Intrafilament chemical densities in DNA: an operational definition based on FASTA sequences

Starting from a single-stranded DNA FASTA sequence, intrafilament chemical densities are introduced based on the direct counting of structural functional groups (NH₂, CH₃, C=O) associated with the nucleobases. The densities are defined as scalar quantities normalized by sequence length and represent the average load of a given chemical group per base along the analyzed ssDNA. The approach does not require construction of the complementary strand and does not refer to the double-helix structure (dsDNA). Although numerous studies have investigated the elemental content of genomes within the framework of stoichiogenomics, a unified operational definition of base-normalized intrafilament chemical densities directly computable from a FASTA sequence does not appear to be formally established. The proposed formulation provides a chemically explicit and formally defined compositional descriptor.