This article focuses only on one specialized aspect of the subject.(October 2014) |
Dynamic combinatorial chemistry (DCC); also known as constitutional dynamic chemistry (CDC) is a method to the generation of new molecules formed by reversible reaction of simple building blocks under thermodynamic control. [3] [4] The library[ further explanation needed ] of these reversibly interconverting building blocks is called a dynamic combinatorial library (DCL). [5] [6] All constituents in a DCL are in equilibrium, and their distribution is determined by their thermodynamic stability within the DCL. The interconversion of these building blocks may involve covalent or non-covalent interactions. When a DCL is exposed to an external influence (such as proteins or nucleic acids), the equilibrium shifts and those components that interact with the external influence are stabilised and amplified, allowing more of the active compound to be formed.
By modern definition, dynamic combinatorial chemistry is generally considered to be a method of facilitating the generation of new chemical species by the reversible linkage of simple building blocks, under thermodynamic control. [4] This principle is known to select the most thermodynamically stable product from an equilibrating mixture of a number of components, a concept commonly utilised in synthetic chemistry to direct the control of reaction selectivity. [7] Although this approach was arguably utilised in the work of Fischer [8] and Werner [9] as early as the 19th century, their respective studies of carbohydrate and coordination chemistry were restricted to rudimentary speculation, requiring the rationale of modern thermodynamics. [10] [11] It was not until supramolecular chemistry revealed early concepts of molecular recognition, complementarity and self-organisation that chemists could begin to employ strategies for the rational design and synthesis of macromolecular targets. [12] The concept of template synthesis was further developed and rationalised through the pioneering work of Busch in the 1960s, which clearly defined the role of a metal ion template in stabilising the desired ‘thermodynamic’ product, allowing for its isolation from the complex equilibrating mixture. [13] [14] Although the work of Busch helped to establish the template method as a powerful synthetic route to stable macrocyclic structures, this approach remained exclusively within the domain of inorganic chemistry until the early 1990s, when Sanders et al. first proposed the concept of dynamic combinatorial chemistry. [4] Their work combined thermodynamic templation in tandem with combinatorial chemistry, to generate an ensemble complex porphyrin and imine macrocycles using a modest selection of simple building blocks.
Sanders then developed this early manifestation of dynamic combinatorial chemistry as a strategy for organic synthesis; the first example being the thermodynamically-controlled macrolactonisation of oligocholates to assemble cyclic steroid-derived macrocycles capable of interconversion via component exchange. [15] Early work by Sanders et al. employed transesterification to generate dynamic combinatorial libraries. In retrospect, it was unfortunate that esters were selected for mediating component exchange, as transesterification processes are inherently slow and require vigorous anhydrous conditions. [4] However, their subsequent investigations identified that both the disulfide and hydrazone covalent bonds exhibit effective component exchange processes and so present a reliable means of generating dynamic combinatorial libraries capable of thermodynamic templation. This chemistry now forms the basis of much research in the developing field of dynamic covalent chemistry, and has in recent years emerged as a powerful tool for the discovery of molecular receptors.
One of the key developments within the field of DCC is the use of proteins (or other biological macromolecules, such as nucleic acids) to influence the evolution and generation of components within a DCL. [16] [17] [18] [19] [20] [21] Protein-directed DCC provides a way to generate, identify and rank novel protein ligands, and therefore have huge potential in the areas of enzyme inhibition and drug discovery. [22]
The development of protein-directed DCC has not been straightforward because the reversible reactions employed must occur in aqueous solution at biological pH and temperature, and the components of the DCL must be compatible with proteins. [16] [22]
Several reversible reactions have been proposed and/or applied in protein-directed DCC. These included boronate ester formation, [23] [24] [25] diselenides-disulfides exchange, [26] disulphide formation, [27] [28] [29] hemithiolacetal formation, [30] [31] hydrazone formation, [32] [33] imine formation [34] [35] [36] and thiol-enone exchange. [37]
For reversible reactions that do not occur in aqueous buffers, the pre-equilibrated DCC approach can be used. The DCL was initially generated (or pre-equilibrated) in organic solvent, and then diluted into aqueous buffer containing the protein target for selection. Organic based reversible reactions, including Diels-Alder [38] and alkene cross metathesis reactions, [39] have been proposed or applied to protein-directed DCC using this method.
Reversible non-covalent reactions, such as metal-ligand coordination, [40] [41] has also been applied in protein-directed DCC. This strategy is useful for the investigation of the optimal ligand stereochemistry to the binding site of the target protein. [42]
Enzyme-catalysed reversible reactions, such as protease-catalysed amide bond formation/hydrolysis reactions [43] and the aldolase-catalysed aldol reactions, [44] [45] have also been applied to protein-directed DCC.
Protein-directed DCC system must be amenable to efficient screening. [16] [22] Several analytical techniques have been applied to the analysis of protein-directed DCL. These include HPLC, [27] [31] [32] [35] mass spectrometry, [24] [28] [29] [33] NMR spectroscopy, [23] [25] [30] and X-ray crystallography. [46]
Although most applications of protein-directed DCC to date involved the use of single protein in the DCL, it is possible to identify protein ligands by using multiple proteins simultaneously, as long as a suitable analytical technique is available to detect the protein species that interact with the DCL components. [47] This approach may be used to identify specific inhibitors or broad-spectrum enzyme inhibitors.
DCC is useful in identifying molecules with unusual binding properties, and provides synthetic routes to complex molecules that aren't easily accessible by other means. These include smart materials, foldamers, self-assembling molecules with interlocking architectures and new soft materials. [4] The application of DCC to detect volatile bioactive compounds, i.e. the amplification and sensing of scent, was proposed in a concept paper. [48] Recently, DCC was also used to study the abiotic origins of life. [49]
A rotaxane is a mechanically interlocked molecular architecture consisting of a dumbbell-shaped molecule which is threaded through a macrocycle. The two components of a rotaxane are kinetically trapped since the ends of the dumbbell are larger than the internal diameter of the ring and prevent dissociation (unthreading) of the components since this would require significant distortion of the covalent bonds.
Ferrocene is an organometallic compound with the formula Fe(C5H5)2. The molecule is a complex consisting of two cyclopentadienyl rings bound to a central iron atom. It is an orange solid with a camphor-like odor, that sublimes above room temperature, and is soluble in most organic solvents. It is remarkable for its stability: it is unaffected by air, water, strong bases, and can be heated to 400 °C without decomposition. In oxidizing conditions it can reversibly react with strong acids to form the ferrocenium cation Fe(C5H5)+2. Ferrocene and the ferrocenium cation are sometimes abbreviated as Fc and Fc+ respectively.
In biochemistry and molecular biology, a binding site is a region on a macromolecule such as a protein that binds to another molecule with specificity. The binding partner of the macromolecule is often referred to as a ligand. Ligands may include other proteins, enzyme substrates, second messengers, hormones, or allosteric modulators. The binding event is often, but not always, accompanied by a conformational change that alters the protein's function. Binding to protein binding sites is most often reversible, but can also be covalent reversible or irreversible.
Supramolecular chemistry refers to the branch of chemistry concerning chemical systems composed of a discrete number of molecules. The strength of the forces responsible for spatial organization of the system range from weak intermolecular forces, electrostatic charge, or hydrogen bonding to strong covalent bonding, provided that the electronic coupling strength remains small relative to the energy parameters of the component. While traditional chemistry concentrates on the covalent bond, supramolecular chemistry examines the weaker and reversible non-covalent interactions between molecules. These forces include hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, pi–pi interactions and electrostatic effects.
In macromolecular chemistry, a catenane is a mechanically interlocked molecular architecture consisting of two or more interlocked macrocycles, i.e. a molecule containing two or more intertwined rings. The interlocked rings cannot be separated without breaking the covalent bonds of the macrocycles. They are conceptually related to other mechanically interlocked molecular architectures, such as rotaxanes, molecular knots or molecular Borromean rings. Recently the terminology "mechanical bond" has been coined that describes the connection between the macrocycles of a catenane. Catenanes have been synthesised in two different ways: statistical synthesis and template-directed synthesis.
Reductive elimination is an elementary step in organometallic chemistry in which the oxidation state of the metal center decreases while forming a new covalent bond between two ligands. It is the microscopic reverse of oxidative addition, and is often the product-forming step in many catalytic processes. Since oxidative addition and reductive elimination are reverse reactions, the same mechanisms apply for both processes, and the product equilibrium depends on the thermodynamics of both directions.
In organic chemistry, the Ugi reaction is a multi-component reaction involving a ketone or aldehyde, an amine, an isocyanide and a carboxylic acid to form a bis-amide. The reaction is named after Ivar Karl Ugi, who first reported this reaction in 1959.
In chemical synthesis, click chemistry is a class of simple, atom-economy reactions commonly used for joining two molecular entities of choice. Click chemistry is not a single specific reaction, but describes a way of generating products that follow examples in nature, which also generates substances by joining small modular units. In many applications, click reactions join a biomolecule and a reporter molecule. Click chemistry is not limited to biological conditions: the concept of a "click" reaction has been used in chemoproteomic, pharmacological, biomimetic and molecular machinery applications. However, they have been made notably useful in the detection, localization and qualification of biomolecules.
The azide-alkyne Huisgen cycloaddition is a 1,3-dipolar cycloaddition between an azide and a terminal or internal alkyne to give a 1,2,3-triazole. Rolf Huisgen was the first to understand the scope of this organic reaction. American chemist Karl Barry Sharpless has referred to this cycloaddition as "the cream of the crop" of click chemistry and "the premier example of a click reaction".
Dynamic covalent chemistry (DCvC) is a synthetic strategy employed by chemists to make complex molecular and supramolecular assemblies from discrete molecular building blocks. DCvC has allowed access to complex assemblies such as covalent organic frameworks, molecular knots, polymers, and novel macrocycles. Not to be confused with dynamic combinatorial chemistry, DCvC concerns only covalent bonding interactions. As such, it only encompasses a subset of supramolecular chemistries.
In chemistry, a foldamer is a discrete chain molecule (oligomer) that folds into a conformationally ordered state in solution. They are artificial molecules that mimic the ability of proteins, nucleic acids, and polysaccharides to fold into well-defined conformations, such as α-helices and β-sheets. The structure of a foldamer is stabilized by noncovalent interactions between nonadjacent monomers. Foldamers are studied with the main goal of designing large molecules with predictable structures. The study of foldamers is related to the themes of molecular self-assembly, molecular recognition, and host–guest chemistry.
In chemistry, mechanically interlocked molecular architectures (MIMAs) are molecules that are connected as a consequence of their topology. This connection of molecules is analogous to keys on a keychain loop. The keys are not directly connected to the keychain loop but they cannot be separated without breaking the loop. On the molecular level, the interlocked molecules cannot be separated without the breaking of the covalent bonds that comprise the conjoined molecules; this is referred to as a mechanical bond. Examples of mechanically interlocked molecular architectures include catenanes, rotaxanes, molecular knots, and molecular Borromean rings. Work in this area was recognized with the 2016 Nobel Prize in Chemistry to Bernard L. Feringa, Jean-Pierre Sauvage, and J. Fraser Stoddart.
The Meerwein–Ponndorf–Verley (MPV) reduction in organic chemistry is the reduction of ketones and aldehydes to their corresponding alcohols utilizing aluminium alkoxide catalysis in the presence of a sacrificial alcohol. The advantages of the MPV reduction lie in its high chemoselectivity, and its use of a cheap environmentally friendly metal catalyst. MPV reductions have been described as "obsolete" owing to the development of sodium borohydride and related reagents.
Molecular binding is an attractive interaction between two molecules that results in a stable association in which the molecules are in close proximity to each other. It is formed when atoms or molecules bind together by sharing of electrons. It often, but not always, involves some chemical bonding.
Smart ligands are affinity ligands selected with pre-defined equilibrium, kinetic and thermodynamic parameters of biomolecular interaction.
Jeremy Keith Morris Sanders is a British chemist and Emeritus Professor in the Department of Chemistry at the University of Cambridge. He is also Editor-in-Chief of Royal Society Open Science. He is known for his contributions to many fields including NMR spectroscopy and supramolecular chemistry. He served as the Pro-Vice-Chancellor for Institutional Affairs at the University of Cambridge, 2011–2015.
A two-dimensional polymer (2DP) is a sheet-like monomolecular macromolecule consisting of laterally connected repeat units with end groups along all edges. This recent definition of 2DP is based on Hermann Staudinger's polymer concept from the 1920s. According to this, covalent long chain molecules ("Makromoleküle") do exist and are composed of a sequence of linearly connected repeat units and end groups at both termini.
Supramolecular catalysis is not a well-defined field but it generally refers to an application of supramolecular chemistry, especially molecular recognition and guest binding, toward catalysis. This field was originally inspired by enzymatic system which, unlike classical organic chemistry reactions, utilizes non-covalent interactions such as hydrogen bonding, cation-pi interaction, and hydrophobic forces to dramatically accelerate rate of reaction and/or allow highly selective reactions to occur. Because enzymes are structurally complex and difficult to modify, supramolecular catalysts offer a simpler model for studying factors involved in catalytic efficiency of the enzyme. Another goal that motivates this field is the development of efficient and practical catalysts that may or may not have an enzyme equivalent in nature.
Targeted covalent inhibitors (TCIs) or Targeted covalent drugs are rationally designed inhibitors that bind and then bond to their target proteins. These inhibitors possess a bond-forming functional group of low chemical reactivity that, following binding to the target protein, is positioned to react rapidly with a proximate nucleophilic residue at the target site to form a bond.
Systems chemistry is the science of studying networks of interacting molecules, to create new functions from a set of molecules with different hierarchical levels and emergent properties.