# 7. Limitations, alternatives, and future directions¶

BpForms has several known limitations. Here we describe these limitations and, where possible suggest alternative solutions to these limitations.

• BpForms has limited ability to represent non-canonical backbones. However, non-canonical backbones can be encoded within monomeric forms.

1. Create another alphabet.
2. Encode the canonical backbone within the monomeric forms of the alphabet (e.g., build an alphabet of nucleotides rather than an alphabet of nucleosides or nucleobases).
3. Add additional monomeric form to the alphabet that encode non-canonical backbones.
• BpForms has limited ability to represent non-canonical backbone-backbone or backbone-monomer bonds. Users can represent non-canonical bonds by encoding non-canonical bonds within instances of bpforms.Monomer that represent multiple monomeric forms. However, the BpForms notation cannot directly describe non-canonical bonds. The BpForms notation could be extended to clearly and compactly describe non-canonical bonds by adding an optional syntax for describing the atoms that bond and the atoms that are displaced by bonds. This syntax could use angle brackets to delimit bonds, a vertical pipe to delimit bonded atoms from displaced atoms, and the IUPAC numbering system to indicate the atoms that bonds and the atoms that are displaced (e.g., A<O4, P6 | H4, O6>C).

• bpforms.BpForm.circular represents the circularity of biopolymers. This attribute must be set programmatically. The BpForms notation cannot not represent the circularity biopolymers.

• BpForms cannot represent the secondary structure (e.g., base pairing) of biopolymers. Extending BpForms to represent secondary structure would enable BpForms to represent single strand breaks, crosslinks, and other deviations from the canonical secondary structures of biopolymers.

• BpForms cannot represent branched biopolymers such as lipids.

Most importantly, BpForms should be extended to represent secondary structure. This would enable BpForms to describe DNA forms involved in DNA damage and repair.