In 3DNA, each base pair (bp) is specified by the identity of its two comprising nucleotides (nts), and their interactions. Some examples are shown below based on the PDB entry 1ehz (the crystal structure of yeast phenylalanine tRNA at 1.93 Å resolution), with the shorthand form on the right:

....>A:...1_:[..G]G-----C[..C]:..72_:A<....  G-C
....>A:...4_:[..G]G-*---U[..U]:..69_:A<....  G-U
....>A:...9_:[..A]A-**+-A[..A]:..23_:A<....  A+A
....>A:..15_:[..G]G-**+-C[..C]:..48_:A<....  G+C
....>A:..26_:[M2G]g-**--A[..A]:..44_:A<....  g-A

Specification of a nucleotide

The nt specification string consists of 6 fields and follows the pattern below, with the number of characters in each field inside the parentheses:

modelNum(4)>chainId(1):ntNum(4)insCode(1):[ntName(3)]baseName(1)

1. modelNum(4) — the model number is up to 4 digits, right-justified, with each leading space replaced by a dot. If no model number is available, as is the case for 1ehz (and virtually all other x-ray crystal structures in the PDB), it is written as .... (4 dots).
2. chainId(1) — the chain id is 1-char long, with space replaced by underscore.
3. ntNum(4) — the nt residue number, handled as for the model number.
4. insCode(1) — insertion code, handled as for the chain id.
5. ntName(3) — the nt residue name is up to 3-char long, right-justified, with each leading space replaced by a dot.
6. baseName(1) — the base name is 1-char long, mapped from ntName(3) following $X3DNA/config/baselist.dat. Note that modified nucleotides are put in lower case to distinguish them from the canonical ones — for example, M2G to g.

For the complementary base in a bp, the order of the 6 fields is reversed — see examples above. To see the full list of nts in a PDB data file, run: find_pair -s 1ehz.pdb stdout (here using 1ehz as an example).

Specification of a base pair

The pattern of a bp is M-xyz-N, where M and N are 1-char base names (as in aforesaid field #6), and the three characters xyz have the following meaning:

• z — the sign of the dot product of the z-axes of the M and N base reference frames. It is positive (+) if the two z-axes point in similar directions, as in Hoogsteen or reverse Watson-Crick bps. Conversely, it is negative (-) when the two z-axes point in opposite directions, as in the canonical Watson-Crick and Wobble bps. See figure below:

• y — it is - if M and N are in a so-called Watson-Crick geometry (the two y-axes of the M and N base reference frames are anti-parallel, so are the two z-axes, whilst the two x-axes are parallel), e.g., the G-U Wobble pair; otherwise, *.
• x — it is - for Watson-Crick bps, otherwise, *.

By design, Watson-Crick bps would be of the pattern M-----N, Wobble bps M-*---N, and non-canonical bps M-**+-N or M-**--N. Thus by browsing through the 3DNA output, users can readily identify these three bp types.

The shortened form is represented as MzN; following aforementioned notation, it can be either M-N or M+N. The relative direction of the two z-axes is critical in effecting 3DNA-calculated bp (and step) parameters, as detailed in the 2003 3DNA NAR paper:

To calculate the six complementary base pair parameters of an M–N pair (Shear, Stretch, Stagger, Buckle, Propeller and Opening), where the two z‐axes run in opposite directions, the reference frame of the complementary base N is rotated about the x2‐axis by 180°, i.e. reversing the y2‐ and z2‐axes in Figure 2a. Under this convention, if the base pair is reckoned as an N–M pair, rather than an M–N pair, the x‐axis parameters (Shear and Buckle) reverse their signs. For an M+N pair, e.g. the Hoogsteen A+U in Figure 2b, the x2‐, y2‐ and z2‐axes do not change sign; thus all six parameters for an N+M pair are of opposite sign(s) from those for an M+N pair.

The M-N and M+N bp designation is unique to 3DNA. In combination with the corresponding 6 bp parameters (shear, stretch, stagger, buckle, propeller, and opening), 3DNA provides a rigorous description of all possible bps. This contrasts and complements with the conventional Saenger scheme and the 3-edge based Leontis/Westhof notation.

The 3DNA M-N vs M+N bp designation is base-centric, without concerning the sugar-phosphate backbone. The chi (χ) torsion angle, which characterizes base/sugar relative orientation, can be in either anti or syn conformation; thus similar backbone(S) can accommodate either M-N or M+N.