Sequence Viewer

The Multalign Viewer tool displays individual sequences and multiple sequence alignments. Sequence alignments can be read from external files (several formats) or created by other tools in Chimera. Structures opened in Chimera are automatically associated with sufficiently similar sequences in the alignment. After association,
   • mousing over a residue in the sequence shows its structure residue number
   • selecting in the sequence selects residues in the structure(s) and vice versa
   • structures can be superimposed using the sequence alignment
Various measures of sequence conservation and structural variation (RMSD) can be computed and shown above the sequences as histograms, and on the structures with color or worm radius. Secondary structure elements can be depicted as colored boxes or regions on the alignment. Regions can also be created by hand.

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Morphing

Different conformations and even different proteins can be compared by morphing from one structure to another. Users can specify the method of coordinate interpolation and how many intermediate structures should be generated. The result is displayed in Chimera's trajectory viewer, MD Movie. The morph can then be saved in coordinate form or recorded as an animation.

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Molecular Graphics

• stick, ball-and-stick, CPK, ribbon, pipe-and-plank, and nucleotide display styles
• molecular surfaces
• highly intuitive rotation, translation, and scaling with the mouse
• Side View for adjusting clipping planes and scaling
• interactive color editing in various color spaces (RGB, CMYK, etc.) with transparency
• ability to save high-resolution images for presentation and publication
• stereo (side-by-side and time-sequential)

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Chemical Knowledge

• determination of atom types in arbitrary molecules, including non-standard residues
• ability to add hydrogen atoms
• high-quality hydrogen bond identification
• selection of atoms/bonds by element, atom type, functional group, amino acid category
• interactive bond rotation, distance and angle measurements

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Showing ConSurf Results

The ConSurf Server provides results as Chimera Web data; after browser configuration, a single click displays the color-coded query structure and multiple sequence alignment with phylogenetic tree and custom headers in a locally installed copy of Chimera (details).

Special thanks to Elana Erez and the Ben-Tal and Pupko groups at Tel Aviv University, and to Fabian Glaser at the Technion.

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Volume Data

Chimera's Volume Viewer displays three-dimensional electron and light microscope data, X-ray density maps, electrostatic potential and other volumetric data. Contour surfaces, meshes and volumetric display styles are provided and thresholds can be changed interactively. Maps can be colored, sliced, segmented, and modifications can be saved. Markers can be placed and structures can be traced. The accompanying image shows a density map of Kelp fly virus from electron microscopy colored radially and with an octant cut out.

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Multiscale Models

The Multiscale Models extension allows Chimera to display large complexes such as virus capsids, ribosomes, and chromatin. It displays the quaternary structure of PDB models and allows subunits to be selected and shown in atomic detail. Matrices are read from PDB files that specify the biological unit. Crystallographic packing can also be shown.

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Screening Docked Molecules

The program DOCK calculates possible binding orientations, given the structures of "ligand" and "receptor" molecules. Typically, a large database of small molecule structures is searched for compounds that may bind the receptor. The Chimera extension ViewDock facilitates interactive selection of promising compounds from the output of DOCK. The molecules can be viewed in the context of the binding site and optionally, screened by number of hydrogen bonds to the receptor. The Dock Prep extension prepares a receptor for input to a docking program by adding hydrogens, assigning partial charges, and performing other related tasks.

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User-Driven Analysis

Users can easily import structure-related data into Chimera in the form of attributes, or values associated with atoms, residues, or models. The data can be imported with Define Attribute and then represented visually with color ranges, atomic radii, or "worm" thickness. Such data can also be manipulated programmatically in Chimera, and in fact Chimera was designed with extensibility and programmability in mind. It is largely implemented in Python, with certain features coded in C++ for efficiency. Python is an easy-to-learn interpreted language with object-oriented features. All of Chimera's functionality is accessible through Python and users can implement their own algorithms and extensions without any Chimera code changes, so any such custom extensions will continue to work across Chimera releases. Many programming examples are provided to assist extension writers.

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Rotamers

Amino acid sidechains adopt different conformational states, or rotamers. Rotamers from the Dunbrack backbone-dependent library or the Richardson "penultimate" library can be viewed, evaluated, and incorporated into structures with the Rotamers tool. A residue can be changed into a different conformation of the same type of amino acid or mutated into a different type. Rotamer torsion angles and library probability values are listed in a dialog, along with (optionally) hydrogen bonds, clashes, and agreement with electron density data. Only rotamers chosen in the list are displayed. When a single rotamer is chosen, it can be incorporated into the structure. The image includes 2D labels.

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Morphing Density Maps

Related density maps can be compared by morphing from one to the other. Several intermediate maps are generated by interpolating between the starting and ending maps. The morph can be viewed interactively and recorded as a movie. The contour level can be adjusted automatically to keep the enclosed volume constant throughout the morph, and other aspects of map display can be adjusted with Volume Viewer.

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Volume Plane Display

Volume data can be shown a single plane (or slab) at a time with the Planes feature in Volume Viewer. Plane display can be set to oscillate along the data X, Y, or Z axis, or the plane location can be specified interactively with a slider.

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Topography

Values in a plane of volume data can be shown as heights normal to the plane (a topographic map). When a single plane is displayed with Volume Viewer, the command topography will plot the values as heights in a surface.

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Coloring by Density

A surface can be colored by density or other volume data. In the image, the surface is clipped and capped, and only the cap is colored by density. Different coloring schemes can be applied.

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Radial Coloring

A surface can be colored radially, that is, by distance from a user-specified point. Additional options include coloring by distance from an axis or a plane. Different coloring schemes can be applied.

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Coloring by Conservation

A structure can be colored to show values of an attribute such as residue conservation. Opening a sequence alignment in Chimera automatically displays it in Multalign Viewer and associates it with any similar structures (a few residue mismatches are allowed). A variety of conservation measures can be computed. The image was created using the PFAM Carb_anhydrase seed alignment PF00194_seed.slx and includes a color key and 2D labels.

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CASTp Pocket Data

Structures and their pocket measurements can be fetched directly from the Computed Atlas of Surface Topography of proteins (CASTp) database or read from local files previously returned by the CASTp server. In Chimera, the pockets and their measurements are shown in a pocket list. Choosing one or more pockets from the list performs actions on the structure, such as zooming in on a pocket, showing its surface, and/or selecting the atoms around it.
The figure shows the four largest pockets by volume identified by CASTp for PDB entry 1ovh, a cavity mutant of T4 lysozyme. The pocket surfaces are colored yellow, orange, pink, and magenta in order of decreasing volume. The largest (yellow) is lysozyme's active site, with two openings. The second largest (orange) is the engineered cavity, completely enclosing a bound aniline derivative. The mutated positions are shown in red. Chloride ions are displayed as green balls.

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Density Display

Electron density maps can be displayed as mesh or solid isosurfaces with Volume Viewer. Contour levels can be adjusted interactively, multiple levels can be shown for a given map, and display can be restricted to a zone around specified atoms. The image (see how-to) shows PDB entry 2fma and its electron density map. Chimera can fetch the map directly from the Electron Density Server.

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B-Factor Coloring

A structure can be colored to show values of an attribute such as atomic B-factor. The image includes a molecular surface that has been clipped and capped, 2D labels, and a color key. Color Zone was used to color the planar cross-section of the surface (see image how-to).

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Structure-Based Sequence Alignment

Given two or more superimposed structures, Match→Align creates a corresponding sequence alignment. The user specifies a distance cutoff for residues allowed to be in the same column of the output alignment. In proteins, the distances are measured between α-carbons. The method is independent of residue types and how the structures were superimposed.
The figure shows a superposition from MatchMaker of five proteins from the SCOP WD40 superfamily and a corresponding sequence alignment from Match→Align, automatically shown in Multalign Viewer. In the sequence alignment, light green and yellow boxes indicate strands and helices, while the headers RMSD and Conservation show spatial and sequence conservation, respectively.

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Superimposing Structures

There are several ways to superimpose structures in Chimera:
•  MatchMaker performs a fit after automatically identifying which residues should be paired. Pairing uses both sequence and secondary structure, allowing similar structures to be superimposed even when their sequence similarity is low to undetectable.
The figure shows five distantly related proteins (pairwise sequence identities <25%) from the SCOP WD40 superfamily before and after MatchMaker superposition with default parameters.
•  Structures can be matched using a pre-existing sequence alignment.
•  The exact atoms to pair can be specified with the match command. This works on any type of structure, while the preceding methods apply only to peptide and nucleotide chains.
•  Structures can be superimposed manually by activating/deactivating them for motion and using the mouse.

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Space Navigator Input Device

Space Navigator is an inexpensive ($60) USB input device from 3Dconnexion for moving and rotating models in 3 dimensions. This animation (8 Mb) shows docking a phage enzyme (PDB 1v0e) into an electron microscopy map (EMDB 1333) using a space navigator and mouse. Works with Windows and Mac Chimera versions (details).

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Nucleotides

Special representations of DNA and RNA can be displayed with the Nucleotides tool or the command nucleotides. Different levels of abstraction are available. The figure shows a ribbon backbone combined with the following sidechain (sugar/base) options:

• ladder rungs
• filled-ring atomic representations
• "lollipops" in which bases are shown as ellipsoids and sugars as tubes

Bases can also be displayed as boxes or elliptical tubes, with or without bumps to indicate orientation. The colors of the special representations will update automatically to match the corresponding atoms.

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Thermal Ellipsoids

Anisotropic B-factors can be shown as ellipsoids, with ellipsoid axes and radii representing the eigenvectors and eigenvalues of the atomic mean-square displacement matrix. Anisotropic B-factors are read from the input coordinate file (for example, from ANISOU records in a PDB file) and can be displayed with the tool Thermal Ellipsoids or the command aniso. The figure shows ellipsoids scaled to enclose 50% probability for the heme and nearby atoms from PDB entry 1a6m.

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