Ultrafast CEST line scanning as a method to quantify mutarotation kinetics

The process of mutarotation of sugars caused by a balanced reaction between their corresponding α and β isomers, has been known for almost 200 years. Still, it remains essential in modern biochemical research, as enzymatic reactions catalyzed by mutarotases are crucial for various pathways in the energy metabolism. In our study a fast magnetic resonance technique based on chemical exchange saturation transfer (CEST) line scanning (LS) was implemented as a method to measure mutarotation kinetics on a 9.4 T small animal MRI scanner. As proof of concept, the isomeric conversion of two hexoses (glucose and galactose) and pentoses (xylose and arabinose) was investigated in an aqueous solution over time. The technique allowed for ultrafast data acquisition without the implementation of complicated encoding schemes and acceleration procedures. Thus, CEST LS provided complete CEST spectra with a frequency step size of 19.6 Hz in less than one minute. For the mutarotation analysis, CEST spectra were acquired over a time duration of four hours and analyzed with four established CEST quantification approaches - based on either asymmetry of CEST spectra or a multi-pool Lorentzian fit. The isomer ratios of the different sugars at equilibrium were determined with an overall accuracy of 94 %, using an adapted 2-side chemical exchange (CE) model. The estimated mutarotation rate constants at 22 °C were in good agreement with conventionally measured reference values, derived from optical and spectroscopic techniques.