Physical and structural studies on the cryocooling of insulin crystals

Reflection profiles were analyzed from microgravity-grown (μg) and earth-grown insulin crystals to measure mosaicity (η) and to reveal mosaic domain structure and composition. The effects of cryocooling on single-domain and multi-domain crystals were compared. The effects of cryocooling on insulin structure were also re-examined. Microgravity crystals were of larger volume, were more homogeneous and were of higher quality than earth crystals. Several μg crystals contained a single mosaic domain which encompassed all or nearly all of the crystal with an η_avg of 0.005°. The earth crystals varied in quality and all contained multiple domains with an η _avg of 0.031°. Cryocooling caused a 43-fold increase in η for μg crystals (η_avg = 0.217°) and an eightfold increase for earth crystals (η_avg = 0.246°). These results indicate that very well ordered crystals are not completely protected from the stresses associated with cryocooling, especially when structural perturbations occur. However, there were differences in the reflection profiles. For multi-mosaic domain crystals, each domain individually broadened and separated from the other domains upon cryocooling. Cryocooling did not cause an increase in the number of domains. A crystal composed of a single domain retained this domain structure and the reflection profiles simply broadened. Therefore, an improved signal-to-noise ratio for each reflection was measured from cryocooled single-domain crystals relative to cryocooled multi-domain crystals. The improved signal from μg crystals, along with the increase in crystal size, facilitated the measurement of the weaker high-resolution reflections. The observed broadening of reflection profiles indicates increased variation in unit-cell parameters, which may be linked to cryocooling-associated structural changes and disorder.