Continuous Protein Crystallization in Mixed-Suspension Mixed-Product-Removal Crystallizers

The
metastable needlelike form of lysozyme can be obtained from
continuous crystallization in mixed-suspension mixed-product-removal
(MSMPR) crystallizers for conditions at which the stable tetragonal
form is obtained from batch crystallization. A 3D-printed airlift
crystallizer and stirred tank crystallizer are used, which both approximate
an MSMPR crystallizer well. The influence of residence time, precipitant
concentration, and agitation mechanism on the attainable solid-state
form is presented. The needlelike crystals appear after nucleation
of the tetragonal form. An earlier transition from the tetragonal form to the needlelike
form is observed in an airlift crystallizer compared to a conventional
stirred crystallizer, which is possibly caused by reduced attrition
and the presence of a gas–liquid interface in the airlift crystallizer.
Furthermore, a long residence time and high precipitant concentration
favor the formation of the needlelike crystals. This work demonstrates
how the mode of operation of well-mixed crystallizers affects the
attainable solid-state form of a model protein. The results generally
show the importance of obtaining a quantitative understanding of the
dynamic interplay between stochastic primary nucleation processes
and deterministic growth and secondary nucleation processes so that
future continuous MSMPR crystallizers can be designed and operated
robustly to produce protein crystals of a desired solid-state form.