Chimera uses atom and residue names, or if these are not "standard," the coordinates of atoms, to determine connectivity and atom types. This information is needed for identification of functional groups, addition of hydrogen atoms when they are not present, and identification of hydrogen bonds. The algorithm and atom types discerned are for the most part adapted from the program IDATM; for a detailed description of IDATM and validation testing, please see
"Determination of Molecular Topology and Atomic Hybridization States from Heavy Atom Coordinates," E.C. Meng and R.A. Lewis, J. Comp. Chem., 12, 891 (1991).
The atom types and algorithm, including extensions of the original method, are described briefly below. Where type definitions are not mutually exclusive, the atom is assigned the most specific type possible; for example, although a carboxylate carbon is also sp2-hybridized, it is assigned the Cac type. Since the categorizations in Chimera differ from those in the original method, the same type may appear in more than one row in the following table.
IDATM atom type | description | |
---|---|---|
Chimera | original | |
C3 | C3 | sp3-hybridized carbon |
C2 | C2 | sp2-hybridized carbon |
Car | C2 | aromatic carbon |
C1 | C1 | sp-hybridized carbon |
Cac | Cac | carboxylate carbon |
N3+ | N3+ | sp3-hybridized nitrogen with formal positive charge |
N3 | N3 | sp3-hybridized nitrogen, neutral |
Npl | Npl | sp2-hybridized but not double-bonded nitrogen (planar) |
N2 | Npl | sp2-hybridized, double-bonded or aromatic nitrogen |
N1 | N1 | sp-hybridized nitrogen |
Nox | Nox | N-oxide nitrogen |
Ntr | Ntr | nitro group nitrogen |
Ng+ | Ng+ | guanidinium nitrogen |
O3 | O3 | sp3-hybridized oxygen |
O2 | O2 | sp2-hybridized oxygen |
Oar | (none) | aromatic oxygen |
O- | O- | carboxylate, phosphate, or sulfate oxygen sharing formal negative charge; nitro group oxygen |
S3+ | S3+ | sp3-hybridized sulfur with formal positive charge |
S3 | S3 | sp3-hybridized sulfur |
S2 | S2 | sp2-hybridized sulfur |
Sac | Sac | sulfate, sulfonate, or sulfamate sulfur |
Son | Sox | sulfone sulfur (>SO2) |
Sxd | Sox | sulfoxide sulfur (>SO) |
S | S | other sulfur |
B | Bac | borate boron |
B | Box | other oxidized boron |
B | B | other boron (not oxidized) |
Pac | Pac | phosphate, phosphonate, or phosphamate phosphorus |
Pox | Pox | P-oxide phosphorus |
P3+ | P3+ | sp3-hybridized phosphorus with formal positive charge |
P | P | other phosphorus |
HC | HC | hydrogen bonded to carbon |
H | H | other hydrogen |
DC | DC | deuterium bonded to carbon |
D | D | other deuterium |
(element symbol) | (element symbol) | atoms of elements not mentioned above |
Brief descriptions of the original algorithm and extensions added during implementation in Chimera are given here.
Many experimentally determined structures of molecules do not include hydrogen atoms. IDATM uses the coordinates of nonhydrogen atoms (plus any hydrogens, if present) to determine the connectivity and hybridization states of atoms within molecules. This knowledge is essential for detailed molecular modeling. The algorithm is hierarchical; the "easiest" assignments are done first and used to aid subsequent assignments. The procedure can be divided into several stages:
In Chimera, a few additional distinctions are made. Carbons that are sp2-hybridized and part of planar ring systems are given an aromatic type. Oxygens within aromatic rings are given an aromatic type. Geometric criteria are used to subdivide sp2-hybridized nitrogens into double-bonded (or aromatic) and non-double-bonded categories. Sulfone and sulfoxide sulfurs are given two different types rather than lumped into a single category.