Abstract
Glulisine is a US Food and Drug Administration (FDA) approved insulin analogue, used for controlling hyperglycaemia in patients with diabetes mellitus (DM). It is fast acting which better approximates physiological insulin secretion, improving patient outcome. Crystallisation of Glulisine was analysed by its crystallisation phase diagram and nucleation-inducing materials. Both the hanging drop vapour diffusion and microbatch-under-oil methods were used and compared. We have shown that the same protein can have different solubility behaviours depending on the nature of the salt in the precipitating agent. In the case of Glulisine with magnesium formate, lowering the precipitant concentration drove the system further into supersaturation resulting in the formation of crystals and precipitation. This was the opposite effect to the usual scenario where raising the precipitant concentration leads to supersaturation. Glulisine with sodium potassium tartrate tetrahydrate (NaKT) followed the expected trend of forming crystals or precipitate at higher concentrations and clear drops at lower concentrations of the precipitant. The outcomes of crystallisation using the different crystallisation methods is also described. Glulisine was successfully crystallised and the crystals diffracted up to a resolution limit of 1.4 Å.
Highlights
High-resolution X-ray diffraction patterns require high-quality crystals and obtaining such crystals is often the bottleneck of the structural determination process [1]
At 1.75 mg/mL Glulisine crystal formation was observed within 16 hours at 0.2 M magnesium formate concentrations in both vapour diffusion and microbatch
At 0.3 M and 0.4 M magnesium formate crystals formed within seven days in microbatch and over two weeks in vapour diffusion
Summary
High-resolution X-ray diffraction patterns require high-quality crystals and obtaining such crystals is often the bottleneck of the structural determination process [1]. Advances in automation and high throughput screening can increase the number of hits obtained but it does not address the core problem of reliance on trial and error [2]. For a more systematic approach to crystallisation, we must consider the principles underlining crystal formation. A variety of parameters such as protein concentration, crystallising agent or precipitant and additives can be adjusted to encourage the formation of crystals. A phase diagram can be experimentally generated and quantify the effect of such modifications. The phase diagram is a 2D representation of the distinct phases that occur and coexist at equilibrium [3]
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