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However, the degradation rates of PFAS vary widely and cannot be explained by differences in C\u2013F bond strength alone. Here, we use quantum chemical calculations to identify the ability of the perfluorocarbon backbone to unwind from its intrinsic helical conformation as an overlooked source of variation in PFAS reduction kinetics. Using the hydrated electron as the reference reductant, we combine the Marcus and Save\u0301ant models to show how helical unwinding shifts the kinetic preference between the stepwise and concerted reductive defluorination pathways, thereby rationalizing the structure-dependent degradation rates reported experimentally. Specifically, longer perfluorocarbon backbones tend to delocalize an excess electron through helical unwinding and form the radical intermediate required for the stepwise pathway. By contrast, the carboxylate headgroup constrains this unwinding, thereby favoring the conventional concerted pathway in which reactivity is governed by C\u2013F bond strength. Nanoscale confinement can provide additional van der Waals stabilization, weakening the conformational constraint imposed by the carboxylate headgroup and lowering the activation free energy for the stepwise pathway. Together, these findings point to molecular conformation as a potential contributor to PFAS reduction kinetics alongside C\u2013F bond strength and raise the prospect of accelerating degradation through microenvironment engineering.<\/jats:p>","DOI":"10.1021\/jacs.6c08421","type":"journal-article","created":{"date-parts":[[2026,9,2]],"date-time":"2026-09-02T05:11:22Z","timestamp":1788325882000},"page":"38754-38764","source":"Crossref","is-referenced-by-count":1,"title":["Helical Unwinding Governs\nDivergent Kinetics in the\nReductive Defluorination of Per- and Polyfluoroalkyl Substances"],"prefix":"10.1021","volume":"148","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-6245-3898","authenticated-orcid":true,"given":"Zhihao","family":"Zhang","sequence":"first","affiliation":[{"name":"Central South University , , ,","place":["Changsha, China, 410083"]},{"name":"Central South University , , ,","place":["Changsha, China, 410083"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8781-7747","authenticated-orcid":true,"given":"Kang","family":"Liu","sequence":"additional","affiliation":[{"name":"Central South University , , ,","place":["Changsha, China, 410083"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0000-5650-4468","authenticated-orcid":true,"given":"Chunli","family":"Wang","sequence":"additional","affiliation":[{"name":"The University of Adelaide , , , ,","place":["Adelaide, South Australia, Australia, 5005"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9007-4817","authenticated-orcid":true,"given":"Min","family":"Liu","sequence":"additional","affiliation":[{"name":"Central South University , , ,","place":["Changsha, China, 410083"]},{"name":"Central South University , , ,","place":["Changsha, China, 410083"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-6600-2055","authenticated-orcid":true,"given":"Zhang","family":"Lin","sequence":"additional","affiliation":[{"name":"Central South University , , ,","place":["Changsha, China, 410083"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"316","published-online":{"date-parts":[[2026,8,28]]},"reference":[{"issue":"1","key":"2026091609120844700_cit1","doi-asserted-by":"publisher","DOI":"10.1038\/s41467-024-49753-5","article-title":"Lithium-Ion\nBattery Components Are at the Nexus of Sustainable Energy and Environmental\nRelease of per- and Polyfluoroalkyl Substances","volume":"15","author":"Guelfo","year":"2024","journal-title":"Nat. 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Commun."},{"issue":"3","key":"2026091609120844700_cit5","doi-asserted-by":"publisher","first-page":"509","DOI":"10.1038\/s41557-025-02057-7","article-title":"Lithium\nMetal-Mediated Electrochemical\nReduction of per- and Poly-Fluoroalkyl Substances","volume":"18","author":"Sarkar","year":"2026","journal-title":"Nat. Chem."},{"issue":"7","key":"2026091609120844700_cit6","doi-asserted-by":"publisher","first-page":"3718","DOI":"10.1021\/acs.est.8b06648","article-title":"Defluorination of Per-\nand Polyfluoroalkyl Substances (PFASs) with\nHydrated Electrons: Structural Dependence and Implications to PFAS\nRemediation and Management","volume":"53","author":"Bentel","year":"2019","journal-title":"Environ. Sci. Technol."},{"issue":"17","key":"2026091609120844700_cit7","doi-asserted-by":"publisher","first-page":"10668","DOI":"10.1021\/acs.est.0c02773","article-title":"Structural Dependence of Reductive\nDefluorination of Linear PFAS Compounds in a UV\/Electrochemical System","volume":"54","author":"Rao","year":"2020","journal-title":"Environ. Sci. Technol."},{"issue":"8","key":"2026091609120844700_cit8","doi-asserted-by":"publisher","first-page":"645","DOI":"10.1021\/acs.estlett.1c00403","article-title":"Interpretation of Reductive\nPFAS Defluorination with Quantum Chemical Parameters","volume":"8","author":"Cheng","year":"2021","journal-title":"Environ. Sci. Technol. 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Technol."},{"issue":"2","key":"2026091609120844700_cit16","doi-asserted-by":"publisher","first-page":"456","DOI":"10.1021\/es702192c","article-title":"The pKa\nValues of PFOA and Other Highly Fluorinated Carboxylic Acids","volume":"42","author":"Goss","year":"2008","journal-title":"Environ. Sci. Technol."},{"issue":"49","key":"2026091609120844700_cit17","doi-asserted-by":"publisher","first-page":"26865","DOI":"10.1021\/acs.est.5c11519","article-title":"Temperature\nEffect on Per- and Polyfluoroalkyl\nSubstance Degradation by Ultraviolet\/Sulfite: Insights on Lamp Heat,\nMolecular Transformation, and Photochemical Principles","volume":"59","author":"Gao","year":"2025","journal-title":"Environ. Sci. Technol."},{"issue":"11","key":"2026091609120844700_cit18","doi-asserted-by":"publisher","first-page":"6957","DOI":"10.1021\/acs.est.0c00961","article-title":"Destruction\nof Per- and Polyfluoroalkyl Substances\n(PFASs) in Aqueous Film-Forming Foam (AFFF) with UV-Sulfite Photoreductive\nTreatment","volume":"54","author":"Tenorio","year":"2020","journal-title":"Environ. Sci. Technol."},{"issue":"5","key":"2026091609120844700_cit19","doi-asserted-by":"publisher","first-page":"351","DOI":"10.1021\/acs.estlett.0c00236","article-title":"Enhanced Degradation\nof Perfluorocarboxylic Acids (PFCAs) by UV\/Sulfite Treatment: Reaction\nMechanisms and System Efficiencies at pH 12","volume":"7","author":"Bentel","year":"2020","journal-title":"Environ. Sci. Technol. Lett."},{"issue":"8","key":"2026091609120844700_cit20","doi-asserted-by":"publisher","first-page":"6736","DOI":"10.1021\/acs.est.5c11944","article-title":"In Situ Growth\nof Polyindole within Montmorillonite Interlayer and Subsequent Efficient\nDestruction of Perfluorooctanoic Acid","volume":"60","author":"Tian","year":"2026","journal-title":"Environ.\nSci. Technol."},{"issue":"23","key":"2026091609120844700_cit21","doi-asserted-by":"publisher","first-page":"16907","DOI":"10.1021\/acs.est.5c14654","article-title":"LDH-Enhanced Degradation of PFASs:\nInfluence of Reactant Configurations within Nanoconfined Spaces on\nHydrated Electron Transport","volume":"60","author":"Feng","year":"2026","journal-title":"Environ. Sci. Technol."},{"issue":"26","key":"2026091609120844700_cit22","doi-asserted-by":"publisher","first-page":"11840","DOI":"10.1021\/jacs.2c04341","article-title":"Integrated\nPhotocatalytic\nReduction and Oxidation of Perfluorooctanoic Acid by Metal-Organic\nFrameworks: Key Insights into the Degradation Mechanisms","volume":"144","author":"Wen","year":"2022","journal-title":"J. Am. Chem. Soc."},{"issue":"20","key":"2026091609120844700_cit23","doi-asserted-by":"publisher","first-page":"11728","DOI":"10.1021\/acs.est.8b02172","article-title":"Degradation of PFOA\nSubstitute: GenX (HFPO\u2013DA Ammonium Salt):\nOxidation with UV\/Persulfate or Reduction with UV\/Sulfite","volume":"52","author":"Bao","year":"2018","journal-title":"Environ. Sci. Technol."},{"issue":"2","key":"2026091609120844700_cit24","doi-asserted-by":"publisher","DOI":"10.1002\/wcms.70019","article-title":"Software Update:\nThe ORCA Program System\u2500Version 6.0","volume":"15","author":"Neese","year":"2025","journal-title":"WIREs Comput. Mol. Sci."},{"issue":"1","key":"2026091609120844700_cit25","doi-asserted-by":"publisher","DOI":"10.1186\/1758-2946-3-8","article-title":"Confab\n- Systematic Generation of Diverse Low-Energy Conformers","volume":"3","author":"O\u2019Boyle","year":"2011","journal-title":"J. Cheminf"},{"issue":"6","key":"2026091609120844700_cit26","doi-asserted-by":"publisher","DOI":"10.1063\/1.5012601","article-title":"B97\u20133c: A Revised\nLow-Cost Variant of the B97-D Density Functional Method","volume":"148","author":"Brandenburg","year":"2018","journal-title":"J. Chem. 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Rev."}],"container-title":["Journal of the American\nChemical Society"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/pubs.acs.org\/doi\/pdf\/10.1021\/jacs.6c08421","content-type":"application\/pdf","content-version":"vor","intended-application":"unspecified"},{"URL":"https:\/\/pubs.acs.org\/jacsat\/article-pdf\/148\/36\/38754\/67622226\/jacs.6c08421.pdf","content-type":"application\/pdf","content-version":"vor","intended-application":"syndication"},{"URL":"https:\/\/pubs.acs.org\/jacsat\/article-pdf\/148\/36\/38754\/67622226\/jacs.6c08421.pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,9,16]],"date-time":"2026-09-16T13:15:39Z","timestamp":1789564539000},"score":1,"resource":{"primary":{"URL":"https:\/\/pubs.acs.org\/jacsat\/article\/148\/36\/38754\/5350916\/Helical-Unwinding-Governs-Divergent-Kinetics-in"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2026,8,28]]},"references-count":40,"journal-issue":{"issue":"36","published-online":{"date-parts":[[2026,8,28]]},"published-print":{"date-parts":[[2026,9,16]]}},"URL":"https:\/\/doi.org\/10.1021\/jacs.6c08421","relation":{},"ISSN":["0002-7863","1520-5126"],"issn-type":[{"value":"0002-7863","type":"print"},{"value":"1520-5126","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,8,28]]}}}