The Twilight Zone as a Representational Artifact: A Technical Perspective on Physicochemical Translation in Protein Sequence Analysis

Standard alignment tools lose statistical reliability below approximately 20% pairwise sequence identity. This threshold, the "twilight zone" (Rost, 1999), is widely treated as a fundamental biological limit. This technical perspective argues it is not. It is an artifact of sequence representation.

The representational root of the problem lies in the 20-letter amino acid alphabet, which encodes evolutionary history rather than physicochemical logic. Conservative substitutions, such as Leu to Ile or Arg to Lys, register as mismatches under any residue-level scoring scheme, including BLOSUM62. Modern tools such as DIAMOND and DIAFold improve sensitivity by incorporating physicochemical properties as scoring weights or prefilters, but they remain anchored in the 20-residue sequence space. When raw residue-level identity collapses into the midnight zone below 10%, no seed is generated and no signal is detected. A scoring matrix cannot amplify what it cannot find. There is nothing to weight, optimize, or accelerate.

The Zappo String Alignment (ZSA) framework, operationalized through the Black-Jack algorithm, addresses this at the architectural level. The 20-letter sequence is translated into a seven-class physicochemical string prior to any alignment operation. Smith-Waterman alignment then operates directly on this translated representation. Conservative substitutions become structural identities by definition. Physicochemical motifs that are completely invisible at the residue level become exact matches in Zappo space.

Applied to SARS-CoV-2 ORF10, an orphan protein with no confirmed homologs and raw pairwise identities of 7.9 to 10.5% against four taxonomically distant partners, the framework recovers a mean Smith-Waterman signal gain of +29.9 percentage points over raw amino acid identity. Through mapping and back-mapping between residue and physicochemical spaces, two continuous high-coverage zones are identified at positions 3 to 8 and 27 to 35, corresponding precisely to the predicted domain boundaries of a four-domain architecture, including candidate functional and therapeutic target sites that remain invisible to all residue-based methods.

The twilight and midnight zones are not biological ceilings. They are the depth at which the 20-letter alphabet runs out of signal. Physicochemical logic persists below that ceiling. This framework provides access to it.