Effect of freezing conditions on distances and their distributions derived from Double Electron Electron Resonance (DEER): A study of doubly-spin-labeled T4 lysozyme

[Display omitted] ► DEER with very rapid freezing yielded broader distributions than regular freezing. ► Neither method of freezing altered the distance maxima. ► Rapid freezing yielded more uniform distribution of spin-labels and enhanced DEER S/N. ► Rapid frozen samples require less cryoprotectant...

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Published in:	Journal of magnetic resonance (1997) Vol. 216; pp. 69 - 77
Main Authors:	Georgieva, Elka R., Roy, Aritro S., Grigoryants, Vladimir M., Borbat, Petr P., Earle, Keith A., Scholes, Charles P., Freed, Jack H.
Format:	Journal Article
Language:	English
Published:	United States Elsevier Inc 01-03-2012
Subjects:	Ampicillin Resistance - genetics Bacteriophage T4 - enzymology Bacteriophage T4 - genetics Bi-label distance Bi-label distance distribution Double Electron Electron Resonance (DEER) Electron Spin Resonance Spectroscopy Freezing Freezing perturbation Glycerol - chemistry Models, Molecular Muramidase - chemistry Muramidase - genetics Mutation Plasmids - genetics Protein Conformation Pulse dipolar spectroscopy (PDS) Rapid freeze quench Spin Labels T4 lysozyme Double Electron Electron Resonance (DEER) Pulse dipolar spectroscopy (PDS) Rapid freeze quench T4 lysozyme Bi-label distance distribution Bi-label distance Freezing perturbation
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Summary:	[Display omitted] ► DEER with very rapid freezing yielded broader distributions than regular freezing. ► Neither method of freezing altered the distance maxima. ► Rapid freezing yielded more uniform distribution of spin-labels and enhanced DEER S/N. ► Rapid frozen samples require less cryoprotectant than regularly frozen samples. ► Bromo-substituted spin-label produced narrower distance distributions.. Pulsed dipolar ESR spectroscopy, DEER and DQC, require frozen samples. An important issue in the biological application of this technique is how the freezing rate and concentration of cryoprotectant could possibly affect the conformation of biomacromolecule and/or spin-label. We studied in detail the effect of these experimental variables on the distance distributions obtained by DEER from a series of doubly spin-labeled T4 lysozyme mutants. We found that the rate of sample freezing affects mainly the ensemble of spin-label rotamers, but the distance maxima remain essentially unchanged. This suggests that proteins frozen in a regular manner in liquid nitrogen faithfully maintain the distance-dependent structural properties in solution. We compared the results from rapidly freeze-quenched (⩽100μs) samples to those from commonly shock-frozen (slow freeze, 1s or longer) samples. For all the mutants studied we obtained inter-spin distance distributions, which were broader for rapidly frozen samples than for slowly frozen ones. We infer that rapid freezing trapped a larger ensemble of spin label rotamers; whereas, on the time-scale of slower freezing the protein and spin-label achieve a population showing fewer low-energy conformers. We used glycerol as a cryoprotectant in concentrations of 10% and 30% by weight. With 10% glycerol and slow freezing, we observed an increased slope of background signals, which in DEER is related to increased local spin concentration, in this case due to insufficient solvent vitrification, and therefore protein aggregation. This effect was considerably suppressed in slowly frozen samples containing 30% glycerol and rapidly frozen samples containing 10% glycerol. The assignment of bimodal distributions to tether rotamers as opposed to protein conformations is aided by comparing results using MTSL and 4-Bromo MTSL spin-labels. The latter usually produce narrower distance distributions.
Bibliography:	ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23
ISSN:	1090-7807 1096-0856
DOI:	10.1016/j.jmr.2012.01.004