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We find a compaction of the unfolded state of a small cold shock protein with increasing temperature in both the presence and the absence of denaturant, with good agreement between the results from single-molecule FRET and dynamic light scattering. Although dissociation of denaturant from the polypeptide chain with increasing temperature accounts for part of the compaction, the results indicate an important role for additional temperature-dependent interactions within the unfolded chain. The observation of a collapse of a similar extent in the extremely hydrophilic, intrinsically disordered protein prothymosin \u03b1 suggests that the hydrophobic effect is not the sole source of the underlying interactions. Circular dichroism spectroscopy and replica exchange molecular dynamics simulations in explicit water show changes in secondary structure content with increasing temperature and suggest a contribution of intramolecular hydrogen bonding to unfolded state collapse.<\/jats:p>","DOI":"10.1073\/pnas.0900622106","type":"journal-article","created":{"date-parts":[[2009,11,20]],"date-time":"2009-11-20T21:37:14Z","timestamp":1258753034000},"page":"20740-20745","update-policy":"https:\/\/doi.org\/10.1073\/pnas.cm10313","source":"Crossref","is-referenced-by-count":228,"title":["Single-molecule spectroscopy of the temperature-induced collapse of unfolded proteins"],"prefix":"10.1073","volume":"106","author":[{"given":"Daniel","family":"Nettels","sequence":"first","affiliation":[{"name":"Biochemisches Institut, Universit\u00e4t Z\u00fcrich, Winterthurerstrasse 190, 8057 Z\u00fcrich, 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