further exploration of these constructs in the context of clinically relevant applications, including drug delivery and in vivo diagnostics.

Click chemistry in complex mixtures: bioorthogonal bioconjugation

The selective chemical modification of biological molecules drives a good portion of modern drug development and fundamental biological research. While a few early examples of reactions that engage amine and thiol groups on proteins helped establish the value of such processes, the development of reactions that avoid most biological molecules so as to achieve selectivity in desired bond-forming events has revolutionized the field. We provide an update on recent developments in bioorthogonal chemistry that highlights key advances in reaction rates, biocompatibility, and applications. While not exhaustive, we hope this summary allows the reader to appreciate the rich continuing development of good chemistry that operates in the biological setting.

Selective chemical protein modification

Chemical modification of proteins is an important tool for probing natural systems, creating therapeutic conjugates and generating novel protein constructs. Site-selective reactions require exquisite control over both chemo- and regioselectivity, under ambient, aqueous conditions. There are now various methods for achieving selective modification of both natural and unnatural amino acids--each with merits and limitations--providing a 'toolkit' that until 20 years ago was largely limited to reactions at nucleophilic cysteine and lysine residues. If applied in a biologically benign manner, this chemistry could form the basis of true Synthetic Biology.

Photoactivated bioconjugation between ortho-azidophenols and anilines: a facile approach to biomolecular photopatterning

Methods for the surface patterning of small molecules and biomolecules can yield useful platforms for drug screening, synthetic biology applications, diagnostics, and the immobilization of live cells. However, new techniques are needed to achieve the ease, feature sizes, reliability, and patterning speed necessary for widespread adoption. Herein, we report an easily accessible and operationally simple photoinitiated reaction that can achieve patterned bioconjugation in a highly chemoselective manner. The reaction involves the photolysis of 2-azidophenols to generate iminoquinone intermediates that couple rapidly to aniline groups. We demonstrate the broad functional group compatibility of this reaction for the modification of proteins, polymers, oligonucleotides, peptides, and small molecules. As a specific application, the reaction was adapted for the photolithographic patterning of azidophenol DNA on aniline glass substrates. The presence of the DNA was confirmed by the ability of the surface to capture living cells bearing the sequence complement on their cell walls or cytoplasmic membranes. Compared to other light-based DNA patterning methods, this reaction offers higher speed and does not require the use of a photoresist or other blocking material.

N-terminal modification of proteins with o-aminophenols

The synthetic modification of proteins plays an important role in chemical biology and biomaterials science. These fields provide a constant need for chemical tools that can introduce new functionality in specific locations on protein surfaces. In this work, an oxidative strategy is demonstrated for the efficient modification of N-terminal residues on peptides and N-terminal proline residues on proteins. The strategy uses o-aminophenols or o-catechols that are oxidized to active coupling species in situ using potassium ferricyanide. Peptide screening results have revealed that many N-terminal amino acids can participate in this reaction, and that proline residues are particularly reactive. When applied to protein substrates, the reaction shows a stronger requirement for the proline group. Key advantages of the reaction include its fast second-order kinetics and ability to achieve site-selective modification in a single step using low concentrations of reagent. Although free cysteines are also modified by the coupling reaction, they can be protected through disulfide formation and then liberated after N-terminal coupling is complete. This allows access to doubly functionalized bioconjugates that can be difficult to access using other methods.

Site-Specific Protein Bioconjugation via a Pyridoxal 5'-Phosphate-Mediated N-Terminal Transamination Reaction

The covalent attachment of chemical groups to proteins is a critically important tool for the study of protein function and the creation of protein-based materials. Methods of site-specific protein modification are necessary for the generation of well defined bioconjugates possessing a new functional group in a single position in the amino acid sequence. This article describes a pyridoxal 5'-phosphate (PLP)-mediated transamination reaction that is specific for the N-terminus of a protein. The reaction oxidizes the N-terminal amine to a ketone or an aldehyde, which can form a stable oxime linkage with an alkoxyamine reagent of choice. Screening studies have identified the most reactive N-terminal residues, facilitating the use of site-directed mutagenesis to achieve high levels of conversion. Additionally, this reaction has been shown to be effective for a number of targets that are not easily accessed through heterologous expression, such as monoclonal antibodies. Curr. Protoc. Chem. Biol. 2:125-134 © 2010 by John Wiley & Sons, Inc.

Viral capsids as self-assembling templates for new materials

The self-assembling protein shells of viruses have provided convenient scaffolds for the construction of many new materials with well-defined nanoscale architectures. In some cases, the native amino acid functional groups have served as nucleation sites for the deposition of metals and semiconductors, leading to organic-inorganic composites with interesting electronic, magnetic, optical, and catalytic properties. Other approaches have involved the covalent modification of the protein monomers, typically with the goal of generating targeting delivery vehicles for drug and imaging cargo. Covalently modified capsid proteins have also been used to generate periodic arrays of chromophores for use in light harvesting and photocatalytic applications. All of these research areas have taken advantage of the low polydispersity, high chemical stability, and intrinsically multivalent properties that are uniquely offered by these biological building blocks.

Controlled integration of gold nanoparticles and organic fluorophores using synthetically modified MS2 viral capsids

The placement of fluorophores in close proximity to metal nanoparticle surfaces is proposed to enhance several photophysical properties of the dyes, potentially leading to improved quantum yields and decreased photobleaching. It is difficult in practice, however, to establish and maintain the nanoscale distances that are required to maximize these effects. The type of metal, size, and shape of the nanoparticle, the physical distance separating the metal nanoparticle from the organic dye, and the spectral properties of the fluorophore itself are all proposed to influence the quantum yield and lifetime. This results in a complex behavior that can lead to either enhanced or quenched fluorescence in different contexts. In this report, we describe a well-defined system that can be used to explore these effects, while physically preventing the fluorophores from contacting the nanoparticle surfaces. The basis of this system is the spherical protein capsid of bacteriophage MS2, which was used to house gold particles within its interior volume. The exterior surface of each capsid was then modified with Alexa Fluor 488 (AF 488) labeled DNA strands. By placing AF 488 dyes at distances of 3, 12, and 24 bp from the surface of capsids containing 10 nm gold nanoparticles, fluorescence intensity enhancements of 2.2, 1.2, and 1.0 were observed, respectively. A corresponding decrease in fluorescence lifetime was observed for each distance. Because of its well-defined and modular nature, this architecture allows the rapid exploration of the many variables involved in metal-controlled fluorescence, leading to a better understanding of this phenomenon.

Osmolyte-mediated encapsulation of proteins inside MS2 viral capsids

The encapsulation of enzymes in nanometer-sized compartments has the potential to enhance and control enzymatic activity, both in vivo and in vitro. Despite this potential, there is little quantitative data on the effect of encapsulation in a well-defined compartment under varying conditions. To gain more insight into these effects, we have characterized two improved methods for the encapsulation of heterologous molecules inside bacteriophage MS2 viral capsids. First, attaching DNA oligomers to a molecule of interest and incubating it with MS2 coat protein dimers yielded reassembled capsids that packaged the tagged molecules. The addition of a protein-stabilizing osmolyte, trimethylamine-N-oxide, significantly increased the yields of reassembly. Second, we found that expressed proteins with genetically encoded negatively charged peptide tags could also induce capsid reassembly, resulting in high yields of reassembled capsids containing the protein. This second method was used to encapsulate alkaline phosphatase tagged with a 16 amino acid peptide. The purified encapsulated enzyme was found to have the same K(m) value and a slightly lower k(cat) value than the free enzyme, indicating that this method of encapsulation had a minimal effect on enzyme kinetics. This method provides a practical and potentially scalable way of studying the complex effects of encapsulating enzymes in protein-based compartments.

Choosing an effective protein bioconjugation strategy

The collection of chemical techniques that can be used to attach synthetic groups to proteins has expanded substantially in recent years. Each of these approaches allows new protein targets to be addressed, leading to advances in biological understanding, new protein-drug conjugates, targeted medical imaging agents and hybrid materials with complex functions. The protein modification reactions in current use vary widely in their inherent site selectivity, overall yields and functional group compatibility. Some are more amenable to large-scale bioconjugate production, and a number of techniques can be used to label a single protein in a complex biological mixture. This review examines the way in which experimental circumstances influence one's selection of an appropriate protein modification strategy. It also provides a simple decision tree that can narrow down the possibilities in many instances. The review concludes with example studies that examine how this decision process has been applied in different contexts.

Rapid chemoselective bioconjugation through oxidative coupling of anilines and aminophenols

A highly efficient protein bioconjugation method is described involving addition of anilines to o-aminophenols in the presence of sodium periodate. The reaction takes place in aqueous buffer at pH 6.5 and can reach high conversion in 2-5 min. The major product was characterized using X-ray crystallography, which revealed that an unprecedented oxidative ring contraction occurs after the coupling step. The compatibility of the reaction with protein substrates has been demonstrated through attachment of small molecules, polymer chains, and peptides to p-aminophenylalanine residues introduced into viral capsids through amber stop codon suppression. Coupling of anilines to o-aminophenol groups derived from tyrosine residues is also described. The compatibility of this method with thiol modification chemistry is shown through attachment of a near-IR fluorescent chromophore to cysteine residues inside the viral capsid shells, followed by attachment of integrin-targeting RGD peptides to anilines on the exterior surface.

Oxidative modification of native protein residues using cerium(IV) ammonium nitrate

A new protein modification strategy has been developed that is based on an oxidative coupling reaction that targets electron-rich amino acids. This strategy relies on cerium(IV) ammonium nitrate (CAN) as an oxidation reagent and results in the coupling of tyrosine and tryptophan residues to phenylene diamine and anisidine derivatives. The methodology was first identified and characterized on peptides and small molecules, and was subsequently adapted for protein modification by determining appropriate buffer conditions. Using the optimized procedure, native and introduced solvent-accessible residues on proteins were selectively modified with polyethylene glycol (PEG) and small peptides. This unprecedented bioconjugation strategy targets these under-utilized amino acids with excellent chemoselectivity and affords good-to-high yields using low concentrations of the oxidant and coupling partners, short reaction times, and mild conditions.

Using synthetically modified proteins to make new materials

The uniquely diverse structures and functions of proteins offer many exciting opportunities for creating new materials with advanced properties. Exploiting these capabilities requires a set of versatile chemical reactions that can attach nonnatural groups to specific locations on protein surfaces. Over the years, we and others have developed a series of new techniques for protein bioconjugation, with a particular emphasis on achieving high site selectivity and yield. Using these reactions, we have been able to prepare a number of new materials with functions that depend on both the natural and the synthetic components. In this Account, we discuss our progress in protein bioconjugation over the past decade, focusing on three distinct projects. We first consider our work to harness sunlight artificially by mimicking features of the photosynthetic apparatus, with its beautifully integrated system of chromophores, electron transfer groups, and catalytic centers. Central to these photosystems are light-harvesting antennae having hundreds of precisely aligned chromophores with positions that are dictated by the proteins within the arrays. Our approach to generating similar arrangements involves the self-assembly of tobacco mosaic virus coat proteins bearing synthetic chromophore groups. These systems offer efficient light collection, are easy to prepare, and can be used to build complex photocatalytic systems through the modification of multiple sites on the protein surfaces. We then discuss protein-based carriers that can deliver drugs and imaging agents to diseased tissues. The nanoscale agents we have built for this purpose are based on the hollow protein shell of bacteriophage MS2. These 27 nm capsids have 32 pores, which allow the entry of relatively large organic molecules into the protein shell without requiring disassembly. Our group has developed a series of chemical strategies that can install dyes, radiolabels, MRI contrast agents, and anticancer drugs on the inside surface of these capsids. We have also developed methods to decorate the external surfaces with binders for specific proteins on cancer cells. As a third research area, our group has developed protein-polymer hybrid materials for water remediation. To reduce the toxicity of heavy metals in living cells, Nature has evolved metallothioneins, which are sulfur-rich polypeptides that bind mercury, cadmium, and other toxic ions at sub-parts-per-billion concentrations. Unfortunately, these proteins are very difficult to incorporate into polymers, largely because typical protein modification reactions target the very cysteine, lysine, and carboxylate-containing residues that are required for their proper function. To address this challenge, we developed a new way to attach these (and many other) proteins to polymer chains by expressing them as part of an N- and C-terminal modification "cassette". The resulting materials retain their selectivity and can remove trace amounts of toxic metal ions from ocean water. Each of these examples has presented a new set of protein bioconjugation challenges that have been met through the development of new reaction methodology. Future progress in the generation of protein-based materials will require scalable synthetic techniques with improved yields and selectivities, inexpensive purification methods for bioconjugates, and theoretical and dynamical treatments for designing new materials through protein self-assembly.

Storable arylpalladium(II) reagents for alkene labeling in aqueous media

We show that arylpalladium(II) reagents linked to biotin and indocyanine dye residues can be prepared by decarboxylative palladation of appropriately substituted electron-rich benzoic acid derivatives. When prepared under the conditions described, these organometallic intermediates are tolerant of air and water, can be stored for several months in solution in dimethyl sulfoxide, and permit biotin- and indocyanine dye-labeling of functionally complex olefinic substrates in water by Heck-type coupling reactions.

A Synthesis of Pseudo-Mauveine and a Homologue

Treatment of N-phenyl- p-phenylenediamine and two equivalents of aniline with potassium dichromate in hot water acidified with a small amount of sulfuric acid gives 3-phenylamino-5-phenyl-7-aminophenazinium sulfate (pseudo-mauveine). The oxidation of N-phenyl- p-phenylenediamine and o-toluidine with potassium dichromate in water with a small amount of sulfuric acid gives 3-phenylamino-5-( o- toluyl)-7-amino-8-methylphenazinium sulfate. Oxidation of aniline and ( o or p) toluidine only gave small quantities of mauveine chromophores. Oxidation of p-toluidine gives a known dimer which gives a purple coloured solution in acid. A hypothesis is described in which N-phenyl- p-phenylenediamine may have been involved in mauveine synthesis in the nineteenth century.

The art of engineering viral nanoparticles

Viral nanotechnology is an emerging and highly interdisciplinary field in which viral nanoparticles (VNPs) are applied in diverse areas such as electronics, energy and next-generation medical devices. VNPs have been developed as candidates for novel materials, and are often described as "programmable" because they can be modified and functionalized using a number of techniques. In this review, we discuss the concepts and methods that allow VNPs to be engineered, including (i) bioconjugation chemistries, (ii) encapsulation techniques, (iii) mineralization strategies, and (iv) film and hydrogel development. With all these techniques in hand, the potential applications of VNPs are limited only by the imagination.

Dual-surface modified virus capsids for targeted delivery of photodynamic agents to cancer cells

Bacteriophage MS2 was used to construct a targeted, multivalent photodynamic therapy vehicle for the treatment of Jurkat leukemia T cells. The self-assembling spherical virus capsid was modified on the interior surface with up to 180 porphyrins capable of generating cytotoxic singlet oxygen upon illumination. The exterior of the capsid was modified with ∼20 copies of a Jurkat-specific aptamer using an oxidative coupling reaction targeting an unnatural amino acid. The capsids were able to target and selectively kill more than 76% of the Jurkat cells after only 20 min of illumination. Capsids modified with a control DNA strand did not target Jurkat cells, and capsids modified with the aptamer were found to be specific for Jurkat cells over U266 cells (a control B cell line). The doubly modified capsids were also able to kill Jurkat cells selectively even when mixed with erythrocytes, suggesting the possibility of using our system to target blood-borne cancers or other pathogens in the blood supply.

Protein chemical modification on endogenous amino acids

Chemical modification of protein is an arduous but fruitful task. Many chemical methods have been developed for such purpose by carefully balancing reactivity and selectivity. Now both chemists and biologists have in hand an arsenal of tools from which they can select a relevant reaction to tackle their problems. This review focuses on the various chemical transformations available for selective modification of proteins. It also provides a brief overview of some of their main applications, including detection of protein interactions, preparation of bioconjugates, and protein microarrays.

Viral capsid DNA aptamer conjugates as multivalent cell-targeting vehicles

Nucleic acid aptamers offer significant potential as convenient and evolvable targeting groups for drug delivery. To attach them to the surface of a genome-free viral capsid carrier, an efficient oxidative coupling strategy has been developed. The method involves the periodate-mediated reaction of phenylene diamine substituted oligonucleotides with aniline groups installed on the outer surface of the capsid shells. Up to 60 DNA strands can be attached to each viral capsid with no apparent loss of base-pairing capabilities or protein stability. The ability of the capsids to bind specific cellular targets was demonstrated through the attachment of a 41-nucleotide sequence that targets a tyrosine kinase receptor on Jurkat T cells. After the installation of a fluorescent dye on the capsid interior, capsids bearing the cell-targeting sequence showed significant levels of binding to the cells relative to those of control samples. Colocalization experiments using confocal microscopy indicated that the capsids were endocytosed and trafficked to lysosomes for degradation. These observations suggest that aptamer-labeled capsids could be used for the targeted drug delivery of acid-labile prodrugs that would be preferentially released upon lysosomal acidification.