Click Chemistry with dPEG® Products Click chemistry is an essential, nearly indispensable tool for bioconjugation reactions. It provides a simple, straightforward route to the creation of numerous different structures and conjugated biomolecules. Since its discovery, the click chemistry reaction has spread into many different fields, sub-fields, and disciplines of chemistry and biology. In this review, we explore the types of click chemistry reactions, discuss some of their strengths and limitations, and review the various types of single molecular weight, discrete PEG (dPEG®)* products for click chemistry applications that Quanta BioDesign makes and sells internationally. If you are unfamiliar with Quanta BioDesign's dPEG® products, please visit the following two pages for a thorough explanation of dPEG® technology and answers to frequently asked questions. What is dPEG®? Frequently Asked Questions (about dPEG® products) Introduction to Click Chemistry In 2001, Hartmuth C. Kolb, M. G. Finn, and K. Barry Sharpless coined the term "click chemistry" to refer to a diverse set of reactions with a shared set of features that make the reactions appear to be "'spring-loaded' for a single trajectory."1 Kolb, Finn, and Sharpless set forth a strict set of criteria that reactions had to meet to be called "click chemistry" reactions. Their paper identified the 1,3-dipolar cycloaddition reaction, which Rolf Huisgen analyzed and reported on in 1967,2,3 as an exemplar of a click chemistry reaction. Although the 1,3-dipolar cycloaddition reaction frequently is named the "Huisgen 1,3-dipolar cycloaddition," the first reported reaction of this type dates back to 1893 and a report by A. Michael.4 Huisgen's 1,3-dipolar cycloaddition between an azide and alkyne proceeds at 100°C and yields a mixture of the 1,4- and 1,5- disubstituted triazoles.2 See Figure 1. Figure 1:The Huisgen 1,3-dipolar cycloaddition. See references 2 and 3. In 2002, Tornøe, Christensen, and Meldal5 and independently, Rostovtsev, Green, Fokin, and Sharpless6 related that copper(I) salts catalyze the rapid, regiospecific formation of 1,4-disubstituted, 1,2,3,-triazoles between azides and terminal alkynes. See Figure 2. This reaction is commonly abbreviated as CuAAC, for "copper(I)-catalyzed azide-alkyne cycloaddition." These two independent reports marked the beginning of the widespread scientific awareness of the power and specificity of click chemistry. * dPEG® is a registered trademark of Quanta BioDesign, Ltd. (reg. no. 3,460,927) for single molecular weight polyethylene glycol products. page 1 of 28 ©2021 by Quanta BioDesign, Ltd. Click Chemistry with dPEG® Products Figure 2: The Copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC). This reaction works for azides and terminal, but not internal, alkynes. See references 5 – 8. What makes a chemical process "click chemistry?" The criteria set out by Kolb, Finn, and Sharpless1 define the rules by which a reaction is or is not a click chemistry reaction. For a process to be "click chemistry," the reaction must: • be modular; • be wide in scope; • give very high chemical yields; • generate only inoffensive byproducts that non-chromatographic methods can remove; and, • be stereospecific (but not necessarily enantioselective) The process should have the following characteristics: • simple reaction conditions; • readily available starting materials and reagents; • no solvent, a benign solvent such as water, or an easily removed solvent; and, • simple, non-chromatographic product isolation (e.g., crystallization or distillation). The principal characteristics of a "click chemistry" reaction product are as follows: • The reaction product is stable under physiological conditions. • The reaction has a substantial thermodynamic driving force (>20 kJ/mole) favoring a single reaction product. In their 2001 paper,1 Kolb, Finn, and Sharpless also identified four classes of chemical transformations that they considered to be click chemistry reactions, which were as follows: 1. cycloadditions of unsaturated species (such as 1,3-dipolar cycloadditions and Diels-Alder reactions); 2. nucleophilic substitution chemistry (including, for example, ring-opening epoxide reactions); 3. non-aldol carbonyl chemistry (examples include oxime and hydrazone bond formation); and 4. additions to carbon-carbon multiple bonds (for instance, epoxidation, sulfenyl halide addition, and the Michael addition). page 2 of 28 ©2021 by Quanta BioDesign, Ltd. Click Chemistry with dPEG® Products Since the seminal 2001 paper by Kolb, Finn, and Sharpless, and subsequent discovery in 2002 of the CuAAC, many more reactions have been discovered or recognized as "click chemistry." These reactions include ruthenium-catalyzed azide-alkyne cycloaddition, strain-promoted azide-alkyne cycloaddition, strain-promoted alkyne-nitrone cycloaddition, photoclick chemistry, sulfur fluoride exchange, inverse electron-demand Diels-Alder, and more. These different types of click chemistry reactions are discussed in the sections below. Types of Click Chemistry: Metal-Catalyzed Click Chemistry Copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) The copper(I)-catalyzed cycloaddition reaction of terminal alkynes to organic azides to form 1,4-disubstituted 1,2,3-triazoles is one of the best known, most popular click chemistry reactions today. The reaction works well on solid supports5 as well as in solution.6 Although copper(I) salts catalyze the reaction, copper(II) salts do not affect the reaction. However, copper(II) salts combined with a reducing agent such as sodium ascorbate form copper(I) salts that can catalyze the reaction. Moreover, metal turnings of pure copper metal catalyze the formation of the 1,4-disubstituted, 1,2,3-triazoles.5,6 The CuAAC works with a broad span of reagents across a wide range of temperatures (0 – 160°C), pH values (4 – 12), and functional groups. The reaction works best in aqueous media, including serum and whole blood. Side reactions are rare and easily managed.6,7 The CuAAC works on terminal alkynes, but not internal ones.7 This limitation is mechanistic. The first step in the CuAAC reaction is the formation of a copper-acetylide complex. This complex then reacts with the three nitrogen atoms of the azide in a stepwise fashion, leading to the 1,2,3-triazole product.8 Copper(I)-catalyzed nitrile oxide-azide cycloaddition (CuNOAC) While studying the CuAAC, Himo et al. found a similar copper(I)-catalyzed reaction that is also a click chemistry reaction.7 Although not as well-known as the azide-alkyne cycloaddition, copper(I) also catalyzes the reaction of terminal alkynes with nitrile oxides to form isoxazoles. Consequently, this reaction is known as the copper(I)- catalyzed nitrile oxide-azide cycloaddition, abbreviated CuNOAC. See Figure 3. Limited investigations have been conducted on this reaction. See references 9, 10, and 11. Figure 3: The Copper(I)-catalyzed Nitrile Oxide-Azide Cycloaddition (CuNOAC). See reference 6. page 3 of 28 ©2021 by Quanta BioDesign, Ltd. Click Chemistry with dPEG® Products Ruthenium-catalyzed azide-alkyne cycloaddition (RuAAC) In 2005, Li Zhang et al. reported a new type of click chemistry reaction catalyzed by ruthenium(II).12,13,14 At 80°C in benzene, in the presence of 5% Ru(OAc)2(PPh3)2, Zhang and colleagues found that benzyl azide and phenylacetylene reacted to form the 1,4- disubstituted, 1,2,3-triazole product. However, by switching to any one of four different ruthenium catalysts – Cp*RuCl(PPh3)2, [Cp*RuCl]4, CpRuCl(COD), or Cp*RuCl(NBD) – the azide-alkyne ligation formed the 1,5-disubstituted product with a terminal alkyne. See Figure 4.a. Further study showed that the RuAAC reaction works with both terminal and internal alkynes. Ruthenium(II)-catalyzed cycloaddition reactions with internal alkynes form 1,4,5-trisubstituted-1,2,3-triazoles.12,14 Although Cu(I)-catalyzed methods for forming trisubstituted 1,2,3-triazoles exist,15 the RuAAC provided the first facile method to 1,4,5- trisubstituted-1,2,3-triazoles. See Figure 4.b. Figure 4: The Ruthenium(II)-catalyzed Azide-Alkyne Cycloaddition (RuAAC) Reaction. a. Ru(II) catalysts and the isomers they form. b. The RuAAC provides a facile route to 1,4,5-trisubstituted-1,2,3- triazoles. See references 12, 13, and 14 for detailed information Although the RuAAC is a "click chemistry" reaction like the CuAAC, there are key differences. First, the solvent must be aprotic. Examples of compatible solvents for the RuAAC include acetone, benzene, dichloromethane, dimethylformamide, and toluene. Protic solvents such as water, methanol, ethanol, and isopropanol, as well as hexanes, diethyl ether, and ethyl acetate, inhibit catalysis. Second, unlike CuAAC, which progresses at room temperature, the RuAAC requires heating to 50 – 80°C to proceed. Third, while the CuAAC accepts a wide variety of copper sources for reaction catalysis (Cu(I) salts, Cu(II) salts plus a reducing agent, and pure copper metal), the RuAAC is quite limited in catalysts that form the 1,5-isomer. page 4 of 28 ©2021 by Quanta BioDesign, Ltd. Click Chemistry with dPEG® Products Other metal-catalyzed azide-alkyne cycloadditions Metals other than copper(I) and ruthenium (II) catalyze azide-alkyne cycloadditions. Researchers have reported 1,3-dipolar azide-alkyne cycloadditions using indium16, iridium17,18,19 nickel20, rhodium21, and zinc22 in the scientific literature. Also, a 2016 report demonstrated a [4+3] azide-alkyne cycloaddition using yttrium triflate.23 Types of Click Chemistry: Bioorthogonal Click Chemistry Bioorthogonality in bioconjugation The toxicity of copper is one of the problems with the CuAAC and potentially with other metal-catalyzed azide-alkyne cycloadditions.24,25,26 Prolonged exposure of living cells to toxic metals such as Cu(I) damages or kills living cells. Attempts have been made to create non-toxic Cu(I) catalysts that permit live cells to be labeled using the CuAAC.27 Also, metal-catalyzed cycloaddition reactions must react in aqueous media. Thus, for example, the RuAAC cannot be used in aqueous media or with living cells because the metal is toxic, and the catalysts do not work in water.