Ligand binding mode to duplex and triplex DNA assessed by combining electrospray tandem mass spectrometry and molecular modeling

The multifaceted roles of mass spectrometric analysis in nucleic acids drug discovery and development

The deceptively simple concepts of mass determination and fragment analysis are the basis for the application of mass spectrometry (MS) to a boundless range of analytes, including fundamental components and polymeric forms of nucleic acids (NAs). This platform affords the intrinsic ability to observe first-hand the effects of NA-active drugs on the chemical structure, composition, and conformation of their targets, which might affect their ability to interact with cognate NAs, proteins, and other biomolecules present in a natural environment. The possibility of interfacing with high-performance separation techniques represents a multiplying factor that extends these capabilities to cover complex sample mixtures obtained from organisms that were exposed to NA-active drugs. This report provides a brief overview of these capabilities in the context of the analysis of the products of NA-drug activity and NA therapeutics. The selected examples offer proof-of-principle of the applicability of this platform to all phases of the journey undertaken by any successful NA drug from laboratory to bedside, and provide the rationale for its rapid expansion outside traditional laboratory settings in support to ever growing manufacturing operations.

Energy-resolved mass spectrometry to investigate nucleobase triplexes – a study applied to triplex-forming artificial nucleobases

This paper discloses the use of an energy-resolved mass spectrometric-based approach to assess the stabilities of base triplexes encompassing artificial nucleobases by using variable energy collision-induced dissociation.

Mass Spectrometry of Nucleic Acid Noncovalent Complexes

Nucleic acids have been among the first targets for antitumor drugs and antibiotics. With the unveiling of new biological roles in regulation of gene expression, specific DNA and RNA structures have become very attractive targets, especially when the corresponding proteins are undruggable. Biophysical assays to assess target structure as well as ligand binding stoichiometry, affinity, specificity, and binding modes are part of the drug development process. Mass spectrometry offers unique advantages as a biophysical method owing to its ability to distinguish each stoichiometry present in a mixture. In addition, advanced mass spectrometry approaches (reactive probing, fragmentation techniques, ion mobility spectrometry, ion spectroscopy) provide more detailed information on the complexes. Here, we review the fundamentals of mass spectrometry and all its particularities when studying noncovalent nucleic acid structures, and then review what has been learned thanks to mass spectrometry on nucleic acid structures, self-assemblies (e.g., duplexes or G-quadruplexes), and their complexes with ligands.

Combining Electrospray Mass Spectrometry (ESI-MS) and Computational Techniques in the Assessment of G-Quadruplex Ligands: A Hybrid Approach to Optimize Hit Discovery

Guanine-rich sequences forming G-quadruplexes (GQs) are present in several genomes, ranging from viral to human. Given their peculiar localization, the induction of GQ formation or GQ stabilization with small molecules represents a strategy for interfering with crucial biological functions. Investigating the recognition event at the molecular level, with the aim of fully understanding the triggered pharmacological effects, is challenging. Native electrospray ionization mass spectrometry (ESI-MS) is being optimized to study these noncovalent assemblies. Quantitative parameters retrieved from ESI-MS studies, such as binding affinity, the equilibrium binding constant, and sequence selectivity, will be overviewed. Computational experiments supporting the ESI-MS investigation and boosting its efficiency in the search for GQ ligands will also be discussed with practical examples. The combination of ESI-MS and in silico techniques in a hybrid high-throughput-screening workflow represents a valuable tool for the medicinal chemist, providing data on the quantitative and structural aspects of ligand–GQ interactions.

Modulation of DNA structure formation using small molecules

Genome integrity is essential for proper cell function such that genetic instability can result in cellular dysfunction and disease. Mutations in the human genome are not random, and occur more frequently at "hotspot" regions that often co-localize with sequences that have the capacity to adopt alternative (i.e. non-B) DNA structures. Non-B DNA-forming sequences are mutagenic, can stimulate the formation of DNA double-strand breaks, and are highly enriched at mutation hotspots in human cancer genomes. Thus, small molecules that can modulate the conformations of these structure-forming sequences may prove beneficial in the prevention and/or treatment of genetic diseases. Further, the development of molecular probes to interrogate the roles of non-B DNA structures in modulating DNA function, such as genetic instability in cancer etiology are warranted. Here, we discuss reported non-B DNA stabilizers, destabilizers, and probes, recent assays to identify ligands, and the potential biological applications of these DNA structure-modulating molecules.

Screening and investigation of triplex DNA binders from Stephania tetrandra S. Moore by a combination of peak area-fading ultra high-performance liquid chromatography with orbitrap mass spectrometry and optical spectroscopies

The identification and screening of triplex DNA binders are important because these compounds, in many cases, are potential anticancer agents as well as promising drug candidates. Therefore, the ability to screen for these compounds in a high-throughput mode could dramatically improve the drug screening process. A method involving a combination of 96-well plate format and peak area-fading ultra high-performance liquid chromatography coupled with Orbitrap mass spectrometry was employed for screening bioactive compounds binding to the triplex DNA from the extracts of Stephania tetrandra S. Moore. Two compounds were screened out and identified as fangchinoline and tetrandrine based on the comparison of retention time and tandem mass spectrometry data with those of standards. The binding mechanisms of fangchinoline and tetrandrine at the molecular level were explored using tandem mass spectrometry, fluorescence spectroscopy, ultraviolet-visible spectroscopy, and circular dichroism. Collision-induced dissociation experiments showed that the complexes with fangchinoline and tetrandrine were dissociated by ligand elimination. According to these measurements, an intercalating binding is the most appropriate binding mode of these two alkaloids to the triplex DNA. The current work provides not only deep insight into alkaloid-triplex DNA complexes but also useful guidelines for the design of efficient anticancer agents.

MS methods to study macromolecule-ligand interaction: Applications in drug discovery

The interaction of small compounds (i.e. ligands) with macromolecules or macromolecule assemblies (i.e. targets) is the mechanism of action of most of the drugs available today. Mass spectrometry is a popular technique for the interrogation of macromolecule-ligand interactions and therefore is also widely used in drug discovery and development. Thanks to its versatility, mass spectrometry is used for multiple purposes such as biomarker screening, identification of the mechanism of action, ligand structure optimization or toxicity assessment. The evolution and automation of the instruments now allows the development of high throughput methods with high sensitivity and a minimized false discovery rate. Herein, all these approaches are described with a focus on the methods for studying macromolecule-ligand interaction aimed at defining the structure-activity relationships of drug candidates, along with their mechanism of action, metabolism and toxicity.

Influence of Linkage Stereochemistry and Protecting Groups on Glycosidic Bond Stability of Sodium Cationized Glycosyl Phosphates

Energy-resolved collision-induced dissociation (ER-CID) experiments of sodium cationized glycosyl phosphate complexes, [GP _x +Na]⁺, are performed to elucidate the effects of linkage stereochemistry (α versus β), the geometry of the leaving groups (1,2-cis versus 1,2-trans), and protecting groups (cyclic versus non-cyclic) on the stability of the glycosyl phosphate linkage via survival yield analyses. A four parameter logistic dynamic fitting model is used to determine CID_50% values, which correspond to the level of rf excitation required to produce 50% dissociation of the precursor ion complexes. Present results suggest that dissociation of 1,2-trans [GP _x +Na]⁺ occurs via a McLafferty-type rearrangement that is facilitated by a syn orientation of the leaving groups, whereas dissociation of 1,2-cis [GP_x+Na]⁺ is more energetic as it involves the formation of an oxocarbenium ion intermediate. Thus, the C1-C2 configuration plays a major role in determining the stability/reactivity of glycosyl phosphate stereoisomers. For 1,2-cis anomers, the cyclic protecting groups at the C4 and C6 positions stabilize the glycosidic bond, whereas for 1,2-trans anomers, the cyclic protecting groups at the C4 and C6 positions tend to activate the glycosidic bond. The C3 O-benzyl (3 BnO) substituent is key to determining whether the sugar or phosphate moiety retains the sodium cation upon CID. For 1,2-cis anomers, the 3 BnO substituent weakens the glycosidic bond, whereas for 1,2-trans anomers, the 3 BnO substituent stabilizes the glycosidic bond. The C2 O-benzyl substituent does not significantly impact the glycosidic bond stability regardless of its orientation. Graphical abstract ᅟ.

Multidimensional Mass Spectrometry of Synthetic Polymers and Advanced Materials

Multidimensional mass spectrometry interfaces a suitable ionization technique and mass analysis (MS) with fragmentation by tandem mass spectrometry (MS² ) and an orthogonal online separation method. Separation choices include liquid chromatography (LC) and ion-mobility spectrometry (IMS), in which separation takes place pre-ionization in the solution state or post-ionization in the gas phase, respectively. The MS step provides elemental composition information, while MS² exploits differences in the bond stabilities of a polymer, yielding connectivity and sequence information. LC conditions can be tuned to separate by polarity, end-group functionality, or hydrodynamic volume, whereas IMS adds selectivity by macromolecular shape and architecture. This Minireview discusses how selected combinations of the MS, MS² , LC, and IMS dimensions can be applied, together with the appropriate ionization method, to determine the constituents, structures, end groups, sequences, and architectures of a wide variety of homo- and copolymeric materials, including multicomponent blends, supramolecular assemblies, novel hybrid materials, and large cross-linked or nonionizable polymers.

A simple heated-capillary modification improves the analysis of non-covalent complexes by Z-spray electrospray ionization

RATIONALE

The observation of intact non-covalent complexes by electrospray ionization mass spectrometry (ESI-MS) hinges on the ability to minimize in-source activation processes that take place during analyte desolvation. We explored the merits of replacing the sampling cone of a standard Z-spray source with a heated capillary that makes the desolvation process slower and more gradual. We employed well-characterized protein-RNA, RNA-RNA, and DNA-DNA assemblies to compare the alternative configurations.

METHODS

Mass analysis evaluated the integrity of the complexes, whereas traveling wave ion mobility experiments assessed the stability of biomolecular structure. Analyses were performed back-to-back on the same samples on a Synapt G2 HDMS equipped with either the standard sampling cone or the heated-capillary apparatus. In each configuration, the source/capillary temperature was varied in controlled fashion, while keeping all other desolvation parameters constant to monitor the in-source dissociation of selected DNA duplexes. Ion mobility data were obtained from the same precursor by using the alternative configurations under the same settings.

RESULTS

Monitoring the percentage of associated complex demonstrated that the heated capillary provided softer desolvation that was more conducive to the detection of intact non-covalent interactions. This configuration failed to produce complete dissociation of 14 bp and 24 bp duplexes, even when the source/capillary temperature was increased well above their solution melting points. Analyzed by IMS-MS, a selected construct displayed just one conformation with the heated capillary, but two with the standard sampling cone.

CONCLUSIONS

The heated capillary minimizes in-source activation processes that can lead to unintended dissociation of complexes and perturbation of biomolecular structure, which rely on the integrity of non-covalent interactions. This effect can be attributed to the attenuation of the supersonic expansion typical of the Z-spray geometry and the greater ability to control the energy imparted to the system. This hardware modification will be expected to benefit the analysis of biomolecular structure performed on this particular instrumental platform.

Nucleic acid ion structures in the gas phase

Nucleic acids are diverse polymeric macromolecules that are essential for all life forms. These biomolecules possess a functional three-dimensional structure under aqueous physiological conditions. Mass spectrometry-based approaches have on the other hand opened the possibility to gain structural information on nucleic acids from gas-phase measurements. To correlate gas-phase structural probing results with solution structures, it is therefore important to grasp the extent to which nucleic acid structures are preserved, or altered, when transferred from the solution to a fully anhydrous environment. We will review here experimental and theoretical approaches available to characterize the structure of nucleic acids in the gas phase (with a focus on oligonucleotides and higher-order structures), and will summarize the structural features of nucleic acids that can be preserved in the gas phase on the experiment time scale.

Ion mobility spectrometry reveals duplex DNA dissociation intermediates

Electrospray ionization (ESI) soft desolvation is widely used to investigate fragile species such as nucleic acids. Tandem mass spectrometry (MS/MS) gives access to the gas phase energetics of the intermolecular interactions in the absence of solvent, by following the dissociation of mass-selected ions. Ion mobility mass spectrometry (IMS) provides indications on the tridimensional oligonucleotide structure by attributing a collision cross section (CCS) to the studied ion. Electrosprayed duplexes longer than eight bases pairs retain their helical structure in a solvent-free environment. However, the question of conformational changes under activation in MS/MS studies remains open. The objective of this study is to probe binding energetics and characterize the unfolding steps occurring prior to oligonucleotide duplex dissociation. Comparing the evolution of CCS with collision energy and breakdown curves, we characterize dissociation pathways involved in CID-activated DNA duplex separation into single strands, and we demonstrate here the existence of stable dissociation intermediates. At fixed duplex length, dissociation pathways were found to depend on the percentage of GC base pairs and on their position in the duplex. Our results show that pure GC sequences undergo a gradual compaction until reaching the dissociation intermediate: A-helix. Mixed AT-GC sequences were found to present at least two conformers: a classic B-helix and an extended structure where the GC tract is a B-helix and the AT tract(s) fray. The dissociation in single strands takes place from both conformers when the AT base pairs are enclosed between two GC tracts or only from the extended conformer when the AT tract is situated at the end(s) of the sequence.

Study on noncovalent complexes of alkaloids with DNA duplex using electrospray ionization mass spectrometry

RATIONALE

DNA is an important molecular target in modern medicine research. Some DNA-binding ligands have been used to treat numerous diseases. Therefore, understanding the interactions of different ligands with DNA and looking for new DNA agents are necessary to develop new drugs.

METHODS

Electrospray ionization mass spectrometry (ESI-MS) in the negative ion mode was used to screen the noncovalent complexes between 11 alkaloids with double helix oligonucleotides at molar ratios 1:1 to 1:4. The relative binding affinities based on the fraction of bound DNA and sequence selectivities of alkaloids towards the duplex were also investigated by ESI-MS. Moreover, tandem mass spectrometry of 5-charged complex ions was used to try to determine DNA-binding modes.

RESULTS

Six alkaloids showed complexation with the selected DNA duplex, i.e., berberine, coptisine, peimine, aconitine, oxysophoridine and cytisine. They showed their binding affinities with d(AACTCCCGGCACAC/GTGTGCCGGGAGTT) in the order of berberine > coptisine > peimine > aconitine, oxysophoridine > cytosine; additional experiments involving collision energy proved this result. Sequence selectivities were not apparent for coptisine, peimine, aconitine and oxysophoridine with four DNA duplexes. The complexes containing berberine and coptisine underwent the predominant loss of the G base. However, for complexes containing the other four alkaloids, they dissociated via the loss of neutral drug. The results confirmed that they may have different binding modes.

CONCLUSIONS

According to experiment data and structural information, the binding mode of individual drugs with DNA was speculated. It was noted that the bindings of alkaloids peimine, aconitine and oxysophoridine with DNA are discovered firstly. This may give a clue to design duplex-binding ligands and be helpful for understanding biological activities of these alkaloids.

Database searching for structural identification of metabolites in complex biofluids for mass spectrometry-based metabonomics

MS and HPLC are commonly used for compound characterization and obtaining structural information; in the field of metabonomics, these two analytical techniques are often combined to characterize unknown endogenous or exogenous metabolites present in complex biological samples. Since the structures of a majority of these metabolites are not actually identified, the result of most metabonomic studies is a list of m/z values and retention times. However, without knowing actual structures, the biological significance of these 'features' cannot be determined. The process of identifying the structures of unknown compounds can be time intensive, costly and frequently requires the use of multiple orthogonal analytical techniques - this laborious procedure seems insurmountable for the long lists of unknowns that must be identified for each study. In addition, the limited sample volume and the extremely low concentration of most endogenous analytes frequently make purification and identification by other instrumentation nearly impossible. This review is intended to explore the problems and progress with current tools that are available for MS-based structure identification for both endogenous and exogenous metabolites.

Evaluation of DNA/Ligand interactions by electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry (ESI-MS) has enabled the detection and characterization of DNA/ligand complexes, including evaluation of both relative binding affinities and selectivities of DNA-interactive ligands. The noncovalent complexes that are transferred from the solution to the gas phase retain the signature of the native species, thus allowing the use of MS to screen DNA/ligand complexes, reveal the stoichiometries of the complexes, and provide insight into the nature of the interactions. Ligands that bind to DNA via metal-mediated modes and those that bind to unusual DNA structures, such as quadruplexes, are amenable to ESI. Chemical probe methods applied to DNA/ligand complexes with ESI-MS detection afford information about ligand-binding sites and conformational changes of DNA that occur upon ligand binding.

CE50: quantifying collision induced dissociation energy for small molecule characterization and identification

Survival yield analysis is routinely used in mass spectroscopy as a tool for assessing precursor ion stability and internal energy. Because ion internal energy and decomposition reaction rates are dependent on chemical structure, we reasoned that survival yield curves should be compound-specific and therefore useful for chemical identification. In this study, a quantitative approach for analyzing the correlation between survival yield and collision energy was developed and validated. This method is based on determining the collision energy (CE) at which the survival yield is 50% (CE(50)) and, further, provides slope and intercept values for each survival yield curve. In initial experiments using a defined set of homologous compounds, we found that CE(50) values were easily determined, quantitative, highly reproducible, and could discriminate between structural and even positional isomers. Further analysis demonstrated that CE(50) values were independent of cone potential and orthogonal to compound mass. Experimentally determined CE(50) values for a diverse set of 54 compounds were correlated to Molconn molecular structure descriptors. The resulting model yielded a statistically significant linear correlation between experimental and calculated CE(50) values and identified several structural characteristics related to precursor ion stability and fragmentation mechanism. Thus, the CE(50) is a promising method for compound identification and discrimination.

A new method for the determination of the relative affinity of a ligand against various DNA sequences by electrospray ionization mass spectrometry. Application to a polyamide minor groove binder

A new method for the determination of the relative affinity of a ligand against various dsDNA sequences is presented by using electrospray ionization time-of-flight mass spectrometry (ESI-QTOF) mass spectrometry. The principle is described here through the complexation of double-stranded DNA by a polyamide ligand including twelve N-methylpyrrole rings. However this method could be applied to other ligands especially when dissociation constants (Kd) are in nanomolar range. This method does not require knowing the ligand concentration accurately. It allows determination of the relative affinity of a ligand against various dsDNA sequences for 1 : 1 complex stoichiometries in a quick manner without labeling.

Identifying nonspecific ligand binding in electrospray ionization mass spectrometry using the reporter molecule method

The application of the reporter molecule (M(rep)) method for identifying nonspecific complexes in the ES-MS analysis of protein-ligand and DNA-ligand interactions in vitro is described. To test the reliability of the method, it was applied to the ES-MS analysis of protein-carbohydrate complexes originating from specific interactions in solution and from nonspecific interactions in the ES process. These control experiments confirm the basic assumptions underlying the M(rep) method, namely that nonspecific ligand binding is a random process, and that the ES droplet histories for specific and nonspecific complexes are distinct. The application of the M(rep) method to the ES-MS analysis of the sequential binding of the ethidium cation, a DNA intercalator, to single and double strand oligodeoxynucleotides is also described, and highlights the general utility of the method.

Mass-spectrometric studies of the interactions of selected metalloantibiotics and drugs with deprotonated hexadeoxynucleotide GCATGC

ESI tandem mass spectrometry is employed in a detailed study of the interactions of a hexameric duplex d(5'GCATGC) with three types of ligated first-row transition metal dications M(2+): metallated bleomycins, singly, doubly, and triply ligated metallophenanthrolines and [M(triethylenetetramine)](2+). The singly, doubly, and triply metallated species were found to dissociate by noncovalent separation into two strands with metal ions attached either to one or to both. Relative gas-phase stabilities of the double-stranded oligodeoxynucleotide (ODN)-M(2+) complexes were found to follow the order Mn(II) > Fe(II) > Co(II) > Ni(II) > Zn(II) > Cu(II). Overall, the presence of metal dications is found to increase the gas-phase stability of the duplex against noncovalent dissociation with the exception of one and three copper dications. An analysis of the dissociation pathways and relative gas-phase stabilities of the species that were investigated provided a basis for the assessment of the possible binding modes between duplex oligonucleotides and metallocomplexes.

Mass spectrometry in systems biology: an overview

As an emerging field, systems biology is currently the talk of the town, which challenges our philosophy in comprehending biology. Instead of the reduction approach advocated in molecular biology, systems biology aims at systems-level understanding of correlations among molecular components. Such comprehensive investigation requires massive information from the "omics" cascade demanding high-throughput screening techniques. Being one of the most versatile analytical methods, mass spectrometry has already been playing a significant role at this early stage of systems biology. In this review, we documented the advances in modern mass spectrometry technologies as well as nascent inventions. Recent applications of mass spectrometry-based techniques and methodologies in genomics, proteomics, transcriptomics and metabolomics will be further elaborated individually. Undoubtedly, more applications of mass spectrometry in systems biology can be expected in the near future.

Evaluation of flavonoids binding to DNA duplexes by electrospray ionization mass spectrometry

In this study, electrospray ionization mass spectrometry (ESI-MS) was used to investigate the binding interactions of ten flavonoid aglycones and ten flavonoid glycosides with DNA duplexes. Relative binding affinities of the flavonoids toward DNA duplexes were estimated based on the fraction of bound DNA. The results revealed that the 4'-OH group of flavonoid aglycones was essential for their DNA-binding properties. Flavonoid glycosides with sugar chain linked on ring A or ring B showed enhanced binding toward the duplexes over their aglycone counterparts, whereas glycosylation of the flavonol quercetin on ring C exhibited a less pronounced effect. The aglycone skeletons and other hydroxyl substitutions on the aglycone also have an effect on the fractions of bound DNA. Upon collision-induced dissociation, the complexes containing flavonoid aglycones underwent the predominant ejection of a neutral ligand molecule, suggesting an intercalative DNA-binding mode. However, for complexes containing flavonoid glycosides, the loss of nucleobase increased to different extents, indicating a stronger binding or different binding mode. The results may provide not only a deeper insight into the DNA-binding properties of flavonoids but also a useful guideline for the design of efficient DNA-binding agents for chemotherapy.

Studies on alkaloids binding to GC-rich human survivin promoter DNA using positive and negative ion electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry (ESI-MS) was used to investigate the binding of 13 alkaloids to two GC-rich DNA duplexes which are critical sequences in human survivin promoter. Negative ion ESI-MS was first applied to screen the binding of the alkaloids to the duplexes. Six alkaloids (including berberine, jatrorrhizine, palmatine, reserpine, berbamine, and tetrandrine) show complexation with the target DNA sequences. Relative binding affinities were estimated from the negative ion ESI data, and the alkaloids show a binding preference to the duplex with higher GC content. Positive ion ESI mass spectra of the complexes were also recorded and compared with those obtained in negative ion mode. Only the 1 : 1 complex with berbamine was observed with lower abundance in the positive ion mass spectrum while complexes with the other alkaloids were absolutely absent. Collision-induced dissociation (CID) experiments indicate that the complexes with the protoberberine alkaloids (berberine, jatrorrhizine, and palmatine) dissociate via base loss and covalent cleavage. In contrast, product ion spectra of the complexes with the alkaloids reserpine, berbamine, and tetrandrine show the predominant loss of a neutral alkaloid molecule, accompanied by base loss and covalent cleavage to a lesser extent. A comparison of the gas-phase behaviors of complexes with the alkaloids to those with the traditional DNA binders has suggested an intercalative binding mode of these alkaloids to the target DNA duplexes.