3DNA is a versatile, integrated software system for the analysis, rebuilding, and visualization of 3D nucleic-acid-containing structures. The software is applicable not only to DNA (as the name 3DNA may imply) but also to complicated RNA structures and DNA-protein complexes. In 3DNA, structural analysis and model rebuilding are two sides of the same coin: the description of the structure is rigorous and reversible, thus allowing for its exact reconstruction based on the derived parameters. 3DNA automatically detects all non-canonical base pairs, base triplets and higher-order associations (collectively termed multiplets), and coaxially stacked helices, provides a comprehensive collection of fiber models of regular DNA and RNA helices, generates highly effective schematic presentations that reveal key features of nucleic-acid structures, performs undisturbed base mutations, and has facilities for the analysis of molecular dynamics simulation trajectories.

DSSR is an integrated software tool for dissecting the spatial structure of RNA. See also DSSR-enhanced visualization of nucleic acid structures in Jmol and DSSR-enabled innovative schematics of 3D nucleic acid structures with PyMOL. DSSR has much to offer. It integrates an unprecedented set of features into a single tool, including analysis/annotation, visualization, and modeling. It delivers a great user experience by solving problems and saving time. It represents what I call a ‘solid software product’. Considering its usability, interoperability, features, and support, DSSR easily stands out among ‘competitors’.

As of version 2.0, DSSR has surpassed 3DNA v2.4. While 3DNA is still actively maintained, no additional features other than bug fixes are planned. See the overview PDF for a brief description of DSSR vs. 3DNA vs. SCHNAaP/SCHNArP. Notably, DSSR 2.0 comes with a professional user manual that documents DSSR features not described in the basic manual or elsewhere. The software is licensed by Columbia University for academic or commercial use. Licensing revenue helps ensure the long-term sustainability of the DSSR project.

The SNAP program has been created from scratch for an integrated characterization of the Structures of Nucleic Acid-Protein complexes in PDB or mmCIF format. It shares the same codebase as DSSR, and works for DNA-protein as well as RNA-protein interactions. DSSR and SNAP serve as the cornerstones of DNA/RNA structural bioinformatics.

All 3DNA-related questions are welcome and should be directed to the 3DNA Forum. For the benefit of the community at large, I do not provide private support of 3DNA via email or personal message. As a general rule, I strive to provide a prompt and concrete response to each and every question posted on the Forum.

More info · Seeing is believing · Cover image · What’s new · 3DNA Forum · Download

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Over 5000 registrations on the 3DNA Forum

As I am writing this blogpost on June 26, 2020, the registrations on the 3DNA Forum has reached 5,054. The numbers were 3,000 on October 15, 2016, 2,000 on on February 3, 2015, and 1,000 on February 27, 2013 respectively. For year 2020, the monthly registrations are 36 (January), 35 (February), 54 (March), 84 (April), 69 (May). As of June 26, the number is 56, which will more than likely pass 60 by the end of this month. The Covid-19 pandemic does not seem to having a negative effect on the registrations.

The over 5,000 registrations are from users all over the world. The 3DNA Forum remains spam free, and all questions are promptly answered. It is functioning well; certainly better than I originally imagined.

Overall, the Forum serves as a virtual platform for me to interact effectively with the ever-increasing user community. I greatly enjoy answering questions, fixing bugs, and making 3DNA/DSSR/SNAP better tools for real-world applications.

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May's article on "The Best Ways to Study DNA and Protein Interactions"

In late March, I was approached by Mike May. He was then writing an article for Biocompare about DNA-protein interactions and asked me to answer a few questions on “What features of 3DNA be used in studying DNA-protein interactions?” and “Please provide 1-2 examples.” Initially, I was a bit surprised by the contact. Thus, I visited his online profile and Amazon Author Page. I also read a couple of his previous publications. Impressed by his track records, I answered his requests and our following communications were as smooth and professional as I could have ever imagined.

The paper The Best Ways to Study DNA and Protein Interactions has now been published, and is freely accessible. It includes the following content:

3DNA creator and maintainer Xiang-Jun Lu mentioned a couple of ways that the software has been used. For example, he noted that “3DNA can analyze all DNA-protein complexes in the Protein Data Bank—PDB—in an automatic, consistent, and robust manner,” and other bioinformatic resources have adopted this feature of 3DNA. He added that scientists have used 3DNA to “understand the structural basis on how transcription factors recognize methylated DNA.” Moreover, 3DNA is continuously developed. A new feature of 3DNA is the automatic identification and comprehensive characterization of G-quadruplexes, a noncanonical DNA structure formed from guanine-rich base sequences.

The bioinformatics resource I used as an example is the paper DNAproDB: an expanded database and web-based tool for structural analysis of DNA–protein complexes by the Rohs lab. The phrase “to understand the structural basis on how transcription factors recognize methylated DNA” refers to the article Toward a mechanistic understanding of DNA methylation readout by transcription factors by the Bussemaker lab. Both works employed DSSR and SNAP, two sophisticated programs I created and maintained over the past ten years, and they have largely obsoleted the original 3DNA suite of programs.

The image I provided is a DSSR-PyMOL schematic based on PDB entry 6LDM. The 6LMD picture features a G-quadruplex, for which DSSR comes with an unmatched set of features (including automatic identification and comprehensive annotations). See the http://g4.x3dna.org/ page for survey results, curated using DSSR, of all G-quadruplexes from the PDB.

This image of a protein-DNA complex (PDB entry 6LDM) shows the protein (purple), the DNA G-quadruplex (green) and thymine (blue). The image was created using the 3DNA-DSSR program and PyMOL. Image courtesy of Xiang-Jun Lu.

DSSR-PyMOL schematic for PDB entry 6ldm

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3DNA-DSSR is linked in the G4-society website

A couple of months ago, I came across the homepage of the newly-established G4 Society on G-quadruplexes (G4s). I checked the “Online tools” section and found a few links to G4 databases and sequence-based predication programs (e.g., G4Hunter). No tools, however, were listed for G4 identification and characterization from 3D atomic coordinates as those deposited in the Protein Data Bank (PDB). So I filled out the contact form and provided a brief description of 3DNA-DSSR, including a link to the website of G4s auto-curated with DSSR from the PDB.

I’ve recently visited the G4-society website again. I am pleased to see that 3DNA-DSSR is now listed under Online tools as a “program for detections/annotations of G4 from atomic coordinates in PDB or PDBx/mmCIF format”. The G4 module of 3DNA-DSSR has been created to streamline the identification and annotation of 3D structures of G4s. The collection of G4s in the PDB, available at G4.x3dna.org, is updated weekly. It represents a unique resource for the G4 community. Hopefully, its value will be more widely appreciated thanks to the link from the G4-society website.

At the G4-society homepage, I noticed the following two items in the “News” section (on December 13, 2019):

The Quadruplex Meeting Report

Meeting report: Seventh International Meeting on Quadruplex Nucleic Acids (Changchun, P.R. China, September 6e9, 2019) written by Jean-Louis Mergny. Reading through the report, I noticed the following:

Jonathan B. Chaires (U. Louisville, KY, USA) provided an overview and historical perspective of the quadruplex field in his inaugural lecture. As of August 2019, the quadruplex field gathers 8467 articles and 253,174 citations in the Science Citation Index. Over 200 G4 structures are available in the PDB.

I did not know how the survey of G4s in the PDB was performed. Based on my data, the PDB-G4 structures was already over 300 as of August 2019. As of December 11, 2019, the number of G4 structures in the PDB is 329. Importantly, the PDB-G4 website compiled using 3DNA-DSSR contains not only citation information but also detailed annotations and schematic images not available elsewhere. Here are a few recent examples:

  • PDB id: 6ge1 — “Unraveling the structural basis for the exceptional stability of RNA G-quadruplexes capped by a uridine tetrad at the 3’ terminus.” by Andralojc et al. in RNA (2019).
  • PDB id: 6gh0 — “Two-quartet kit* G-quadruplex is formed via double-stranded pre-folded structure.” by Kotar et al. in Nucleic Acids Res. (2019).
  • PDB id: 6e8u — “Structure and functional reselection of the Mango-III fluorogenic RNA aptamer.” by Trachman et al. in Nat. Chem. Biol. (2019).
  • PDB id: 6ac7 —“Structure of a (3+1) hybrid G-quadruplex in the PARP1 promoter.” by Sengar et al. in Nucleic Acids Res. (2019).

The Important Paper

A guide to computational methods for G-quadruplex prediction by Emilia Puig Lombardi and Arturo Londoňo-Vallejo in Nucleic Acids Res. (2019), which presents an updated overview of G4 prediction algorithms. I am impressed by the large number of sequence-based G4 prediction software tools, including the most recent G4-iM Grinder. Nevertheless, as noted by the authors in the concluding remarks, “All computational G-quadruplex prediction approaches have their drawbacks and limitations despite the recent advances in the field and the introduction of validation steps based on experimental data.”

The G4 module in 3DNA-DSSR belongs to a completely different category of software tool. It does not ‘predict’ G4 propensity/stability from a base sequence, but identify and annotate G4s in a 3D atomic coordinate file. It complements sequence-based predicting tools by gaining insights into the 3D G4 structures and refining folding rules to improve performance of prediction tools. Based on my knowledge, the 3D G4 structures contains features that are not captured by any of the sequence-based prediction tools.

While reading the review article, I found Fig. 1 informative (see below). The right side of Fig. 1A shows a “cartoon representation of the Oxytricha telomeric DNA G4 crystal structure (PDB accession 1JPQ (112))” using PyMOL. In comparison, the cartoon-block image auto-generated via 3DNA-DSSR and PyMOL for PDB id: 1jpq is shown at the bottom. The DSSR-PyMOL version is obviously different, presumably simpler and more informative, from that illustrated in Fig. 1A.

Figure 1. From guanines to G-quadruplexes

3DNA-DSSR cartoon-block schematic for PDB entry 1jpq, rendered with PyMOL

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ONZ classification of G-tetrads

Recently I read the article Topology-based classification of tetrads and quadruplex structures in Bioinformatics by Popenda et al. In this work, the authors proposed an ONZ classification scheme of G-tetrads in intramolecular G-quadruplexes (G4) as shown below (Fig. 2 in the publication):

ONZ classification of G-tetrads in intramolecular G-quadruplexes

I am glad to find that DSSR has been used as a component in their computational tool ElTetrado to automatically identify and classify tetrads and quadruplexes.

Structures from both sets were analysed using self-implemented programs along with DSSR software from the 3DNA suite (Lu et al. (2015)). From DSSR, we acquired the information about base pairs and stacking.

I like the ONZ classification scheme: it is simple in concept yet provides a new perspective for the topologies of G-tetrads in intramolecular G4 structures. So I implemented the idea in DSSR v1.9.8-2019oct16, with this feature available via the --g4-onz option. Note that ElTetrado, according to the authors, is applicable to ONZ classifications of general types of tetrads and quadruplexes. The DSSR implementation of ONZ classifications, on the other hand, is strictly limited to G-tetrads in intramolecular G4 structures.

The DSSR ONZ classification results match the ones reported in Figs. 1, 5, and 6 of the Popenda et al. paper. For example, for PDB entry 6H1K (Fig. 6), the relevant results with the --g4-onz option and without it are listed below:

# x3dna-dssr -i=6h1k.pdb --g4-onz
List of 3 G-tetrads
   1 glyco-bond=s--- groove=w--n planarity=0.149 type=planar Z- nts=4 GGGG A.DG1,A.DG20,A.DG16,A.DG27
   2 glyco-bond=-sss groove=w--n planarity=0.136 type=planar Z+ nts=4 GGGG A.DG2,A.DG19,A.DG15,A.DG26
   3 glyco-bond=--s- groove=-wn- planarity=0.307 type=other  O+ nts=4 GGGG A.DG17,A.DG21,A.DG25,A.DG28
# ---------------------------------------
# x3dna-dssr -i=6h1k.pdb 
#   without option --g4-onz
List of 3 G-tetrads
   1 glyco-bond=s--- groove=w--n planarity=0.149 type=planar nts=4 GGGG A.DG1,A.DG20,A.DG16,A.DG27
   2 glyco-bond=-sss groove=w--n planarity=0.136 type=planar nts=4 GGGG A.DG2,A.DG19,A.DG15,A.DG26
   3 glyco-bond=--s- groove=-wn- planarity=0.307 type=other  nts=4 GGGG A.DG17,A.DG21,A.DG25,A.DG28

With the --json option, the ONZ classification results are always available. An example is shown below for PDB entry 6H1K (Fig. 6):

# x3dna-dssr -i=6h1k.pdb --json | jq -c '.G4tetrads[] | [.nts_long, .topo_class]'
["A.DG1,A.DG20,A.DG16,A.DG27","Z-"]
["A.DG2,A.DG19,A.DG15,A.DG26","Z+"]
["A.DG17,A.DG21,A.DG25,A.DG28","O+"]

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H-bonds reported by DSSR and SNAP

I recently read a short communication by Pavel Afonine, titled phenix.hbond: a new tool for annotation hydrogen bonds in the July 2019 issue of the Computational Crystallography Newsletter (CCN). It appears that every bioinformatics tool (e.g., PyMOL or Jmol) has its own implementation of an algorithm on calculating H-bonds, one of the fundamental stabilizing forces of proteins and DNA/RNA structures. So does 3DNA/DSSR, as noted in my 2014-04-11 blogpost Get hydrogen bonds with DSSR.

Both DSSR and SNAP have the --get-hbond option, and they use the same underlying algorithm. However, the default output from the two programs differs: DSSR reports the H-bonds within nucleic acids, and SNAP covers only those at the DNA/RNA-protein interface. Using the PDB entry 1oct as an example, Running DSSR on it with the --get-hbond option gives 33 H-bonds in the DNA duplex, while SNAP reports 38 H-bonds at the DNA-protein interface. By design, the default output caters for the most-common use case of each program.

Under the scene, however, there exist variations in the seemingly simple --get-hbond option. One can attach text ‘nucleic’ (or ‘nuc’, ‘nt’), as in --get-hbond-nucleic, to output H-bonds within nucleic acids. Similarly, --get-hbond-protein (or ‘amino’, ‘aa’) would output H-bonds within proteins. Not surprisingly, the --get-hbond-nt-aa option would list H-bonds in nucleic acids and proteins, including those at their interface. These variations apply to both DSSR and SNAP, even though some are redundant with the default.

Notably, in combination with --json, the --get-hbond option by default would output all H-bonds, as if --get-hbond-nt-aa has been set. For PDB entry 1oct, DSSR or SNAP would report 208 H-bonds. Moreover, the JSON output has a residue_pair field for each identified H-bond, with values like "nt:nt", "nt:aa", or "aa:aa". Using 1oct as an example,

# x3dna-dssr -i=1oct.pdb --get-hbond --json | jq '.hbonds[0]'
{
  "index": 1,
  "atom1_serNum": 34,
  "atom2_serNum": 608,
  "donAcc_type": "standard",
  "distance": 3.304,
  "atom1_id": "O6@A.DG202",
  "atom2_id": "N4@B.DC230",
  "atom_pair": "O:N",
  "residue_pair": "nt:nt"
}
# x3dna-dssr -i=1oct.pdb --get-hbond --json | jq '.hbonds[60]'
{
  "index": 61,
  "atom1_serNum": 462,
  "atom2_serNum": 1187,
  "donAcc_type": "standard",
  "distance": 3.692,
  "atom1_id": "O2@B.DT223",
  "atom2_id": "NH2@C.ARG102",
  "atom_pair": "O:N",
  "residue_pair": "nt:aa"
}
# x3dna-dssr -i=1oct.pdb --get-hbond --json | jq '.hbonds[100]'
{
  "index": 101,
  "atom1_serNum": 791,
  "atom2_serNum": 818,
  "donAcc_type": "standard",
  "distance": 2.871,
  "atom1_id": "N@C.THR26",
  "atom2_id": "OD2@C.ASP29",
  "atom_pair": "N:O",
  "residue_pair": "aa:aa"
}

In the above three cases, using SNAP instead of DSSR would give the same results.

Also, one can take advantage of the residue_pair value to filter H-bonds by type. For example, the following command would extract only H-bonds at the DNA-protein interface (38 occurrences, same as the number noted above):

x3dna-snap -i=1oct.pdb --get-hbond --json | jq '.hbonds[] | select(.residue_pair=="nt:aa")'

Back to the phenix.hbond tool, the author noted that:

Running phenix.hbond requires atomic model in PDB or mmCIF format with all hydrogen atoms added, as well as ligand restraint files if the model contains unknown to the library items.

While there is no particular reason why this should not work for all bio-macromolecules, currently phenix.hbond is only optimized and tested to work with proteins, which is the limitation that will be removed in future.

In contrast, the H-bond identification algorithm in DSSR/SNAP does not require hydrogen atoms. In fact, hydrogen atoms are simply ignored if they exist. As shown above, the H-bond method as implemented in DSSR/SNAP works for DNA, RNA, protein, or their complexes. This does not necessarily mean that the 3DNA way is superior to other similar tools. It just works well in my hand, and it may serve as a pragmatic choice for other users.

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5CM and 5MC, two forms of 5-methylcytosine in the PDB

In the PDB, the ligand identifiers 5MC and 5CM all refer to 5-methylcytosine, but differ in the sugar moieties the base is attached to. Chemically, 5MC is 5-methyl-2’-deoxycytidine-5’-monophosphate as in DNA, and 5MC is 5-methylcytidine-5’-monophosphate. See the molecular images shown below.

Web 3DNA 2.0 highlighted in the cover of the NAR'19 webserTwo forms of 5-methylcytosine in PDB: 5CM and 5MC

The 5-methyl group is named C5A in 5CM and CM5 in 5MC, respectively, for non-obvious reasons other than conventions. For comparison, the methyl-group in thymine of DNA is named C7, as for example in PDB id 355d. It is worth noting that DSSR is able to handle all such variations in atom or residue names.

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DNA conformational changes play a force-generating role during bacteriophage genome packaging

A paper titled DNA Conformational Changes Play a Force-Generating Role during Bacteriophage Genome Packaging has just been officially published in the Biophysical Journal (Volume 116, Issue 11, P2172-2180, June 04, 2019). I am glad to have the opportunity to collaborate with Kim Sharp, Gino Cingolani and Stephen Harvey on this interesting project that has big implications in understanding the mechanism of bacteriophage genome packaging. The abstract of the paper is shown below:

Motors that move DNA, or that move along DNA, play essential roles in DNA replication, transcription, recombination, and chromosome segregation. The mechanisms by which these DNA translocases operate remain largely unknown. Some double-stranded DNA (dsDNA) viruses use an ATP-dependent motor to drive DNA into preformed capsids. These include several human pathogens as well as dsDNA bacteriophages—viruses that infect bacteria. We previously proposed that DNA is not a passive substrate of bacteriophage packaging motors but is instead an active component of the machinery. We carried out computational studies on dsDNA in the channels of viral portal proteins, and they reveal DNA conformational changes consistent with that hypothesis. dsDNA becomes longer (“stretched”) in regions of high negative electrostatic potential and shorter (“scrunched”) in regions of high positive potential. These results suggest a mechanism that electrostatically couples the energy released by ATP hydrolysis to DNA translocation: The chemical cycle of ATP binding, hydrolysis, and product release drives a cycle of protein conformational changes. This produces changes in the electrostatic potential in the channel through the portal, and these drive cyclic changes in the length of dsDNA as the phosphate groups respond to the protein’s electrostatic potential. The DNA motions are captured by a coordinated protein-DNA grip-and-release cycle to produce DNA translocation. In short, the ATPase, portal, and dsDNA work synergistically to promote genome packaging.

Significantly, our work is highlighted in a “New and Notable” article, May the Road Rise to Meet You: DNA Deformation May Drive DNA Translocation by Paul Jardine (Volume 116, Issue 11, Pages 2060-2061, 4 June 2019):

Regardless of what drives conformational change in the portal, the idea that the linear DNA substrate is deformed in a way that makes it an energetic participant in its own movement opens new possibilities for how motors work. Large paddling or rotational motions by motor components may not be required if linear motion can be achieved by stretching or compressing the linear substrate, with rectified, cyclic conformational changes in the DNA rather than lever motions doing the work. If borne out by experiments, further simulation, and more structural information, this proposed mechanism may require a reappraisal of how we think about translocating motors.

For this project, I developed the x3dna-search program to survey similar fragments of single-stranded or double helical structures in the PDB.

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The article on G.A pairs in ACS Biochemistry

After many years of efforts, it is a great pleasure to see our paper Effects of Noncanonical Base Pairing on RNA Folding: Structural Context and Spatial Arrangements of G·A Pairs published in ACS Biochemistry. The abstract is shown below:

Noncanonical base pairs play important roles in assembling the three-dimensional structures critical to the diverse functions of RNA. These associations contribute to the looped segments that intersperse the canonical double-helical elements within folded, globular RNA molecules. They stitch together various structural elements, serve as recognition elements for other molecules, and act as sites of intrinsic stiffness or deformability. This work takes advantage of new software (DSSR) designed to streamline the analysis and annotation of RNA three-dimensional structures. The multiscale structural information gathered for individual molecules, combined with the growing number of unique, well-resolved RNA structures, makes it possible to examine the collective features deeply and to uncover previously unrecognized patterns of chain organization. Here we focus on a subset of noncanonical base pairs involving guanine and adenine and the links between their modes of association, secondary structural context, and contributions to tertiary folding. The rigorous descriptions of base-pair geometry that we employ facilitate characterization of recurrent geometric motifs and the structural settings in which these arrangements occur. Moreover, the numerical parameters hint at the natural motions of the interacting bases and the pathways likely to connect different spatial forms. We draw attention to higher-order multiplexes involving two or more G·A pairs and the roles these associations appear to play in bridging different secondary structural units. The collective data reveal pairing propensities in base organization, secondary structural context, and deformability and serve as a starting point for further multiscale investigations and/or simulations of RNA folding.

Sample G.A pair characterized by DSSR

This work represents a multifaceted, fundamental application enabled by DSSR. Even at the base-pair (bp) level, DSSR provides unique features that complement the Leontis-Westhof (LW) notation of 12 geometric types.

At the review stage, we were asked by a referee to comment on the differences between DSSR and LW on bp classifications. The following paragraph in the “DISCUSSION” section of the paper is our response, expanded on the original writing that focused on DSSR’s capabilities:

Qualitative descriptions of noncanonical RNA base pairing, pioneered by Leontis and Westhof9,41 and linked in this work to the rigid-body parameters of interacting bases, have proven valuable in deciphering the connections between RNA primary, secondary, and tertiary structures. The present categorization is based on the positions of the hydrogen-bonded atoms with respect to a standard, embedded base reference frame30 defined in terms of an idealized Watson−Crick base pair. The major- and minor-groove base edges used here correspond in most cases to what are termed the Hoogsteen and sugar edges in the Leontis−Westhof scheme (one can compare the two classification schemes in Table S2). The + and − symbols introduced in 3DNA24 and DSSR27 unambiguously distinguish the relative orientations of the two bases. The trans and cis designations used in the earlier literature, however, are qualitative in nature and often uncertain. There are many “nc” (near cis, as in ncWW) and “nt” (near trans, as in ntSH) annotations listed in the RNA Structure Atlas; see, for example, the base-pair interactions in the sarcin−ricin domain of E. coli 23S rRNA found by entering PDB entry 1msy at http://rna.bgsu.edu/rna3dhub/pdb. The assignment of qualitative descriptors of RNA associations on the basis of atomic identity alone is generally not clear-cut. Numerical differences in the rigid-body parameters are critical to differentiating pairing schemes that share a common hydrogen bond, e.g., the G(N3)···A(N6) interaction found in m−WII and m−MI arrangements of G and A (Table 1 and Figures 4 and S3). The numerical data also provide a basis for following conformational transitions and may potentially be of value in making functional and other meaningful distinctions among RNA base pairs.

See also a recent thread Noncanonical base pair standards on the 3DNA Forum and the section titled “3.2.2 Base pairs” in the DSSR User Manual.

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Web 3DNA 2.0 paper published in NAR

It is a great pleasure to announce the publication of Web 3DNA 2.0 for the analysis, visualization, and modeling of 3D nucleic acid structures in Nucleic Acids Research (NAR). The paper will appear in the web server issue of NAR in July 2019. At nine-page in length and with several new structural parameters, this w3DNA 2.0 paper is certainly not a typical NAR web-server publication. It represents a significant contribution to the field of 3D nucleic acids structural bioinformatics, and will undoubtedly push the popularity of 3DNA to a new level.

The abstract is shown below:

Web 3DNA (w3DNA) 2.0 is a significantly enhanced version of the widely used w3DNA server for the analysis, visualization, and modeling of 3D nucleic-acid-containing structures. Since its initial release in 2009, the w3DNA server has continuously served the community by making commonly-used features of the 3DNA suite of command-line programs readily accessible. However, due to the lack of updates, w3DNA has clearly shown its age in terms of modern web technologies and it has long lagged behind further developments of 3DNA per se. The w3DNA 2.0 server presented here overcomes all known shortcomings of w3DNA while maintaining its battle-tested characteristics. Technically, w3DNA 2.0 implements a simple and intuitive interface (with sensible defaults) for increased usability, and it complies with HTML5 web standards for broad accessibility. Featurewise, w3DNA 2.0 employs the most recent version of 3DNA, enhanced with many new functionalities, including: the automatic handling of modified nucleotides; a set of ‘simple’ base-pair and step parameters for qualitative characterization of non-Watson–Crick double- helical structures; new structural parameters that integrate the rigid base plane and the backbone phosphate group, the two nucleic acid components most reliably determined with X-ray crystallography; in silico base mutations that preserve the backbone geometry; and a notably improved module for building models of single-stranded RNA, double- helical DNA, Pauling triplex, G-quadruplex, or DNA structures ‘decorated’ with proteins. The w3DNA 2.0 server is freely available, without registration, at http://web.x3dna.org.

Moreover, details on reproducing our reported results are available in a dedicated section ‘web 3DNA 2.0 (http://web.x3dna.org)’ on the 3DNA Forum.

Graphical abstract of web 3DNA 2.0

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