XANES measurements of the rate of radiation damage to selenomethionine side chains

Holton, J.M.

doi:10.1107/S0909049506048898

Figure 5
Some experimental parameters protect SeMet from radiation damage. The `protection factor' is the fractional increase in the `half-dose' (D½) of SeMet when the indicated `protective measure' is used. The D½ is the X-ray dose that will damage half of the SeMet in the sample. The `not ice' column shows the protection factor of rapidly cooling a solution of SeMet to form an amorphous glassy solid as opposed to cooling it slowly to form ice Ih (droplet 110 in Table 1

). The `not nano-ice' column shows the protection factor of the same amorphous condition over the same solution cooled slightly slower to form the nano-crystalline solid discussed in the text (droplet 130). `Low pH' is the protection factor of preparing the amorphous sample with 1 M acetic acid added to the solution over adding 0.6 M NaOH (droplets 39 and 52). `Ascorbate' is the protection factor of using 1 M ascorbic acid instead of acetic acid (droplets 20 and 21). `Nitrate' is the protection factor of using 1 M nitric acid instead of acetic acid (droplets 25 and 21). `Low temperature' is the protection factor of running the experiment at 93 K instead of 140 K in 0.6 M NaOH (droplets 86 and 88). All other experiments in this figure were run at 93 K. See Fig. 6

for details. `In peptide' is the protection factor of unfolded GCN4-N16A-p1 peptide at 20 mM (80 g L⁻¹) boiled in 9.5 M urea over free SeMet at 25 mM also boiled in 9.5 M urea with 80 g L⁻¹ insulin. `Folded' is the protection factor of folded GCN4-N16A-p1 peptide in solution (no urea) over the same concentration of unfolded peptide (boiled in 9.5 M urea). `Crystallized' is the protection factor for GCN4-N16A-p1 peptide crystals over the folded peptide in solution. `GCN4 xtal' is the protection factor of a crystal of the GCN4-N16A-p1 peptide over free SeMet in the crystal cryoprotectant. This combines the previous three protective measures. `Not NE1 xtal' is the protection factor of a crystal of free SeMet in solution over the SeMet side chains in the E1 domain of NEDD8, but note that not all crystals are protective. `Ice vs GCN4' is the protection factor of the largest observed D½ (43.2 MGy) over the smallest observed D½ (5 MGy). This final protection factor is 750% and dwarfs all the others, so it is indicated by a number over the graph. Although individual protection factors are small enough to be difficult to measure, the effects were generally additive and the range of observed D½ values for SeMet does vary widely.

JOURNAL OF
SYNCHROTRON
RADIATION

ISSN: 1600-5775

Volume 14| Part 1| December 2007| Pages 51-72

doi:10.1107/S0909049506048898

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