The Organic Topical Group holds two-three meetings a year at the New York Academy of Science as part of the “Chemical Biology Discussion Group”.  These meetings are regularly attended by 70-80 students postdoctoral fellows and faculty members from the Tri-state area schools including members of Albert Einstein College of Medicine, City College, Columbia, Hunter College, NYU, Polytechnic, Princeton, Rockefeller, Sloan Kettering, Stony Brook, and Yale University.  These meetings are highlighted on the web pages of ACS Chemical Biology and Nature Chemical Biology.   Following meetings were held in the 2008-2009 academic year:

The year-end meeting of the Chemical Biology Discussion Group, held June 1, 2009, highlighted the diverse spokes of chemical biology around a central theme: biomolecular recognition.  In his keynote talk, Adrian Whitty described an effort to find small molecules that disrupt protein?protein interactions.  Shorter student and postdoc talks covered attempts to diversify the palette of small molecules available for drug discovery, new biomolecular switches, a new antibiotic target, chemical analogs for studying DNA damage, sorting cellular proteins, and a synthetic motor built from and fueled by strands of DNA.  While very different in scope, all linked back to the central issue: harnessing the chemistry of biology for new applications.

Speakers: Adrian Whitty (Boston University);  Renato Bauer (Memorial Sloan Kettering Cancer Center);  Angelo Guainazzi (Stony Brook University);  Scott Lefurgy (Albert Einstein College of Medicine, Yeshiva University);  Guillaume Charron (The Rockefeller University)

Developing small molecule inhibitors of protein-protein interaction (PPI) interfaces remains among the most difficult challenges facing contemporary drug discovery. In this talk I will discuss some of the factors that determine the "druggability" of PPI targets, what we have learned to date about the strengths and weaknesses of fragment-based approaches for addressing such targets, and what to look for in target sites and in fragment hits to determine which are most likely to be advanceable to pharmaceutically-relevant lead compounds. The talk will be illustrated using unpublished data obtained during a multi-target collaboration between Biogen Idec and Sunesis Pharmaceuticals, co-led by the author, in which Sunesis's proprietary Tethering® technology was used to search for leads against TNFa and other highly challenging protein-protein interaction targets.

Diversity-oriented synthesis (DOS) is a major research area through which the potentials of organic synthesis are currently being tapped to impact biology and medicine. In practice, collections of compounds derived from DOS are screened in a high-throughput manner to find novel small molecules that interact with target proteins in biochemical assays or that modulate cellular pathways in phenotypic assays. Here, we present a DOS strategy that exploits optically active t-butylsulfinamides as lynchpins for the transition metal-mediated cyclizations of enynes or diynes. The required enynes and diynes were synthesized enantioselectively in three steps and, upon treatment with transition metal-based reagents, produced functionalized mono- and bicycles as end products. The present work addresses reactivity patterns of important cycloaddition and cycloisomerization reactions in terms of yield, regioselectivity, and diastereoselectivity, and also demonstrates how a strategically designed synthetic route can rapidly yield novel architectures for biological evaluation. Our particular strategy gives rise to eight different scaffolds based on those found in polycyclic terpenoid and alkaloid natural products.

Nitrogen mustards (NM) are a group of bifunctional alkylating agents that react with the N(7) atom of guanine residues forming interstrand crosslinks (ICLs). ICLs are very cytotoxic since they inhibit vital cellular processes such as transcription and replication by covalently linking two opposite DNA strands. Despite the importance of ICL-forming agents in cancer chemotherapy, the mechanism by which these lesions are repaired remains poorly understood. A major impediment in studying ICLs repair has been the limited availability of well-defined substrates. We have developed a new strategy that enables the synthesis of defined site-specific NM-like ICLs in high yields and purity. Our strategy relies on the incorporation of ICL precursors bearing reactive aldehyde functionality on complementary strands of DNA, followed by ICL formation via double reductive amination. The synthetic substrates, which bear chemical modification with respect to therapeutic NM ICLs, were validated through molecular dynamic studies, confirming that the mimic had identical structural features to its natural counterpart. Our synthetic approach furthermore allows for the synthesis of major groove ICLs with different amount of distortion, providing unique and valuable tools for biochemical and cell biological studies of ICL repair.

Biological bipedal motors, such as kinesin, myosin, and dynein are all examples of coordinated activity between two motor domains that lead to processive linear movement along directionally polar tracks. How such directed motion emerges from domain coordination is a major issue in the effort to create synthetic molecular motors that can cyclically bias Brownian motion using chemical energy as input (1). Synthetic DNA walking devices (2 - 5) are useful systems to explore these questions, due to DNA's programmability and structural robustness. A benchmark goal is the design and construction of controlled autonomous translocators, for example to use in synthetic molecular assembly procedures that emulate nucleic acid polymerases or the ribosome.

To address this problem, we have contructed an autonomous bipedal walker made of DNA that walks along a directionally polar DNA track that is consumed during the walking cycle. This device displays true motor behavior by coordinating the stepping cycle of its two legs as it walks along its track; it does this by having its leading leg catalyze the release of its trailing leg. The release signal, sent from the leading leg to the trailing leg, is mediated by metastable DNA fuel strand complexes (4 - 7), and aided by the structural asymmetry of the track. The basis of our demonstration entails crosslinking aliquots of the walker covalently to its track in successive walking states, showing that the walker can complete a full walking cycle on a stiff linear track whose length could be extended for longer walks.

Streptococcus pneumoniae is a leading cause of death among children worldwide. The increasing prevalence of multi-drug resistant S. pneumoniae continually requires new approaches to combat this threat. Our laboratory has discovered an antibiotic target in this organism—mevalonate kinase (MK), which catalyzes the first step in the conversion of mevalonate to the isoprenoid building block, isopentenyl diphosphate. Mevalonate kinase is potently, allosterically inhibited by diphosphomevalonate (DPM), whereas human MK is not inhibited by DPM. To assess the spectrum of DPM inhibition, MK homologs from pathogenic bacteria were assessed for their sensitivity to DPM. Surprisingly, these homologs are inhibited via a completely different mechanism that appears to hinge on the oligomeric state of the enzyme. This result suggests that DPM may be an exquisitely narrow-spectrum antibiotic capable of killing numerous subspecies of S. pneumoniae without affecting even their closest bacterial relatives. To extend DPM inhibition to a downstream target in the mevalonate pathway, DPM analogs were designed to inactivate DPM decarboxylase by producing a highly reactive carbocation immediately prior to decarboxylation. The absence of covalent adduct formation suggests that, counter to existing dogma, the decarboxylation transition state is concerted. Determination of the transition state structure is underway.

Directed evolution techniques have matured over recent years and high affinity binders are readily discoverable using numerous protein scaffolds, such as peptides, antibodies and repeat proteins just to name a few. It would be advantageous to be able to control the binding event with an orthogonal effector. Intrinsically disordered proteins are able to form ordered secondary and tertiary structures upon binding a ligand. We describe a Repeat in Toxin (RTX) motif from the adenylate cyclase toxin of Bordetella pertussis. This motif is comprised of 8 glycine and aspartic acid rich nonamers. Calcium binding causes this unstructured protein to form a parallel beta-helix where the calcium binding causes the first six residues to form a turn and last three form a beta-strand. These assemble into a beta-helix, where the strands form parallel beta sheets that present two highly variable residues. This work explores the calcium-induced RTX transition from a disordered to ordered state. CD, FRET and fluorescent spectroscopic methods are used to study this RTX motif and assess potential application as a useful scaffold for designing allosterically controlled biomolecular recognition.

Salmonella enterica serovars are a group of Gram-negative facultative intracellular bacteria that infect a wide variety of animals. Salmonella infections are common in humans, causing typhoid fever and gastrointestinal diseases, and are an important public health concern worldwide. Once inside macrophages, Salmonella reside in a niche for their proliferation, Salmonella-containing vacuoles (SCVs), maintained by secreted bacterial protein effectors that modulate the composition of SCVs. Protein lipidation is believed to be an important process in maintaining SCVs since lipidated protein in host cells are differentially recruited or excluded from the SCVs. The lipidation of proteins has traditionally been studied with radioactive lipids, which are cumbersome to use and present low specificity, limiting the detection of less abundant lipidated proteins. Here, we present new chemical tools designed for the detection and identification of fatty-acylated proteins during Salmonella infection. Proteomic analysis of these changes should reveal insights into the specific role of secreted bacterial proteins effectors in reorganizing SCVs.

At the Center for Chemical Methodology and Library Development at Boston University (http://cmld.bu.edu), Professor Porco and coworkers have recently focused on identification of reactions leading to complex chemotypes. Reaction development is generally guided by problems in total synthesis or interest in developing transformations of broad scope and utility. Chemical methodology development has increasingly relied on systematic evaluation of catalysts and other variables including solvent, temperature, and ligands. Screening has increased the efficiency of reaction development but has generally been focused on specific transformations. An emerging but underdeveloped method for chemical reaction discovery involves high-throughput screening. A few examples have been reported in which new reactions were discovered through screening of either multicomponent systems or reaction partners and catalysts. As a part of our overall interest in the synthesis of new structural frameworks, we have initiated a program to identify novel chemical transformations using both "multidimensional screening" and "reaction discovery" approaches. In this approach, substrates are reacted with various catalysts and reaction partners in an array format and analyzed for unique reaction processes. In this lecture, we will report our recent studies on this mode of reaction screening and identification of several new transformations discovered during initial screening efforts.

The thyroid hormone receptors (TR) responds directly to circulating thyroid hormones to maintain homeostatic balance, particularly for energy metabolism, temperature regulation, and lipid metabolism. The signaling pathways regulated by the TR are very complex and the selective pharmacological regulation of those pathways is difficult to achieve. In an effort to better understand the events underlying regulation of signaling and provide for more closely tuned pharmacological approaches we have developed a set of tools for studying and regulating TR signaling. High throughput screening afforded several novel chemotypes that inhibited the interaction of liganded TR with its requisite cofactors. Careful lead optimization has allowed conversion of one of these hits into a validated leads useful in cellular studies and potentially in animal models.

In the past decade, fragment-based ligand discovery has established itself as a powerful method to identify drug leads and chemical probes. In contrast to conventional high-throughput screens, which typically require tens of thousands to millions of compounds to identify hits, fragment-based approaches require only hundreds to thousands of very small molecules, or fragments. This reduction in library size makes it easier for academic laboratories and small companies to initiate projects. Moreover, since there are fewer possible small molecules than large molecules, the strategy promotes more efficient exploration of the vastness of chemical diversity.