Assessing the purity of model glycoconjugate vaccines by low field NMR†

Glycoconjugate vaccines are of growing importance to modern healthcare where they provide an opportunity for high efficacy prophylactic treatment against a growing number of infectious bacterial diseases. Unfortunately, their preparation is highly complex and involves multiple stages of analysis prior to product release. Such analyses must quantify the degree of successful conjugation and the amount of relevant co-expressed and co-purified process impurities (i.e. cell-wall polysaccharide). Whilst nuclear magnetic resonance (NMR) spectroscopy can be used for these assessments, the cost of high field systems is significant and hence there is a need to evaluate the performance of low-cost benchtop apparatus. Here, we set a goal of achieving a satisfactory analysis within 20 min on a series of model glycoconjugates and sought to use hyperpolarization methods based on signal amplification by reversible exchange (SABRE) to enable higher sample throughput. Our analyses demonstrate that a 1 Tesla (T) benchtop NMR can achieve satisfactory dextran-conjugation analysis results without the need for hyperpolarization, although SABRE hyperpolarization offers a route to improvement. The assessment of the common impurity cell-wall polysaccharide proved more challenging, and its hyperpolarization failed due to the necessary solvent system. At high field satisfactory analyses were possible at 10 wt%, 5 wt%, 1 wt% and 0.5 wt% loadings where the resulting signals are distinguishable. However, at 1 T signal overlap precluded simple signal integration and a T₁ filter was implemented. This allowed the overlapping signal contributions to be differentiated and made quantification possible for the 10 wt% sample where signal to noise ratios remained high.