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The presence of low energy or red chlorophylls (chls) is nearly universal in this core antenna, and these red chls are known to extend the light harvesting capacity and control the rate of energy migration in PSI. Surprisingly, the significance of the structural position of the red chls in PSI remains unknown. In this study, we introduced mutations that generate specific red sites in the PSI core antenna, consistent with those observed in natural species. Using CryoEM, we characterized the structural changes at these red sites. Using transient absorption spectroscopy, we measured the effects of red chlorophyll at specific sites on energy transfer and trapping in PSI to identify their functional significance. While one site funneled energy to the reaction center, the other slowed equilibration in the antenna. These opposing effects indicate different functions are associated with the different red chlorophyll location, reconciling previous seemingly incompatible suggestions as to their functional role.<\/jats:p>","DOI":"10.64898\/2026.10.01.756128","type":"posted-content","created":{"date-parts":[[2026,10,5]],"date-time":"2026-10-05T03:00:13Z","timestamp":1791169213000},"source":"Crossref","is-referenced-by-count":0,"title":["Red states modulate energy localization between the antenna and trap in PSI"],"prefix":"10.64898","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-3181-0236","authenticated-orcid":false,"given":"Marvin","family":"Asido","sequence":"first","affiliation":[{"name":"Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0008-0369-7460","authenticated-orcid":false,"given":"Halima","family":"Khatun","sequence":"additional","affiliation":[{"name":"School of Molecular Sciences, Arizona State University, Tempe, AZ, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2018-9261","authenticated-orcid":false,"given":"Anton","family":"Khmelnitskiy","sequence":"additional","affiliation":[{"name":"Biodesign Institute, Arizona State University, Tempe, AZ 85801, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3866-0077","authenticated-orcid":false,"given":"Mike","family":"Reppert","sequence":"additional","affiliation":[{"name":"Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-7746-2981","authenticated-orcid":false,"given":"Gabriela S","family":"Schlau-Cohen","sequence":"additional","affiliation":[{"name":"Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5072-0928","authenticated-orcid":false,"given":"Yuval","family":"Mazor","sequence":"additional","affiliation":[{"name":"School of Molecular Sciences, Arizona State University, Tempe, AZ, USA"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"54368","reference":[{"key":"2026100806550834000_2026.10.01.756128v1.1","doi-asserted-by":"publisher","DOI":"10.1529\/biophysj.105.079483"},{"key":"2026100806550834000_2026.10.01.756128v1.2","doi-asserted-by":"publisher","DOI":"10.1063\/5.0198443\/19889971\/154111_1_5.0198443.AM.PDF"},{"key":"2026100806550834000_2026.10.01.756128v1.3","doi-asserted-by":"publisher","DOI":"10.1104\/pp.24.1.1"},{"key":"2026100806550834000_2026.10.01.756128v1.4","doi-asserted-by":"publisher","DOI":"10.1016\/S0005-2728(05)80317-2"},{"key":"2026100806550834000_2026.10.01.756128v1.5","doi-asserted-by":"publisher","DOI":"10.1038\/nature02200"},{"key":"2026100806550834000_2026.10.01.756128v1.6","unstructured":"Blankenship R. 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