Halogen bonds (X-bonds) are shown to be geometrically perpendicular to and energetically independent of hydrogen bonds (H-bonds) that share a common carbonyl oxygen acceptor. This orthogonal relationship is accommodated by the in-plane and out-of-plane electronegative potentials of the oxygen, which are differentially populated by H- and X-bonds. Furthermore, the local conformation of a peptide helps to define the geometry of the H-bond and thus the oxygen surface that is accessible for X-bonding. These electrostatic and steric forces conspire to impose a strong preference for the orthogonal geometry of X- and H-bonds. Thus, the optimum geometry of an X-bond can be predicted from the pattern of H-bonds in a folded protein, enabling X-bonds to be introduced to improve ligand affinities without disrupting these structurally important interactions. This concept of orthogonal molecular interactions can be exploited for the rational design of halogenated ligands as inhibitors and drugs, and in biomolecular engineering.

The halogen bond, a noncovalent interaction involving polarizable chlorine, bromine, or iodine molecular substituents, is now being exploited to control the assembly of small molecules in the design of supramolecular complexes and new materials. We demonstrate that a halogen bond formed between a brominated uracil and phosphate oxygen can be engineered to direct the conformation of a biological molecule, in this case to define the conformational isomer of a four-stranded DNA junction when placed in direct competition against a classic hydrogen bond. As a result, this bromine interaction is estimated to be Ϸ2-5 kcal/mol stronger than the analogous hydrogen bond in this environment, depending on the geometry of the halogen bond. This study helps to establish halogen bonding as a potential tool for the rational design and construction of molecular materials with DNA and other biological macromolecules.biomolecular engineering ͉ DNA structure ͉ molecular interactions H alogen bonds have recently seen a resurgence of interest as a tool for ''bottom-up'' molecular design. Chlorines, bromines, and iodines in organic and inorganic compounds are known to polarize along their covalent bonds to generate an electropositive crown; the halogen thus acts as a Lewis acid to pair with Lewis bases, including oxygens and nitrogens. These electrostatic pairs, originally called charge-transfer bonds (1), are now known as halogen bonds (X-bonds), recognizing their similarities to hydrogen bonds (H-bonds) in their strength and directionality (2). In chemistry, X-bonds are being exploited in the design and engineering of supramolecular assemblies (3) and molecular crystals (for review, see ref. 4), with an iodine X-bond estimated to be Ϸ3.5 kcal/mol more stable than an O-H⅐⅐⅐O H-bond in organic crystals (5).The X-bond, however, has not generally been a part of the biologist's lexicon. Although halogens are widely used in drug design and to probe molecular interactions, X-bonds have only recently been recognized as a distinct interaction in ligand recognition and molecular folding and in the assembly of proteins and nucleic acids (6, 7). With the growing application of biological molecules (biomolecule), particularly nucleic acids (for review, see ref. 8), in the design of nanomechanical devices, we ask here whether specific X-bonds can be engineered to direct conformational switching in a biomolecule.To compare X-and H-bonds in the complex environment of a biomolecule, we have designed a crystallographic assay to determine whether an intramolecular X-bond could be engineered to direct the conformational isomerization of a DNA Holliday junction by competing an X-bond against a classic H-bond and, consequently, we are able to compare the stabilization energies afforded by these two types of interactions. The stacked-X form of the DNA Holliday junction (Fig. 1), seen in high-salt solutions (9) and in crystal structures (10 -12), is a simple and well controlled biomolecular assay system that can isomerize between two nearly isoenergetic an...

Interest in noncovalent interactions involving halogens, particularly halogen bonds (X-bonds), has grown dramatically in the past decade, propelled by the use of X-bonding in molecular engineering and drug design. However, it is clear that a complete analysis of the structure-energy relationship must be established in biological systems to fully exploit X-bonds for biomolecular engineering. We present here the first comprehensive experimental study to correlate geometries with their stabilizing potentials for fluorine (F), chlorine (Cl), bromine (Br), or iodine (I) X-bonds in a biological context. For these studies, we determine the single-crystal structures of DNA Holliday junctions containing halogenated uracil bases that compete X-bonds against classic hydrogen bonds (H-bonds), estimate the enthalpic energies of the competing interactions in the crystal system through crystallographic titrations, and compare the enthalpic and entropic energies of bromine and iodine X-bonds in solution by differential scanning calorimetry. The culmination of these studies demonstrates that enthalpic stabilization of X-bonds increases with increasing polarizability from F to Cl to Br to I, which is consistent with the σ-hole theory of X-bonding. Furthermore, an increase in the X-bonding potential is seen to direct the interaction toward a more ideal geometry. However, the entropic contributions to the total free energies must also be considered to determine how each halogen potentially contributes to the overall stability of the interaction. We find that bromine has the optimal balance between enthalpic and entropic energy components, resulting in the lowest free energy for X-bonding in this DNA system. The X-bond formed by iodine is more enthalpically stable, but this comes with an entropic cost, which we attribute to crowding effects. Thus, the overall free energy of an X-bonding interaction balances the stabilizing electrostatic effects of the σ-hole against the competing effects on the local structural dynamics of the system.

The intracellular effects and overall efficacies of anticancer therapies can vary significantly by tumor type. To identify patterns of drug-induced gene modulation that occur in different cancer cell types, we measured gene expression changes across the NCI-60 cell line panel after exposure to 15 anticancer agents. The results were integrated into a combined database and set of interactive analysis tools, designated the NCI Transcriptional Pharmacodynamics Workbench (NCI TPW), that allows exploration of gene expression modulation by molecular pathway, drug target, and association with drug sensitivity. We identified common transcriptional responses across agents and cell types and uncovered gene expression changes associated with drug sensitivity. We also demonstrated the value of this tool for investigating clinically-relevant molecular hypotheses and identifying candidate biomarkers of drug activity. The NCI TPW, publicly available at https://tpwb.nci.nih.gov, provides a comprehensive resource to facilitate understanding of tumor cell characteristics that define sensitivity to commonly used anticancer drugs.