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Characterization of creatine guanidinium proton exchange by water‐exchange (WEX) spectroscopy for absolute‐pH CEST imaging in vitro
Author(s) -
Goerke Steffen,
Zaiss Moritz,
Bachert Peter
Publication year - 2014
Publication title -
nmr in biomedicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.278
H-Index - 114
eISSN - 1099-1492
pISSN - 0952-3480
DOI - 10.1002/nbm.3086
Subject(s) - chemistry , arrhenius equation , reaction rate constant , proton , analytical chemistry (journal) , creatine , nuclear magnetic resonance , aqueous solution , activation energy , nuclear magnetic resonance spectroscopy , atmospheric temperature range , kinetics , chromatography , thermodynamics , stereochemistry , biochemistry , physics , quantum mechanics
Chemical exchange saturation transfer (CEST) enables indirect detection of small metabolites in tissue by MR imaging. To optimize and interpret creatine‐CEST imaging we characterized the dependence of the exchange‐rate constant k sw of creatine guanidinium protons in aqueous creatine solutions as a function of pH and temperature T in vitro . Model solutions in the low pH range (pH = 5–6.4) were measured by means of water‐exchange (WEX)‐filtered 1 H NMR spectroscopy on a 3 T whole‐body MR tomograph. An extension of the Arrhenius equation with effective base‐catalyzed Arrhenius parameters yielded a general expression for k sw (pH, T ). The defining parameters were identified as the effective base‐catalyzed rate constant k b,eff (298.15 K) = (3.009 ± 0.16) × 10 9 Hz l/mol and the effective activation energy E A,b,eff = (32.27 ± 7.43) kJ/mol at a buffer concentration of c buffer = (1/15) M. As expected, a strong dependence of k sw on temperature was observed. The extrapolation of the exchange‐rate constant to in vivo conditions (pH = 7.1, T = 37 °C) led to the value of the exchange‐rate constant k sw = 1499 Hz. With the explicit function k sw (pH, T ) available, absolute‐pH CEST imaging could be realized and experimentally verified in vitro . By means of our calibration method it is possible to adjust the guanidinium proton exchange‐rate constant k sw to any desired value by preparing creatine model solutions with a specific pH and temperature. Copyright © 2014 John Wiley & Sons, Ltd.