Comparison of slam-freezing and high-pressure freezing effects on the DNA cholesteric liquid crystalline structure.
Using in parallel electron microscopy of ultrathin frozenhydrated sections and freeze-fracture replicas, we compare the ultrastructural consequences of two freezing techniques: slam-freezing at liquid helium temperature and high-pressure freezing, on a model system, the DNA cholesteric liquid crystalline phase. Both freezing techniques are able to vitrify DNA liquid crystalline solutions containing up to 85% water, but induce structural rearrangements of the molecular organization. The cholesteric structure is preserved by the slam-freezing method despite the formation of periodic distortions induced by the mechanical compressive stress. In contrast, high-pressure freezing does not preserve the structure of the liquid crystal: the long-range cholesteric stratification disappears, and the local continuous twist between molecules is modified. These results show that vitrification, though necessary, may not be a sufficient token of preservation of the native state of hydrated materials. We discuss the possible origins of the molecular rearrangements that have time to occur in the specimens as a result of the low freezing rate permitted by the high-pressure freezing process.
Leforestier
A
A
Laboratoire de Physique des Solides, Université Paris-Sud, Orsay, France.
Richter
K
K
Livolant
F
F
Dubochet
J
J
D003460:Crystallization; D004247:DNA / Q000737:chemistry*; D005614:Freeze Fracturing; D005615:Freezing; D011312:Pressure
D004247:DNA