DNA-Compatible Diazo-Transfer Reaction in Aqueous Media Suitable for DNA-Encoded Chemical Library Synthesis

DNA-Compatible N-Formylation of Amines by Using TMSCF2Br

DNA-encoded libraries (DELs) have emerged as powerful tools in drug discovery. Protected amino acids serve as essential building blocks in the construction of DELs, resulting in the widespread presence of amino groups within these libraries. N-formylation of free amines not only enhances the activity of lead compounds but also functions as an effective amino-protecting strategy. In this study, we introduce trimethyl(bromodifluoromethyl)silane (TMSCF2Br) as a novel N-formylation reagent for DEL synthesis. This approach demonstrates robustness in DEL-compatible synthesis and enables library diversification through functional group transformation (FGT). Additionally, we achieved efficient removal of formyl groups, enabling the formyl group to be strategically used for on-DNA amino protection orthogonal to Fmoc and Boc groups.

Next-Generation Protein Sequencing and individual ion mass spectrometry enable complementary analysis of interleukin-6

The vast complexity of the proteome currently overwhelms any single analytical technology in capturing the full spectrum of proteoform diversity. In this study, we evaluated the complementarity of two cutting-edge proteomic technologies-single-molecule protein sequencing and individual ion mass spectrometry-for analyzing recombinant human IL-6 (rhIL-6) at the amino acid, peptide, and intact proteoform levels. For single-molecule protein sequencing, we employ the recently released Platinum® instrument. Next-Generation Protein Sequencing™ (NGPS™) on Platinum utilizes cycles of N-terminal amino acid recognizer binding and aminopeptidase cleavage to enable parallelized sequencing of single peptide molecules. We found that NGPS produces single amino acid coverage of multiple key regions of IL-6, including two peptides within helices A and C which harbor residues that reportedly impact IL-6 function. For top-down proteoform evaluation, we use individual ion mass spectrometry (I2MS), a highly parallelized orbitrap-based charge detection MS platform. Single ion detection of gas-phase fragmentation products (I2MS2) gives significant sequence coverage in key regions in IL-6, including two regions within helices B and D that are involved in IL-6 signaling. Together, these complementary technologies deliver a combined 52% sequence coverage, offering a more complete view of IL-6 structural and functional diversity than either technology alone. This study highlights the synergy of complementary protein detection methods to more comprehensively cover protein segments relevant to biological interactions.

Chemical Stability of mRNA/cDNA Complexes: Defining the Limits of mRNA Display

Messenger RNA (mRNA) display is being increasingly adopted for peptide drug candidate discovery. While many conditions have been reported for the affinity enrichment step and in some cases for peptide modification, there is still limited understanding about the versatility of peptide-puromycin-mRNA/cDNA (complementary DNA) complexes. This work explores the chemical stability of mRNA/cDNA hybrid complexes under a range of different fundamental chemical conditions as well as with peptide modification conditions reported in an mRNA display setting. We further compare the stability of full complexes originating from two different mRNA display systems (RaPID and cDNA-TRAP). Overall, these complexes were found to be stable under a broad range of conditions, with some edge conditions benefitting from encoding directly in cDNA rather than mRNA. This should allow for more and broader exploitation of late-stage peptide modification chemistry in mRNA display, with confidence regarding the stability of encoding, and potentially better hit-finding campaigns as a result.

FSO2N3-Mediated On-DNA Diazo-Transfer Chemistry

DNA-encoded library (DEL) is a powerful hit selection technique in both basic science and innovative drug discovery. In this study, we report a robust and straightforward DNA-compatible diazo-transfer reaction utilizing FSO2N3 as the diazo-transfer reagent in solution. This reaction demonstrates high conversions and facile operation while being metal-free and maintaining high levels of DNA fidelity. It is also compatible with a wide range of substrates, allowing for convenient access to both aliphatic and aromatic amines. Consequently, it will further enrich the DEL chemistry toolbox.

Automated Synthesis for the Safe Production of Organic Azides from Primary Amines

Described herein is the development of an automated and reproducible process for the conversion of primary amines to organic azides utilizing prepacked capsules containing all the required reagents, including imidazole-1-sulfonyl azide tetrafluoroborate. Apart from manually loading the primary amine into the reaction vessel, the entire reaction and product isolation process can be achieved automatically, with no further user involvement, and delivers the desired organic azide in high purity. This practical and simple automated capsule-based method offers a convenient and safe way of generating organic azides without handling or exposure of potentially explosive reagents.

Protein-templated ligand discovery via the selection of DNA-encoded dynamic libraries

DNA-encoded chemical libraries (DELs) have become a powerful technology platform in drug discovery. Dual-pharmacophore DELs display two sets of small molecules at the termini of DNA duplexes, thereby enabling the identification of synergistic binders against biological targets, and have been successfully applied in fragment-based ligand discovery and affinity maturation of known ligands. However, dual-pharmacophore DELs identify separate binders that require subsequent linking to obtain the full ligands, which is often challenging. Here we report a protein-templated DEL selection approach that can identify full ligand/inhibitor structures from DNA-encoded dynamic libraries (DEDLs) without the need for subsequent fragment linking. Our approach is based on dynamic DNA hybridization and target-templated in situ ligand synthesis, and it incorporates and encodes the linker structures in the library, along with the building blocks, to be sampled by the target protein. To demonstrate the performance of this method, 4.35-million- and 3.00-million-member DEDLs with different library architectures were prepared, and hit selection was achieved against four therapeutically relevant target proteins.

Evolution of chemistry and selection technology for DNA-encoded library

DNA-encoded chemical library (DEL) links the power of amplifiable genetics and the non-self-replicating chemical phenotypes, generating a diverse chemical world. In analogy with the biological world, the DEL world can evolve by using a chemical central dogma, wherein DNA replicates using the PCR reactions to amplify the genetic codes, DNA sequencing transcripts the genetic information, and DNA-compatible synthesis translates into chemical phenotypes. Importantly, DNA-compatible synthesis is the key to expanding the DEL chemical space. Besides, the evolution-driven selection system pushes the chemicals to evolve under the selective pressure, i.e., desired selection strategies. In this perspective, we summarized recent advances in expanding DEL synthetic toolbox and panning strategies, which will shed light on the drug discovery harnessing in vitro evolution of chemicals via DEL.

Mask and Release Strategy-Enabled Diversity-Oriented Synthesis for DNA-Encoded Library

An ideal DNA-encoded library (DEL) selection requires the library to consist of diverse core skeletons and cover chemical space as much as possible. However, the lack of efficient on-DNA synthetic approaches toward core skeletons has greatly restricted the diversity of DEL. To mitigate this issue, this work disclosed a "Mask & Release" strategy to streamline the challenging on-DNA core skeleton synthesis. N-phenoxyacetamide is used as a masked phenol and versatile directing group to mediate diversified DNA-compatible C-H functionalization, introducing the 1st-dimensional diversity at a defined site, and simultaneously releasing the phenol functionality, which can facilitate the introduction of the 2nd diversity. This work not only provides a set of efficient syntheses toward DNA-conjugated drug-like core skeletons such as ortho-alkenyl/sulfiliminyl/cyclopropyl phenol, benzofuran, dihydrobenzofuran but also provides a paradigm for on-DNA core skeleton synthetic method development.

Impact of organic chemistry conditions on DNA durability in the context of DNA-encoded library technology

High-power screening (HPS) technologies, such as DNA-encoded library (DEL) technology, could exponentially increase the dimensions of the chemical space accessible for drug discovery. The intrinsic fragile nature of DNA is associated with cumbersome limitations and DNA durability (e.g., depurination, loss of phosphate groups, adduct formation) is compromised in numerous organic chemistry conditions that require empirical testing. An atlas of reaction conditions (temperature, pH, solvent/buffer, ligands, oxidizing reagents, catalysts, scavengers in function of time) that have been systematically tested in multiple combinations, indicates precisely limits useful for DEL construction. More importantly, this approach could be used broadly to effectively evaluate DNA-compatibility of any novel on-DNA chemical reaction, and it is compatible with different molecular methodologies. This atlas and the general approach presented, by allowing novel reaction conditions to be performed in presence of DNA, should greatly help in expanding the DEL chemical space as well as any field involving DNA durability.

Enolate-Azide [3 + 2]-Cycloaddition Reaction Suitable for DNA-Encoded Library Synthesis

The DNA-encoded chemical library (DEL) is a powerful hit selection technique in either basic science or innovative drug discovery. With the aim to circumvent the issue concerning DNA barcode damage in a conventional on-DNA copper-catalyzed azide-alkyne cycloaddition reaction (CuAAC), we have successfully developed the first DNA-compatible enolate-azide [3 + 2] cycloaddition reaction. The merits of this DEL chemistry include metal-free reaction and high DNA fidelity, high conversions and easy operation, broad substrate scope, and ready access to the highly substituted 1,4,5-trisubstituted triazoles. Thus, it will not only further enrich the DEL chemistry toolbox but also will have great potential in practical DEL synthesis.

Diversity-Oriented Synthesis (DOS) of On-DNA Peptidomimetics from Acid-Derived Phosphonium Ylides

The DNA-encoded library (DEL) technology represents a revolutionary drug-discovery tool with unprecedented screening power originating from the association of combinatorial chemistry and DNA barcoding. The chemical diversity of DELs and its chemical space will be further expanded as new DNA-compatible reactions are introduced. This work introduces the use of DOS in the context of on-DNA peptidomimetics. Wittig olefination of aspartic acid-derived on-DNA Wittig ylide, combined with a broad substrate scope of aldehydes, led to formation of on-DNA α ${\alpha }$ , β ${\beta }$ -unsaturated ketones. The synthesis of on-DNA multi-peptidyl-ylides was performed by incorporating sequential amino acids onto a monomeric ylide. Di-, tri- and tetrameric peptidyl-ylides were validated for Wittig olefination and led to on-DNA α ${\alpha }$ , β ${\beta }$ -unsaturated-based peptidomimetics, an important class of intermediates. One on-DNA aryl Wittig ylide was also developed and applied to Wittig olefination for synthesis of on-DNA chalcone-based molecules. Furthermore, DOS was used successfully with electron-deficient peptidomimetics and led to the development of different heterocyclic cores containing on-DNA peptidomimetics.

Metal-Free and Open-Air Arylation Reactions of Diaryliodonium Salts for DNA-Encoded Library Synthesis

A successful DNA-encoded library (DEL) will consist of diverse skeletons and cover chemical space as comprehensive as possible to fully realize its potential in drug discovery and chemical biology. However, the lack of versatile on-DNA arylation methods for phenols that are less nucleophilic and reactive poses a great hurdle for DEL to include diaryl ether, a privileged chemotype in pharmaceuticals and natural products. This work describes the use of "substrate activation" approach to address the arylation of DNA-conjugated phenols. Diaryliodonium salt, a highly electrophilic and reactive arylation reagent, is employed as Ar+ sources to ensure highly selective on-DNA arylation of phenols and oximes with both high yields and DNA fidelity. Notably, the new on-DNA arylation reaction can be applied to the late-stage modification of peptides containing tyrosine side-chain and to synthesize DNA-tagged analogues of existing drug molecules such as sorafenib, a known pan-kinase inhibitor. The new on-DNA diaryliodonium salts chemistry affords a greater flexibility in DEL design and synthesis.

Opportunities for Expanding Encoded Chemical Diversification and Improving Hit Enrichment in mRNA-Displayed Peptide Libraries

DNA-encoded small-molecule libraries and mRNA displayed peptide libraries both use numerically large pools of oligonucleotide-tagged molecules to identify potential hits for protein targets. They differ dramatically, however, in the 'drug-likeness' of the molecules that each can be used to discover. We give here an overview of the two techniques, comparing some advantages and disadvantages of each, and suggest areas where particularly mRNA display can benefit from adopting advances developed with DNA-encoded small molecule libraries. We outline cases where chemical modification of the peptide library has already been used in mRNA display, and survey opportunities to expand this using examples from DNA-encoded small molecule libraries. We also propose potential opportunities for encoding such reactions within the mRNA/cDNA tag of an mRNA-displayed peptide library to allow a more diversity-oriented approach to library modification. Finally, we outline alternate approaches for enriching target-binding hits from a pooled and tagged library, and close by detailing several examples of how an adjusted mRNA-display based approach could be used to discover new 'drug-like' modified small peptides.

Azide-modified Nucleosides as Versatile Tools for Bioorthogonal Labeling and Functionalization

Azide-modified nucleosides are important building blocks for RNA and DNA functionalization by click chemistry based on azide-alkyne cycloaddition. This has put demand on synthetic chemistry to develop approaches for the preparation of azide-modified nucleoside derivatives. We review here the available methods for the synthesis of various nucleosides decorated with azido groups at the sugar residue or nucleobase, their incorporation into oligonucleotides and cellular RNAs, and their application in azide-alkyne cycloadditions for labelling and functionalization.

Photochemical C-H arylation of heteroarenes for DNA-encoded library synthesis

DNA-encoded library (DEL) technology has emerged as a time- and cost-efficient technique for the identification of therapeutic candidates in the pharmaceutical industry. Although several reaction classes have been successfully validated in DEL environments, there remains a paucity of DNA-compatible reactions that harness building blocks (BBs) from readily available substructures bearing multifunctional handles for further library diversification under mild, dilute, and aqueous conditions. In this study, the direct C-H carbofunctionalization of medicinally-relevant heteroarenes can be accomplished via the photoreduction of DNA-conjugated (hetero)aryl halides to deliver reactive aryl radical intermediates in a regulated fashion within minutes of blue light illumination. A broad array of electron-rich and electron-poor heteroarene scaffolds undergo transformation in the presence of sensitive functional groups.

On-DNA-1,2,3-Triazole Formation via Click Reaction

Copper-catalyzed azide-alkyne cycloaddition (CuAAC) provides a simple and convenient strategy to synthesize diverse 1,2,3-triazoles for drug discovery. Described herein is a protocol for the CuSO₄-catalyzed cycloaddition between alkynes and DNA-appended azides.

The expanding reaction toolkit for DNA-encoded libraries

Over the past decade, DNA-encoded libraries (DELs) have emerged as a leading platform for small molecule drug discovery among pharmaceutical companies, biotech companies and academic drug hunters alike. This revolutionary technology has tremendous potential that is yet to be fully realized, as the exploration of therapeutically relevant chemical space is fueled by the ever-expanding repertoire of DNA-compatible reactions used to construct the libraries. Advances in direct coupling reactions, like photo-catalytic cross couplings, unique cyclizations such as the formation of 1,2,4-oxadiazoles, and new functional group transformations are valuable contributions to the DEL reaction toolkit, and indicate where future reaction development efforts should focus in order to maximize the productivity of DELs.

Convertible and Constrained Nucleotides: The 2'-Deoxyribose 5'-C-Functionalization Approach, a French Touch

Many strategies have been developed to modulate the biological or biotechnical properties of oligonucleotides by introducing new chemical functionalities or by enhancing their affinity and specificity while restricting their conformational space. Among them, we review our approach consisting of modifications of the 5'-C-position of the nucleoside sugar. This allows the introduction of an additional chemical handle at any position on the nucleotide chain without disturbing the Watson-Crick base-pairing. We show that 5'-C bromo or propargyl convertible nucleotides (CvN) are accessible in pure diastereoisomeric form, either for nucleophilic displacement or for CuAAC conjugation. Alternatively, the 5'-carbon can be connected in a stereo-controlled manner to the phosphate moiety of the nucleotide chain to generate conformationally constrained nucleotides (CNA). These allow the precise control of the sugar/phosphate backbone torsional angles. The consequent modulation of the nucleic acid shape induces outstanding stabilization properties of duplex or hairpin structures in accordance with the preorganization concept. Some biological applications of these distorted oligonucleotides are also described. Effectively, the convertible and the constrained approaches have been merged to create constrained and convertible nucleotides (C2NA) providing unique tools to functionalize and stabilize nucleic acids.

DNA-Encoded Chemical Libraries: A Comprehensive Review with Succesful Stories and Future Challenges

DNA-encoded chemical libraries (DELs) represent a versatile and powerful technology platform for the discovery of small-molecule ligands to protein targets of biological and pharmaceutical interest. DELs are collections of molecules, individually coupled to distinctive DNA tags serving as amplifiable identification barcodes. Thanks to advances in DNA-compatible reactions, selection methodologies, next-generation sequencing, and data analysis, DEL technology allows the construction and screening of libraries of unprecedented size, which has led to the discovery of highly potent ligands, some of which have progressed to clinical trials. In this Review, we present an overview of diverse approaches for the generation and screening of DEL molecular repertoires. Recent success stories are described, detailing how novel ligands were isolated from DEL screening campaigns and were further optimized by medicinal chemistry. The goal of the Review is to capture some of the most recent developments in the field, while also elaborating on future challenges to further improve DEL technology as a therapeutic discovery platform.

On-DNA Derivatization of Quinoxalin-2-ones by Visible-Light-Triggered Alkylation with Carboxylic Acids

An efficient visible-light-induced alkylation of DNA-tagged quinoxaline-2-ones was described. The methodology demonstrated moderate-to-excellent conversions under mild conditions. The reaction was found to be tolerant with both N-protected α-amino acids and aliphatic carboxylic acids and could be applied to the synthesis of focused DNA-encoded quinoxalin-2-one libraries.

DNA-Encoded Chemistry: Drug Discovery from a Few Good Reactions

Click chemistry, proposed nearly 20 years ago, promised access to novel chemical space by empowering combinatorial library synthesis with a "few good reactions". These click reactions fulfilled key criteria (broad scope, quantitative yield, abundant starting material, mild reaction conditions, and high chemoselectivity), keeping the focus on molecules that would be easy to make, yet structurally diverse. This philosophy bears a striking resemblance to DNA-encoded library (DEL) technology, the now-dominant combinatorial chemistry paradigm. This review highlights the similarities between click and DEL reaction design and deployment in combinatorial library settings, providing a framework for the design of new DEL synthesis technologies to enable next-generation drug discovery.

DNA-Compatible Click Reaction Employing In Situ Generated Azides from Boronic Acids

An efficient method for the synthesis of DNA-conjugated 1,2,3-triazoles is copper (II) [Cu(II)-β-cyclodextrin]-mediated Huisgen cycloaddition ("click reaction") of DNA-conjugated alkynes with azides. However, a diverse array of building blocks is required to produce useful DNA encoded libraries, and the commercial availability of azides is limited. The method described herein generates azides in situ from aryl borates and TMSN₃ , which then further react with DNA-conjugated terminal alkynes. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Conjugation of PEG linker to DNA headpiece Basic Protocol 2: DNA conjugated terminal alkyne preparation Basic Protocol 3: DNA compatible one-pot click reaction Basic Protocol 4: LCMS monitoring.

Development of DNA-compatible hydroxycarbonylation reactions using chloroform as a source of carbon monoxide

A robust palladium-catalyzed hydroxycarbonylation of aryl halides on DNA has been developed. Instead of Mo(CO)₆ as a source of carbon monoxide as previously described in the literature, chloroform was used as a surrogate in this report for the purpose of avoiding to use a large excess of molybdenum reagent which is not totally soluble in aqueous reaction mixtures.

Amine-to-Azide Conversion on Native RNA via Metal-Free Diazotransfer Opens New Avenues for RNA Manipulations

A major challenge in the field of RNA chemistry is the identification of selective and quantitative conversion reactions on RNA that can be used for tagging and any other RNA tool development. Here, we introduce metal-free diazotransfer on native RNA containing an aliphatic primary amino group using the diazotizing reagent fluorosulfuryl azide (FSO2N3). The reaction provides the corresponding azide-modified RNA in nearly quantitatively yields without affecting the nucleobase amino groups. The obtained azido-RNA can then be further processed utilizing well-established bioortho-gonal reactions, such as azide-alkyne cycloadditions (Click) or Staudinger ligations. We exemplify the robustness of this approach for the synthesis of peptidyl-tRNA mimics and for the pull-down of 3-(3-amino-3-carboxypropyl)uridine (acp3U)- and lysidine (k2C)-containing tRNAs of an Escherichia coli tRNA pool isolated from cellular extracts. Our approach therefore adds a new dimension to the targeted chemical manipulation of diverse RNA species.

Improved Diazo-Transfer Reaction for DNA-Encoded Chemistry and Its Potential Application for Macrocyclic DEL-Libraries

DNA-encoded libraries (DEL) are increasingly being used to identify new starting points for medicinal chemistry in drug discovery. Herein, we discuss the development of methods that allow the conversion of both primary amines and anilines, attached to DNA, to their corresponding azides in excellent yields. The scope of these diazo-transfer reactions was investigated, and a proof-of-concept has been devised to allow for the synthesis of macrocycles on DNA.

Amine-to-Azide Conversion on Native RNA via Metal-Free Diazotransfer Opens New Avenues for RNA Manipulations

A major challenge in the field of RNA chemistry is the identification of selective and quantitative conversion reactions on RNA that can be used for tagging and any other RNA tool development. Here, we introduce metal-free diazotransfer on native RNA containing an aliphatic primary amino group using the diazotizing reagent fluorosulfuryl azide (FSO2 N3 ). The reaction provides the corresponding azide-modified RNA in nearly quantitatively yields without affecting the nucleobase amino groups. The obtained azido-RNA can then be further processed utilizing well-established bioortho-gonal reactions, such as azide-alkyne cycloadditions (Click) or Staudinger ligations. We exemplify the robustness of this approach for the synthesis of peptidyl-tRNA mimics and for the pull-down of 3-(3-amino-3-carboxypropyl)uridine (acp3 U)- and lysidine (k2 C)-containing tRNAs of an Escherichia coli tRNA pool isolated from cellular extracts. Our approach therefore adds a new dimension to the targeted chemical manipulation of diverse RNA species.

Developments in Photoredox-Mediated Alkylation for DNA-Encoded Libraries

Recently, DNA-encoded library (DEL) technology has emerged as an innovative screening modality for the rapid discovery of therapeutic candidates in pharmaceutical settings. This platform enables a cost-effective, time-efficient, and large-scale assembly and interrogation of billions of small organic ligands against a biological target in a single experiment. An outstanding challenge in DEL synthesis is the necessity for water-compatible transformations under ambient conditions. To access uncharted chemical space, the adoption of photoredox catalysis in DELs, including Ni-catalyzed manifolds and radical/polar crossover reactions, has enabled the construction of novel structural scaffolds through regulated odd-electron intermediates. Herein, a critical discussion of the validation of photoredox-mediated alkylation in DEL environments is presented. Current synthetic gaps are highlighted and opportunities for further development are speculated upon.

Solution-Phase Fmoc-Based Peptide Synthesis for DNA-Encoded Chemical Libraries: Reaction Conditions, Protecting Group Strategies, and Pitfalls

Peptide drug discovery has shown a resurgence since 2000, bringing 28 non-insulin therapeutics to the market compared to 56 since its first peptide drug, insulin, in 1923. While the main method of discovery has been biological display-phage, mRNA, and ribosome-the synthetic limitations of biological systems has restricted the depth of exploration of peptide chemical space. In contrast, DNA-encoded chemistry offers the synergy of large numbers and ribosome-independent synthetic flexibility for the fast and deeper exploration of the same space. Hence, as a bridge to building DNA-encoded chemical libraries (DECLs) of peptides, we have developed substrate-tolerant amide coupling reaction conditions for amino acid monomers, performed a coupling screen to illustrate such tolerance, developed protecting group strategies for relevant amino acids and reported the limitations thereof, developed a strategy for the coupling of α,α-disubstituted alkenyl amino acids relevant to all-hydrocarbon stapled peptide drug discovery, developed reaction conditions for the coupling of tripeptides likely to be used in DECL builds, and synthesized a fully deprotected DNA-decamer conjugate to illustrate the potency of the developed methodology for on-DNA peptide synthesis.

Selective Fragments for the CREBBP Bromodomain Identified from an Encoded Self-assembly Chemical Library

DNA-encoded chemical libraries (DECLs) are collections of chemical moieties individually coupled to distinctive DNA barcodes. Compounds can be displayed either at the end of a single DNA strand (i. e., single-pharmacophore libraries) or at the extremities of two complementary DNA strands (i. e., dual-pharmacophore libraries). In this work, we describe the use of a dual-pharmacophore encoded self-assembly chemical (ESAC) library for the affinity maturation of a known 4,5-dihydrobenzodiazepinone ring (THBD) acetyl-lysine (KAc) mimic for the cyclic-AMP response element binding protein (CREB) binding protein (CREBBP or CBP) bromodomain. The new pair of fragments discovered from library selection showed a sub-micromolar affinity for the CREBBP bromodomain in fluorescence polarization and ELISA assays, and selectivity against BRD4(1).

An overview of DNA-encoded libraries: A versatile tool for drug discovery

DNA-encoded libraries (DELs) are collections of small molecules covalently attached to amplifiable DNA tags carrying unique information about the structure of each library member. A combinatorial approach is used to construct the libraries with iterative DNA encoding steps, facilitating tracking of the synthetic history of the attached compounds by DNA sequencing. Various screening protocols have been developed which allow protein target binders to be selected out of pools containing up to billions of different small molecules. The versatile methodology has allowed identification of numerous biologically active compounds and is now increasingly being adopted as a tool for lead discovery campaigns and identification of chemical probes. A great focus in recent years has been on developing DNA compatible chemistries that expand the structural diversity of the small molecule library members in DELs. This chapter provides an overview of the challenges and accomplishments in DEL technology, reviewing the technological aspects of producing and screening DELs with a perspective on opportunities, limitations, and future directions.

Homogeneous and Functional Group Tolerant Ring-Closing Metathesis for DNA-Encoded Chemical Libraries

Reaction heterogeneity, poor pH control, and catalyst decomposition in the ring-closing metathesis (RCM) of DNA-chemical conjugates lead to poor yields of the cyclized products. Herein we address these issues with a RCM reaction system that includes a novel aqueous solvent combination to enable reaction homogeneity, an acidic buffer system which masks traditionally problematic functional groups, and a decomposition-resistant catalyst which maximizes conversion to the cyclized product. Additionally, we provide a systematic study of the substrate scope of the on-DNA RCM reaction, a demonstration of its applicability to a single-substrate DNA-encoded chemical library that includes sequencing analysis, and the first successful stapling of an unprotected on-DNA [i, i+4] peptide.