Cannot understand what a base pair type means from web-3DNA output

[sowmya]

Dear Dr. Xiang,

I was analyzing the base pair type distribution among a dataset of RNAs that I have collected for my study using the pair.csv file for those PDB IDs obtained using web-3DNA. When I compiled the list of unique base pair types I noticed certain base pairs with a P, which are: P+A, A-P, P-G, P+G, C-P and P-A.
I got information on the other base pair types from the x3DNA website: http://x3dna.org/highlights/named-base-pairs, but I'm unable to find information on the above mentioned base pairs. Can you share some resources to understand what these named base pairs mean and how they are identified (in terms of the structural orientation of the bases and other parameters)?

Thank you
- Sowmya

[xiangjun]

See the paper "DSSR: an integrated software tool for dissecting the spatial structure of RNA" (https://doi.org/10.1093/nar/gkv716).

Identification of nucleotides

        Note that pseudouridine (PSU) is shortened to ‘P’, due to its special C1′–C5 glycosidic link- age (Figure 2).

The M–N versus M+N relative base orientations