Unlocking precision in aptamer engineering: a case study of the thrombin binding aptamer illustrates why modification size, quantity, and position matter.

The thrombin binding aptamer (TBA) is a prototypical platform used to understand the impact of chemically-modified nucleotides on aptamer stability and target affinity. To provide structural insight into the experimentally-observed effects of modification size, location, and number on aptamer performance, long time-scale molecular dynamics (MD) simulations were performed on multiple binding orientations of TBA-thrombin complexes that contain a large, flexible tryptophan thymine derivative (T-W) or a truncated analogue (T-K). Depending on modification position, T-W alters aptamer-target binding orientations, fine-tunes aptamer-target interactions, strengthens networks of nucleic acid-protein contacts, and/or induces target conformational changes to enhance binding. The proximity and 5'-to-3' directionality of nucleic acid structural motifs also play integral roles in the behavior of the modifications. Modification size can differentially influence target binding by promoting more than one aptamer-target binding pose. Multiple modifications can synergistically strengthen aptamer-target binding by generating novel nucleic acid-protein structural motifs that are unobtainable for single modifications. By studying a diverse set of modified aptamers, our work uncovers design principles that must be considered in the future development of aptamers containing chemically-modified nucleotides for applications in medicine and biotechnology, highlighting the value of computational studies in nucleic acids research.

Stable Interstrand Cross-Links Generated from the Repair of 1,N6-Ethenoadenine in DNA by α-Ketoglutarate/Fe(II)-Dependent Dioxygenase ALKBH2.

DNA cross-links severely challenge replication and transcription in cells, promoting senescence and cell death. In this paper, we report a novel type of DNA interstrand cross-link (ICL) produced as a side product during the attempted repair of 1,N6-ethenoadenine (εA) by human α-ketoglutarate/Fe(II)-dependent enzyme ALKBH2. This stable/nonreversible ICL was characterized by denaturing polyacrylamide gel electrophoresis analysis and quantified by high-resolution LC-MS in well-matched and mismatched DNA duplexes, yielding 5.7% as the highest level for cross-link formation. The binary lesion is proposed to be generated through covalent bond formation between the epoxide intermediate of εA repair and the exocyclic N6-amino group of adenine or the N4-amino group of cytosine residues in the complementary strand under physiological conditions. The cross-links occur in diverse sequence contexts, and molecular dynamics simulations rationalize the context specificity of cross-link formation. In addition, the cross-link generated from attempted εA repair was detected in cells by highly sensitive LC-MS techniques, giving biological relevance to the cross-link adducts. Overall, a combination of biochemical, computational, and mass spectrometric methods was used to discover and characterize this new type of stable cross-link both in vitro and in human cells, thereby uniquely demonstrating the existence of a potentially harmful ICL during DNA repair by human ALKBH2.

Thieno[3,2-b]thiophene for the Construction of Far-Red Molecular Rotor Hemicyanines as High-Affinity DNA Aptamer Fluorogenic Reporters.

The construction of far-red fluorescent molecular rotors (FMRs) is an imperative task for developing nucleic acid stains that have superior compatibility with cellular systems and complex matrices. A typical strategy relies on the methine extension of asymmetric cyanines, which unfortunately fails to produce sensitive rotor character. To break free from this paradigm, we have synthesized far-red hemicyanines using a dimethylamino thieno[3,2-b]thiophene donor. The resultant probes, designated as ATh2Ind and ATh2Btz, possess excitation maxima (λmax) of >600 nm and have been rigorously characterized by NMR, electrochemistry, and computational methods. The dyes possess alternating charge patterns like indodicarbocyanine (Cy5), but with twisted intramolecular charge transfer (TICT) rotational barriers at 60°, akin to the classical FMR thiazole orange (TO1). ATh2Btz also displays cyanine characteristics, enhancing its response upon binding to nucleic acids and allowing for efficient staining of cellular nuclei. When binding to the DNA aptamer for quinine (MN4), ATh2Btz exhibits a Kd of 17 nM, a 660-fold light-up response, brightness (Φfl x εmax) of ∼37,000 M-1cm-1, and λex/λem of 655/677 nm. The resulting far-red DNA-based MN4-ATh2Btz platform has been termed "pomegranate."

Neutral Adducts of Molybdenum Hexafluoride Structure and Bonding in MoF6(NC5H5) and MoF6(NC5H5)2.

The first Lewis acid base adducts of MoF6 and an organic base have been synthesized, i.e., MoF6(NC5H5) and MoF6(NC5H5)2. These adducts are structurally characterized with X-ray crystallography, showing that both adducts adopt capped trigonal prismatic structures. The MoF6(NC5H5) and MoF6(NC5H5)2 adducts are fluxional on the NMR time scale at room temperature. Two different fluorine environments could be resolved by 19F NMR spectroscopy at -80 °C for the 1:2 adduct, MoF6(NC5H5)2, whereas MoF6(NC5H5) remains fluxional at that temperature. Density functional theory (DFT) calculations aide the assignment of the infrared and Raman spectra. Natural Bond Order and Molecular Electrostatic Potential analyses elucidate the structures and properties of the MoF6 pyridine adducts. Regions of significantly higher molecular electrostatic potential, i.e., σ-holes, in trigonal prismatic compared to octahedral MoF6 rationalize the capped trigonal prismatic geometry of the adducts. Whereas MoF6(NC5H5) is stable at room temperature under exclusion of moisture, MoF6(NC5H5)2 decomposes at 60 °C in pyridine solvent, and the solid slowly decomposes at room temperature after 24 h.

Fluoride-Ion Donor Properties of AsF5.

Arsenic pentafluoride undergoes ligand-induced autoionization in the presence of 1,10-phenanthroline (phen) in a SO2ClF solution to form the donor-stabilized [AsF4(phen)][AsF6] salt. Reacting [AsF4(phen)][AsF6] with the strong Lewis acid SbF5·SO2 yields the mixed arsenic-antimony salt [AsF4(phen)][Sb2F11]. These salts are the first examples of crystallographically characterized donor-stabilized [AsF4]+ cations. The analogous reaction of AsF5 and 2,2'-bipyridine (bipy) does not result in autoionization but leads to the formation of the neutral 2:1 adduct (AsF5)2·bipy. The gas-phase and solution fluoride-ion affinities of [AsF4]+ and [SbF4]+ were calculated, revealing them to be incredibly strong Lewis acids. Density functional theory calculations and natural bond orbital analysis show that significant electron-pair donation from phen to the As center in [AsF4(phen)]+ occurs and quenches the extreme electrophilicity of the [AsF4]+ cation.

Is Metal Stabilization of the Leaving Group Required or Can Lysine Facilitate Phosphodiester Bond Cleavage in Nucleic Acids? A Computational Study of EndoV.

Endonuclease V (EndoV) is a single-metal-dependent enzyme that repairs deaminated DNA nucleobases in cells by cleaving the phosphodiester bond, and this enzyme has proven to be a powerful tool in biotechnology and medicine. The catalytic mechanism used by EndoV must be understood to design new disease detection and therapeutic solutions and further exploit the enzyme in interdisciplinary applications. This study has used a mixed molecular dynamics (MD) and quantum mechanics/molecular mechanics (QM/MM) approach to compare eight distinct catalytic pathways and provides the first proposed mechanism for bacterial EndoV. The calculations demonstrate that mechanisms involving either direct or indirect metal coordination to the leaving group of the substrate previously proposed for other nucleases are unlikely for EndoV, regardless of the general base (histidine, aspartate, and substrate phosphate moiety). Instead, distinct catalytic pathways are characterized for EndoV that involve K139 stabilizing the leaving group, a metal-coordinated water stabilizing the transition structure, and either H214 or a substrate phosphate group activating the water nucleophile. In silico K139A and H214A mutational results support the newly proposed roles of these residues. Although this is a previously unseen combination of general base, general acid, and metal-binding architecture for a one-metal-dependent endonuclease, our proposed catalytic mechanisms are fully consistent with experimental kinetic, structural, and mutational data. In addition to substantiating a growing body of literature, suggesting that one metal is enough to catalyze P-O bond cleavage in nucleic acids, this new fundamental understanding of the catalytic function will promote the exploration of new and improved applications of EndoV.

Elucidation of the catalytic mechanism of a single-metal dependent homing endonuclease using QM and QM/MM approaches: the case study of I-PpoI.

Homing endonucleases (HEs) are highly specific DNA cleaving enzymes, with I-PpoI having been suggested to use a single metal to accelerate phosphodiester bond cleavage. Although an I-PpoI mechanism has been proposed based on experimental structural data, no consensus has been reached regarding the roles of the metal or key active site amino acids. This study uses QM cluster and QM/MM calculations to provide atomic-level details of the I-PpoI catalytic mechanism. Minimal QM cluster and large-scale QM/MM models demonstrate that the experimentally-proposed pathway involving direct Mg2+ coordination to the substrate coupled with leaving group protonation through a metal-activated water is not feasible due to an inconducive I-PpoI active site alignment. Despite QM cluster models of varying size uncovering a pathway involving leaving group protonation by a metal-activated water, indirect (water-mediated) metal coordination to the substrate is required to afford this pathway, which renders this mechanism energetically infeasible. Instead, QM cluster models reveal that the preferred pathway involves direct Mg2+-O3' coordination to stabilize the charged substrate and assist leaving group departure, while H98 activates the water nucleophile. These calculations also underscore that both catalytic residues that directly interact with the substrate and secondary amino acids that position or stabilize these residues are required for efficient catalysis. QM/MM calculations on the solvated enzyme-DNA complex verify the preferred mechanism, which is fully consistent with experimental kinetic, structural, and mutational data. The fundamental understanding of the I-PpoI mechanism of action, gained from the present work can be used to further explore potential uses of this enzyme in biotechnology and medicine, and direct future computational investigations of other members of the understudied HE family.

Distinctive Formation of a DNA-Protein Cross-Link during the Repair of DNA Oxidative Damage: Insights into Human Disease from MD Simulations and QM/MM Calculations.

Reactive oxygen species damage DNA and result in health issues. The major damage product, 8-oxo-7,8-dihydroguanine (8oG), is repaired by human adenine DNA glycosylase homologue (MUTYH). Although MUTYH misfunction is associated with a genetic disorder called MUTYH-associated polyposis (MAP) and MUTYH is a potential target for cancer drugs, the catalytic mechanism required to develop disease treatments is debated in the literature. This study uses molecular dynamics simulations and quantum mechanics/molecular mechanics techniques initiated from DNA-protein complexes that represent different stages of the repair pathway to map the catalytic mechanism of the wild-type MUTYH bacterial homologue (MutY). This multipronged computational approach characterizes a DNA-protein cross-linking mechanism that is consistent with all previous experimental data and is a distinct pathway across the broad class of monofunctional glycosylase repair enzymes. In addition to clarifying how the cross-link is formed, accommodated by the enzyme, and hydrolyzed for product release, our calculations rationalize why cross-link formation is favored over immediate glycosidic bond hydrolysis, the accepted mechanism for all other monofunctional DNA glycosylases to date. Calculations on the Y126F mutant MutY highlight critical roles for active site residues throughout the reaction, while investigation of the N146S mutant rationalizes the connection between the analogous N224S MUTYH mutation and MAP. In addition to furthering our knowledge of the chemistry associated with a devastating disorder, the structural information gained about the distinctive MutY mechanism compared to other repair enzymes represents an important step for the development of specific and potent small-molecule inhibitors as cancer therapeutics.

Posttranscriptional modifications at the 37th position in the anticodon stem-loop of tRNA: structural insights from MD simulations.

Transfer RNA (tRNA) is the most diversely modified RNA. Although the strictly conserved purine position 37 in the anticodon stem-loop undergoes modifications that are phylogenetically distributed, we do not yet fully understand the roles of these modifications. Therefore, molecular dynamics simulations are used to provide molecular-level details for how such modifications impact the structure and function of tRNA. A focus is placed on three hypermodified base families that include the parent i6A, t6A, and yW modifications, as well as derivatives. Our data reveal that the hypermodifications exhibit significant conformational flexibility in tRNA, which can be modulated by additional chemical functionalization. Although the overall structure of the tRNA anticodon stem remains intact regardless of the modification considered, the anticodon loop must rearrange to accommodate the bulky, dynamic hypermodifications, which includes changes in the nucleotide glycosidic and backbone conformations, and enhanced or completely new nucleobase-nucleobase interactions compared to unmodified tRNA or tRNA containing smaller (m1G) modifications at the 37th position. Importantly, the extent of the changes in the anticodon loop is influenced by the addition of small functional groups to parent modifications, implying each substituent can further fine-tune tRNA structure. Although the dominant conformation of the ASL is achieved in different ways for each modification, the molecular features of all modified tRNA drive the ASL domain to adopt the functional open-loop conformation. Importantly, the impact of the hypermodifications is preserved in different sequence contexts. These findings highlight the likely role of regulating mRNA structure and translation.

Harnessing a 4-Formyl-Aniline Handle to Tune the Stability of a DNA Aptamer-Protein Complex via Fluorescent Surrogates.

Interactions between DNA aptamers and protein targets hold promise for the development of pharmaceuticals and diagnostics. As such, the utilization of fluorescent nucleobase surrogates in studying aptamer-protein interactions is a powerful tool due to their ability to provide site-specific information through turn-on fluorescence. Unfortunately, previously described turn-on probes serving as nucleobase replacements have only been strongly disruptive to the affinity of aptamer-protein interactions. Herein, we present a modified TBA15 aptamer for thrombin containing a fluorescent surrogate that provides site-specific turn-on emission with low nanomolar affinity. The modification, referred to as AnBtz, was substituted at position T3 and provided strong turn-on emission (Irel ≈ 4) and brightness (ε·Φ > 20 000 cm-1 M-1) with an apparent dissociation constant (Kd) of 15 nM to afford a limit of detection (LOD) of 10 nM for thrombin in 20% human serum. The probe was selected through a modular "on-strand" synthesis process that utilized a 4-formyl-aniline (4FA) handle. Using this platform, we were able to enhance the affinity of the final aptamer conjugate by ∼30-fold in comparison with the initial conjugate design. Molecular dynamics simulations provide insight into the structural basis for this phenomenon and highlight the importance of targeting hydrophobic protein binding sites with fluorescent nucleobase surrogates to create new contacts with protein targets.

Interaction Modes Between SF4 and Ketones; Study of Intermediates in Deoxofluorination Reactions.

Interactions between ketones and SF4 are studied for the simplest ketone, acetone, and the bulky polycyclic 2-adamantanone. Acetone forms the 1 : 2 adduct SF4 ⋅ [O=C(CH3 )2 ]2, as well as the dimeric 1 : 1 adducts [SF4 ⋅ O=C(CH3 )2 ]2 as identified by low-temperature Raman spectroscopy and, for the latter, X-ray crystallography. In both adducts, SF4 acts as a double chalcogen-bond donor to two keto groups. In contrast 2-adamantanone does not form an isolable solid adduct with SF4 ; in the presence of HF, however, it forms SF4 ⋅ O=C10 H14 O ⋅ HF, which comprises chains with weak S-O and S-FH chalcogen bonds in the crystal structure. Sulfur tetrafluoride in this compound is readily lost at -85 °C, leading to the isolation of C10 H14 O ⋅ HF at low temperature. Density functional theory (DFT) calculations aid in vibrational assignments and serve to describe the interactions of the keto group with SF4 and HF, as well as interactions between SF4 with HF. It is found that separate and combined CO-HF and CO-SF4 chalcogen bonds do not polarize the C=O group to any significant degree.

Anatomy of noncovalent interactions between the nucleobases or ribose and π-containing amino acids in RNA-protein complexes.

A set of >300 nonredundant high-resolution RNA-protein complexes were rigorously searched for π-contacts between an amino acid side chain (W, H, F, Y, R, E and D) and an RNA nucleobase (denoted π-π interaction) or ribose moiety (denoted sugar-π). The resulting dataset of >1500 RNA-protein π-contacts were visually inspected and classified based on the interaction type, and amino acids and RNA components involved. More than 80% of structures searched contained at least one RNA-protein π-interaction, with π-π contacts making up 59% of the identified interactions. RNA-protein π-π and sugar-π contacts exhibit a range in the RNA and protein components involved, relative monomer orientations and quantum mechanically predicted binding energies. Interestingly, π-π and sugar-π interactions occur more frequently with RNA (4.8 contacts/structure) than DNA (2.6). Moreover, the maximum stability is greater for RNA-protein contacts than DNA-protein interactions. In addition to highlighting distinct differences between RNA and DNA-protein binding, this work has generated the largest dataset of RNA-protein π-interactions to date, thereby underscoring that RNA-protein π-contacts are ubiquitous in nature, and key to the stability and function of RNA-protein complexes.

Effects of a Second Local DNA Damage Event on the Toxicity of the Human Carcinogen 4-Aminobiphenyl: A Molecular Dynamics Study of a Damaged DNA Structure.

Exposure of humans to carcinogenic aromatic amines (AAs) occurs daily. AAs are bioactivated in cells into products that attack DNA, primarily leading to N-linked C8-dG adducts. Previous work on DNA containing a single AA-derived adduct (monoadducted DNA) has shown a structure-function relationship between the damaged DNA conformation and cellular outcomes. However, relatively little is known about the conformation and biological outcomes of DNA containing two bulky adducts (diadducted DNA) in close proximity. To fill this current void in the literature, the present work uses quintuplet 0.5 µs MD simulations to understand the structural impact of DNA exposure to the potent bladder carcinogen 4-aminobiphenyl (ABP), which is found in cigarette smoke and select dyes, and results in the widely studied N-linked ABPdG adduct. Specifically, 18 unique DNA duplexes were investigated that contain one or two ABPdG adducts in the anti and/or syn glycosidic orientation(s) in all combinations of three G positions in the NarI mutation hotspot for AAs (5'-G1G2CG3CC). Monoadducted DNA displays sequence-dependent conformational heterogeneity, with the G1 site having the greatest anti preference, and highlights the range of helical structures associated with the syn lesion orientation [i.e., stacked (S), intercalated (I), and wedge (W) conformations]. Diadducted DNA results in interesting lesion separation effects on the conformational heterogeneity, including a greater anti preference for neighboring adducts (G1G2) and a greater syn preference for next-nearest neighbor damaged sites (G2G3) compared to monoadducted DNA. As a result, an increase in the number of ABPdG adducts changes the conformational heterogeneity of ABP-exposed DNA depending on the relative positions of the lesions and thereby could result in increased or decreased toxicity upon human exposure to elevated levels of ABP.

The metal dependence of single-metal mediated phosphodiester bond cleavage: a QM/MM study of a multifaceted human enzyme.

Nucleases catalyze the cleavage of phosphodiester bonds in nucleic acids using a range of metal cofactors. Although it is well accepted that many nucleases rely on two metal ions, the one-metal mediated pathway is debated. Furthermore, one-metal mediated nucleases maintain activity in the presence of many different metals, but the underlying reasons for this broad metal specificity are unknown. The human apurinic/apyrimidinic endonuclease (APE1), which plays a key role in DNA repair, transcription regulation, and gene expression, is a prototypical example of a one-metal dependent nuclease. Although Mg2+ is the native metal cofactor, APE1 remains catalytically active in the presence of several metals, with the rate decreasing as Mg2+ > Mn2+ > Ni2+ > Zn2+, while Ca2+ completely abolished the activity. The present work uses quantum mechanics-molecular mechanics techniques to map APE1-facilitated phosphodiester bond hydrolysis in the presence of these metals. The structural differences in stationary points along the reaction pathway shed light on the interplay between several factors that allow APE1 to remain catalytically active for various metals, with the trend in the barrier heights correlating with the experimentally reported APE1 catalytic activity. In contrast, Ca2+ significantly changes the metal coordination and active site geometry, and thus completely inhibits catalysis. Our work thereby provides support for the controversial single-metal mediated phosphodiester bond cleavage and clarifies uncertainties regarding the role of the metal and metal identity in this important reaction. This information is key for future medicinal and biotechnological applications including disease diagnosis and treatment, and protein engineering.

Multiscale computational investigations of the translesion synthesis bypass of tobacco-derived DNA adducts: critical insights that complement experimental biochemical studies.

Among the numerous agents that damage DNA, tobacco products remain one of the most lethal and result in the most diverse set of DNA lesions. This perspective aims to provide an overview of computational work conducted to complement experimental biochemical studies on the mutagenicity of adducts derived from the most potent tobacco carcinogen, namely 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (nicotine-derived nitrosaminoketone or NNK). Lesions ranging from the smallest methylated thymine derivatives to the larger, flexible pyridyloxobutyl (POB) guanine adducts are considered. Insights are obtained from density functional theory (DFT) calculations and molecular dynamics (MD) simulations into the damaged nucleobase and nucleoside structures, the accommodation of the lesions in the active site of key human polymerases, the intrinsic base pairing potentials of the adducts, and dNTP incorporation opposite the lesions. Overall, the computational data provide atomic level information that can rationalize the differential mutagenic properties of tobacco-derived lesions and uncover important insights into the impact of adduct size, nucleobase, position, and chemical composition of the bulky moiety.

Stabilisation of [WV F4 ]+ by N- and P-Donor Ligands: Second-Order Jahn-Teller Effects in Octacoordinate d1 Complexes.

The first isolated examples of cationic fluoridotungsten(V) complexes are reported as octacoordinate [WF4 (L)4 ]+ (L=C5 H5 N, P(CH3 )3 ). The [WF4 (NC5 H5 )4 ]+ cation is synthesised as its [O3 SCF3 ]- salt upon reaction of WF5 (NC5 H5 )2 with [(CH3 )3 Si(NC5 H5 )][O3 SCF3 ] in excess C5 H5 N, whereas [WF4 {P(CH3 )3 }4 ]+ is accessed directly from WF6 upon reaction with (CH3 )3 SiO3 SCF3 and excess P(CH3 )3 . These salts were characterised by X-ray crystallography and Raman spectroscopy in the solid state. New geometry indices for octacoordinate complexes (τ8 and τ8 ') are introduced, allowing for the facile differentiation of trigonal-dodecahedral (TD) and square-antiprismatic (SA) geometries. This has disambiguated the SA geometries of [WF4 (L)4 ]+ and the geometries of a series of previously reported d0 and d1 MA4 B4 complexes. Computational (DFT-B3LYP) studies of [WF4 (PH3 )4 ]2+/+ and related model systems demonstrate the occurrence of a second-order Jahn-Teller (SOJT) distortion from TD in d0 complexes to SA in d1 complexes, with the degree of SOJT stabilisation being most significant in 5d complexes containing fluorido ligands and monodentate neutral donors.

Donor-Stabilised [SbF4 ]+ : SbF5 as a Fluoride-Ion Donor.

Antimony pentafluoride is a strong Lewis acid and fluoride-ion acceptor that has not previously demonstrated any discreet fluoride-ion donor properties. The first donor-stabilised [SbF4 ]+ cations were prepared from the autoionisation of SbF5 in the presence of bidentate N-donor ligands 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen) as their [SbF6 ]- salts. The [SbF4 (N-N)][Sb2 F11 ] (N-N=bipy, phen) salts were synthesised by the addition of one equivalent of SbF5 ⋅SO2 to [SbF4 (N-N)][SbF6 ] in liquid SO2. The salts show remarkable stability and were characterised by Raman spectroscopy and multinuclear NMR spectroscopy. The crystal structures of [SbF4 (phen)][SbF6 ] ⋅ 3CH3 CN and [SbF4 (phen)][SbF6 ] ⋅ 2SO2 were determined, showing distorted octahedral cations. DFT calculations and NBO analyses reveal that significant degree of electron-pair donation from N to Sb stabilizes [SbF4 ]+ with the Sb-N bond strength being approximately two thirds of that of the Sb-F bonds in these cations and the cationic charge being primarily ligand-centred.

Structural Rationalization for the Nonmutagenic and Mutagenic Bypass of the Tobacco-Derived O4-4-(3-Pyridyl)-4-oxobut-1-yl-thymine Lesion by Human Polymerase η: A Multiscale Computational Study.

Tobacco-derived pyridyloxobutyl (POB) DNA adducts are unique due to the large size and flexibility of the alkyl chain connecting the pyridyl ring to the nucleobase. Recent experimental work suggests that the O4-4-(3-pyridyl)-4-oxobut-1-yl-T (O4-POB-T) lesion can undergo both nonmutagenic (dATP) and mutagenic (dGTP) insertion by the translesion synthesis (TLS) polymerase (pol) η in human cells. Interestingly, the mutagenic rate for O4-POB-T replication is reduced compared to that for the smaller O4-methylthymine (O4-Me-T) lesion, and O4-POB-T yields a different mutagenic profile than the O2-POB-T variant (dTTP insertion). The present work uses a combination of density functional theory calculations and molecular dynamics simulations to probe the impact of the size and flexibility of O4-POB-T on pol η replication outcomes. Due to changes in the Watson-Crick binding face upon damage of canonical T, O4-POB-T does not form favorable hydrogen-bonding interactions with A. Nevertheless, dATP is positioned for insertion in the pol η active site by a water chain to the template strand, which suggests a pol η replication pathway similar to that for abasic sites. Although a favorable O4-POB-T:G mispair forms in the pol η active site and DNA duplexes, the inherent dynamical nature of O4-POB-T periodically disrupts interstrand hydrogen bonding that would otherwise facilitate dGTP insertion and stabilize damaged DNA duplexes. In addition to explaining the origin of the experimentally reported pol η outcomes associated with O4-POB-T replication, comparison to structural data for the O4-Me-T and O2-POB-T adducts highlights an emerging common pathway for the nonmutagenic replication of thymine alkylated lesions by pol η, yet underscores the broader impacts of bulky moiety size, flexibility, and position on the associated mutagenic outcomes.

Chalcogen versus Dative Bonding in [SF3]+ Lewis Acid-Base Adducts: [SF3(NCCH3)2]+, [SF3(NC5H5)2]+, and [SF3(phen)]+ (phen = 1,10-phenanthroline).

The Lewis-acid behavior of [SF3][MF6] (M = Sb, As) salts toward mono- and bidentate nitrogen bases was explored. Reactions of [SF3][MF6] with excesses of CH3CN and C5H5N yielded [SF3(L)2]+ (L = CH3CN, C5H5N) salts, whereas the reaction of [SF3][SbF6] with equimolar 1,10-phenanthroline (phen) in CH3CN afforded [SF3(phen)][SbF6]·2CH3CN. Salts of these cations were characterized by low-temperature X-ray crystallography and Raman spectroscopy in the solid state as well as by 19F NMR spectroscopy in solution. In the solid state, the geometries of [SF3(NC5H5)2]+ and [SF3(phen)]+ are square pyramids with negligible cation-anion contacts, whereas the coordination of CH3CN and [SbF6]- to [SF3]+ in [SF3(NCCH3)2][SbF6] results in a distorted octahedral coordination sphere with a minimal perturbation of the trigonal-pyramidal SF3 moiety. 19F NMR spectroscopy revealed that [SF3(L)2]+ is fluxional in excess L at -30 °C, whereas [SF3(phen)]+ is rigid in CH2Cl2 at -40 °C. Density functional theory (DFT-B3LYP) calculations suggest that the S-N bonds in [SF3(NC5H5)2]+ and [SF3(phen)]+ possess substantial covalent character and result in a regular AX5E VSEPR geometry, whereas those in [SF3(NCCH3)2]+ are best described as S···N chalcogen-bonding interactions via σ-holes on [SF3]+, which is consistent with the crystallographic data.

Lewis Acid Behavior of MoF5 and MoOF4: Syntheses and Characterization of MoF5(NCCH3), MoF5(NC5H5)n, and MoOF4(NC5H5)n (n = 1, 2).

The Lewis acid-base adducts MoF5(NC5H5)n and MoOF4(NC5H5)n (n = 1, 2) were synthesized from the reactions of MoF5 and MoOF4 with C5H5N and structurally characterized by X-ray crystallography. Whereas the crystal structures of MoF5(NC5H5)2 and MoOF4(NC5H5)2 are isomorphous containing pentagonal-bipyramidal molecules, the fluorido-bridged, heptacoordinate [MoF5(NC5H5)]2 dimer differs starkly from monomeric, hexacoordinate MoOF4(NC5H5). For the weaker Lewis base CH3CN, only the 1:1 adduct, MoF5(NCCH3), could be isolated. All adducts were characterized by Raman spectroscopy in conjunction with vibrational frequency calculations. Multinuclear NMR spectroscopy revealed an unprecedented isomerism of MoOF4(NC5H5)2 in solution, with the pyridyl ligands occupying adjacent or nonadjacent positions in the equatorial plane of the pentagonal bipyramid. Paramagnetic MoF5(NC5H5)2 was characterized by electron paramagnetic resonance (EPR) spectroscopy as a dispersion in solid adamantane as well as in a diamagnetic host lattice of MoOF4(NC5H5)2; EPR parameters were computed using ZORA with the BPW91 functional using relativistic all-electron wave functions for Mo and simulated using EasySpin. Density functional theory calculations (B3LYP) and natural bond orbital analyses were conducted to elucidate the distinctive bonding and structural properties of all adducts reported herein and explore fundamental differences observed in the Lewis acid behavior of MoF5 and MoOF4.