{
  "abstract": "Aromatic side chains carry the optical and redox chemistry of peptides, and lengthening one is widely expected to extend its conjugation and narrow its HOMO-LUMO gap. We test that expectation with plane-wave density functional theory on 22 isolated neutral peptides NH2-(X)n-COOH (X = Phe, Tyr, Trp, Cys; n = 1 to 5, and n = 10 for Cys and Trp). The gap does narrow, at the same rate for all three aromatic residues, i.e., -0.17 to -0.18 eV per residue, even though benzene, phenol and indole are very different chromophores. Residue-resolved projections reveal why: nothing delocalises. Every aromatic frontier orbital is 97 to 100 % confined to a single residue and the HOMO participation number saturates at one ring. Each residue instead keeps its own local gap: for Trp it is invariant to within 0.03 eV over n = 2 to 10, while over that same span the global gap falls by 1.19 eV. The global gap is therefore an extreme-value functional of the residue site energies, gap(n) =-{Delta}(n), whose alignment penalty accounts for the entire effect: the polar backbone generates a monotonic internal electrostatic ramp that leaves the two gap edges at opposite ends of the chain, 32 [A] apart. We set out the resulting design rules for peptide-based bio-organic semiconductors.",
  "abstract_shown": true,
  "affiliations": [
    "ShenZhen UnibindMD Computation Biotech Co., Ltd."
  ],
  "article_type": "New results",
  "arxiv": null,
  "author_total": 2,
  "authors": [
    {
      "name": "X. Wu"
    },
    {
      "affs": [
        0
      ],
      "name": "W. Fu"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/bngptycqv73ct.bib",
  "cited_by": 0,
  "doi": "10.64898/2026.10.05.756722",
  "fulltext_url": "https://www.biorxiv.org/content/10.64898/2026.10.05.756722v1.full",
  "id": "bngptycqv73ct",
  "issns": [],
  "landing_url": "https://www.biorxiv.org/content/10.64898/2026.10.05.756722v1",
  "language": null,
  "licence": "CC-BY",
  "licence_label": "CC BY",
  "licence_source": "article",
  "licence_url": "https://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": null,
  "native_subjects": [
    [
      "biorxiv",
      "biochemistry"
    ],
    [
      "openalex",
      "2208|Organic Electronics and Photovoltaics"
    ]
  ],
  "openalex": "W7220662280",
  "pdf_url": "https://www.biorxiv.org/content/10.64898/2026.10.05.756722v1.full.pdf",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-05",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/bngptycqv73ct.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:26:16Z",
      "native_id": "10.64898/2026.10.05.756722",
      "source": "biorxiv",
      "url": "https://www.biorxiv.org/content/10.64898/2026.10.05.756722v1"
    }
  ],
  "subjects": [
    {
      "field": "biochemistry",
      "origin": "biorxiv:biochemistry",
      "subfield": "general"
    }
  ],
  "title": "Peptide frontier-orbital gaps narrow by backbone electrostatics",
  "title_text": "Peptide frontier-orbital gaps narrow by backbone electrostatics",
  "type": "preprint",
  "updated_at": "2026-10-30T12:26:16Z",
  "url": "https://research.draconitic.com/paper/bngptycqv73ct",
  "venue": "bioRxiv",
  "versions": []
}