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The traditionally rare enol\u2013imino tautomer for nucleobases was found to be the major species for KP1212, providing a structural support for the tautomer hypothesis. We further found that KP1212 is significantly protonated at physiological pH with a pK\n                    <jats:sub>a<\/jats:sub>\n                    of 7. The protonated KP1212 was shown to be mutagenic, revealing a bimodal mutagenic property of KP1212. The results could prove instrumental in developing the next-generation antiviral treatments.\n                  <\/jats:p>","DOI":"10.1073\/pnas.1415974112","type":"journal-article","created":{"date-parts":[[2015,3,2]],"date-time":"2015-03-02T22:13:24Z","timestamp":1425334404000},"page":"3229-3234","update-policy":"https:\/\/doi.org\/10.1073\/pnas.cm10313","source":"Crossref","is-referenced-by-count":17,"title":["Two-dimensional IR spectroscopy of the anti-HIV agent KP1212 reveals protonated and neutral tautomers that influence pH-dependent mutagenicity"],"prefix":"10.1073","volume":"112","author":[{"given":"Chunte Sam","family":"Peng","sequence":"first","affiliation":[{"name":"Departments of aChemistry and"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Bogdan I.","family":"Fedeles","sequence":"additional","affiliation":[{"name":"Departments of aChemistry and"},{"name":"Biological Engineering and"},{"name":"Center for Environmental Health Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139; and"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Vipender","family":"Singh","sequence":"additional","affiliation":[{"name":"Departments of aChemistry and"},{"name":"Biological Engineering and"},{"name":"Center for Environmental Health Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139; and"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Deyu","family":"Li","sequence":"additional","affiliation":[{"name":"Departments of aChemistry and"},{"name":"Biological Engineering and"},{"name":"Center for Environmental Health Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139; and"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Tiffany","family":"Amariuta","sequence":"additional","affiliation":[{"name":"Biological Engineering and"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"John M.","family":"Essigmann","sequence":"additional","affiliation":[{"name":"Departments of aChemistry and"},{"name":"Biological Engineering and"},{"name":"Center for Environmental Health Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139; and"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Andrei","family":"Tokmakoff","sequence":"additional","affiliation":[{"name":"Department of Chemistry, Institute for Biophysical Dynamics, and James Franck Institute, The University of Chicago, Chicago, IL 60637"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"341","published-online":{"date-parts":[[2015,3,2]]},"reference":[{"key":"e_1_3_3_1_2","doi-asserted-by":"publisher","DOI":"10.1073\/pnas.212514799"},{"key":"e_1_3_3_2_2","doi-asserted-by":"publisher","DOI":"10.1016\/j.coviro.2011.09.001"},{"key":"e_1_3_3_3_2","doi-asserted-by":"publisher","DOI":"10.1021\/jm400653j"},{"key":"e_1_3_3_4_2","doi-asserted-by":"publisher","DOI":"10.1073\/pnas.96.4.1492"},{"key":"e_1_3_3_5_2","doi-asserted-by":"publisher","DOI":"10.1073\/pnas.91.14.6609"},{"key":"e_1_3_3_6_2","doi-asserted-by":"publisher","DOI":"10.1073\/pnas.95.7.3578"},{"key":"e_1_3_3_7_2","doi-asserted-by":"publisher","DOI":"10.2165\/00063030-200317030-00003"},{"key":"e_1_3_3_8_2","doi-asserted-by":"publisher","DOI":"10.1016\/j.antiviral.2005.03.004"},{"key":"e_1_3_3_9_2","doi-asserted-by":"publisher","DOI":"10.1016\/j.antiviral.2004.12.004"},{"key":"e_1_3_3_10_2","doi-asserted-by":"publisher","DOI":"10.1371\/journal.pone.0015135"},{"key":"e_1_3_3_11_2","doi-asserted-by":"publisher","DOI":"10.1038\/82191"},{"key":"e_1_3_3_12_2","doi-asserted-by":"publisher","DOI":"10.1128\/JVI.02346-12"},{"key":"e_1_3_3_13_2","doi-asserted-by":"publisher","DOI":"10.1073\/pnas.1405635111"},{"key":"e_1_3_3_14_2","doi-asserted-by":"publisher","DOI":"10.1038\/171964b0"},{"key":"e_1_3_3_15_2","doi-asserted-by":"publisher","DOI":"10.1038\/263285a0"},{"key":"e_1_3_3_16_2","doi-asserted-by":"publisher","DOI":"10.1073\/pnas.1303235110"},{"key":"e_1_3_3_17_2","first-page":"39","article-title":"Study of transient chemical reactions by NMR. 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