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The intrinsic exchange rates of the hydroxyl and amino protons at pH 7.0\u00b0C and 36\u00b0C were found to be in the range 700 to about 10,000 s<jats:sup>\u22121<\/jats:sup>. In addition, the exchange catalysis by phosphate, carbonate, carboxyl\u2010, and amino\u2010groups was investigated. The presence of these exchange catalysts at physiological concentrations increased the proton exchange rates from hydroxyl and amino groups several fold. The proton exchange rates are sufficiently fast that the total magnetization transfer between biomolecules and free bulk water is not rate limited by the proton exchange rate, but by the intramolecular cross\u2010relaxation rates between the exchangeable and nonexchangeable protons of the biomolecules. Since the cross\u2010relaxation rates between surface hydration water molecules and biomolecules are usually vanishingly small because of too rapid exchange with the free bulk water, it is proposed that the contrast in MR images is a fingerprint of the number of the exchangeable protons from OH and NH groups of the tissue, as far as the contrast depends on the magnetization transfer between biomolecules and water.<\/jats:p>","DOI":"10.1002\/mrm.1910350106","type":"journal-article","created":{"date-parts":[[2007,3,11]],"date-time":"2007-03-11T13:35:03Z","timestamp":1173620103000},"page":"30-42","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":308,"title":["Proton exchange rates from amino acid side chains\u2014 implications for image contrast"],"prefix":"10.1002","volume":"35","author":[{"given":"Edvards","family":"Liepinsh","sequence":"first","affiliation":[]},{"given":"Gottfried","family":"Otting","sequence":"additional","affiliation":[]}],"member":"311","published-online":{"date-parts":[[2005,12,12]]},"reference":[{"key":"e_1_2_1_2_2","doi-asserted-by":"publisher","DOI":"10.1002\/mrm.1910100113"},{"key":"e_1_2_1_3_2","doi-asserted-by":"publisher","DOI":"10.1002\/mrm.1910090304"},{"key":"e_1_2_1_4_2","doi-asserted-by":"publisher","DOI":"10.1016\/0079-6565(90)80008-6"},{"key":"e_1_2_1_5_2","doi-asserted-by":"publisher","DOI":"10.1002\/mrm.1910290114"},{"key":"e_1_2_1_6_2","doi-asserted-by":"publisher","DOI":"10.1002\/mrm.1910300606"},{"key":"e_1_2_1_7_2","doi-asserted-by":"publisher","DOI":"10.1080\/00268979200101491"},{"key":"e_1_2_1_8_2","doi-asserted-by":"publisher","DOI":"10.1080\/00268979200101501"},{"key":"e_1_2_1_9_2","doi-asserted-by":"publisher","DOI":"10.1002\/mrm.1910290311"},{"key":"e_1_2_1_10_2","doi-asserted-by":"publisher","DOI":"10.1002\/mrm.1910110304"},{"key":"e_1_2_1_11_2","doi-asserted-by":"publisher","DOI":"10.1002\/mrm.1910180122"},{"key":"e_1_2_1_12_2","doi-asserted-by":"publisher","DOI":"10.1016\/0022-2364(90)90361-C"},{"key":"e_1_2_1_13_2","doi-asserted-by":"publisher","DOI":"10.1016\/0167-4838(94)90162-7"},{"key":"e_1_2_1_14_2","doi-asserted-by":"publisher","DOI":"10.1002\/mrm.1910170203"},{"key":"e_1_2_1_15_2","volume-title":"Biochemistry and the Central Nervous System","author":"McLlwain H.","year":"1985"},{"key":"e_1_2_1_16_2","doi-asserted-by":"publisher","DOI":"10.1126\/science.1948083"},{"key":"e_1_2_1_17_2","doi-asserted-by":"publisher","DOI":"10.1007\/BF02192808"},{"key":"e_1_2_1_18_2","doi-asserted-by":"publisher","DOI":"10.1093\/nar\/20.24.6549"},{"key":"e_1_2_1_19_2","doi-asserted-by":"publisher","DOI":"10.1016\/0003-9861(88)90092-6"},{"key":"e_1_2_1_20_2","doi-asserted-by":"publisher","DOI":"10.1006\/jmbi.1994.0055"},{"key":"e_1_2_1_21_2","doi-asserted-by":"publisher","DOI":"10.1006\/jmbi.1994.0056"},{"key":"e_1_2_1_22_2","doi-asserted-by":"publisher","DOI":"10.1021\/j100095a038"},{"key":"e_1_2_1_23_2","doi-asserted-by":"publisher","DOI":"10.1006\/jmbi.1993.1350"},{"key":"e_1_2_1_24_2","doi-asserted-by":"publisher","DOI":"10.1021\/ja00187a050"},{"key":"e_1_2_1_25_2","doi-asserted-by":"publisher","DOI":"10.1002\/ange.19630751202"},{"key":"e_1_2_1_26_2","unstructured":"G.Wagner \u201cKonformation und Dynamik von Protease\u2010In\u2010hibitoren:1H NMR\u2010Studien \u201d Ph.D. thesis No. 5992 ETH\u2010Z\u00fcrich 1977."},{"key":"e_1_2_1_27_2","doi-asserted-by":"publisher","DOI":"10.1051\/epn\/19861701011"},{"key":"e_1_2_1_28_2","first-page":"4764","article-title":"Rate and mechanism of proton exchange in aqueous solutions of phosphate buffer","volume":"86","author":"Luz Z.","year":"1964","journal-title":"J. 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