Modelling the acid/base 1H NMR chemical shift limits of metabolites in human urine

Introduction Despite the use of buffering agents the H-1 NMR spectra of biofluid samples in metabolic profiling investigations typically suffer from extensive peak frequency shifting between spectra. These chemical shift changes are mainly due to differences in pH and divalent metal ion concentrations between the samples. This frequency shifting results in a correspondence problem: it can be hard to register the same peak as belonging to the same molecule across multiple samples. The problem is especially acute for urine, which can have a wide range of ionic concentrations between different samples. Objectives To investigate the acid, base and metal ion dependent H-1 NMR chemical shift variations and limits of the main metabolites in a complex biological mixture. Methods Urine samples from five different individuals were collected and pooled, and pre-treated with Chelex-100 ion exchange resin. Urine samples were either treated with either HCl or NaOH, or were supplemented with various concentrations of CaCl2, MgCl2, NaCl or KCl, and their H-1 NMR spectra were acquired. Results Nonlinear fitting was used to derive acid dissociation constants and acid and base chemical shift limits for peaks from 33 identified metabolites. Peak pH titration curves for a further 65 unidentified peaks were also obtained for future reference. Furthermore, the peak variations induced by the main metal ions present in urine, Na+, K+, Ca2+ and Mg2+, were also measured. Conclusion These data will be a valuable resource for H-1 NMR metabolite profiling experiments and for the development of automated metabolite alignment and identification algorithms for H-1 NMR spectra.