{"record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"19adef10ba27da593dd6ed81e853957fe22c8ffa01ad736221436603bd6291d9","created":"2026-10-15T23:43:22Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["4a9084622d681c20bd8298a4e37cbf2b8d4e1775d949304dc4c12b2a99c82a0d","ce59fb6cd812fc05906d0c60a71c75ea22bc686bcf38e7c3ef7010834d98ee7b"],"salt":"1452f0a137d0bef723994d67b8e95cefd3e4f56011f1460e3cfde62d48de8950","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"mechanism","assisted_by":[],"external_id":"bio.protein-folding.speed-limit","kind":"well-posed","literature_status":"partially-resolved","n":"2","parents":[],"plain":"The fastest proteins fold in about a microsecond, and theory and experiment suggest a floor of roughly N/100 microseconds for a chain of N amino acids. The question is which physical process sets this floor: the chain moving through water, or friction inside the chain itself.","posed_since":"2004","precise":"For ultrafast folders (villin headpiece, WW domains; folding times $\\sim 1 \\text{ to } 10\\ \\mathrm{microseconds}$), determine the prefactor $k_0$ in the Kramers rate $k = k_0 \\exp(-dG/k_B T)$, with the empirical estimate $\\tau_{\\min} \\sim N/100\\ \\mathrm{microseconds}$ for $N$ residues (Kubelka, Hofrichter and Eaton, 2004), and separate solvent from internal friction through the viscosity dependence $\\tau = a\\eta + b$ at fixed stability ($\\eta = \\text{solvent viscosity}$). An answer gives $k_0$ and the internal-friction term $b$ with their molecular origin (backbone dihedral barriers, side-chain contacts) and reproduces measured $\\eta$-dependences.","problem_ref":"4a9084622d681c20bd8298a4e37cbf2b8d4e1775d949304dc4c12b2a99c82a0d","references":"","settled_by":"Viscosity-dependent single-molecule transition-path and folding-time measurements on ultrafast folders matched by all-atom or coarse-grained simulations that identify the friction source.","status_note":"Single-molecule measurements since 2012 bound transition-path times at microseconds; the molecular origin of internal friction is still disputed as of 2026, to this survey's knowledge.","title":"What sets the folding speed limit and internal friction?","topic_ref":null},"attested":{"attestation":{"batch":null,"client_id":null,"id_token_sha256":null,"kind":"platform"},"record_hash":"48c70d17fce18072845a759fb2e617a81c67220ea1200c54ec25d4721d0baa3e","schema":"pubphys.attested/1"},"envelope":{"attested_hash":"a1a5ed651d8e4ad374367cedce3362610f8a2ea6691755bbd1266ae88384d6a6","platform_signature":{"key_id":"c6afc19b31429869751f06879c75cd64ea92654423d15b44be775bf1310a60da","sig":"myyrRxTZq21O6jMMBJ_BhuKVlTfJVEclmPcxgrFMa4Ylos5qfWmYR0XW49_RXHJAMHE5hwyjwuRg7NjVkyTrBA"},"schema":"pubphys.envelope/1"},"record_hash":"48c70d17fce18072845a759fb2e617a81c67220ea1200c54ec25d4721d0baa3e","leaf_index":2624}