1. 1.0 0.0 fraction unconverted peak valley

2. XANES probe for damage rate fluence (10 15 photons/mm 2 ) fraction unconverted 0.0 0.2 0.4 0.6 0.8 1.0 0 20 40 60 80 100 Se cross-section at 12680 eV 8.6 17 26 34 dose (MGy) half-dose

3. No damage in the “dark” Elapsed time (hours) Peak cross section (cm 2 /g) 215 230 245 260 0 5 10 15 20 25

4. “protection factor” of nitrate ion Absorbed Dose (MGy) Fraction unconverted 25mM SeMet in 25% glycerol 0.0 0.2 0.4 0.6 0.8 1.0 0 20 40 60 80 100 Half-dose = 27.4 ± 1 MGy 1 M nitric acid Half-dose = 16.2 ± 0.8 MGy 1 M acetic acid Protection factor: 69% ± 10%

5. 660%

6. Temperature dependence Temperature (K) Half-dose (MGy) 25mM SeMet in 0.6 M NaOH 25% glycerol 5 6 7 8 9 10 11 12 13 14 90 100 110 120 130 140 150 160

7. Concentration dependence SeMet concentration (mM) Half-dose (MGy) SeMet in 0.6 M NaOH 25% glycerol at 93 K 4 6 8 10 12 14 1 10 100 1000

8. SeMet damage mechanism Se CH3 Se N Low-energy electron CH3 Se X-ray N I R

9. Concentration dependence SeMet concentration (mM) Half-dose (MGy) SeMet in 0.6 M NaOH 25% glycerol at 93 K 4 6 8 10 12 14 1 10 100 1000

10. 1.0 0.0

11. XANES probe for damage rate fluence (10 15 photons/mm 2 ) fraction unconverted 0.0 0.2 0.4 0.6 0.8 1.0 0 50 100 150 200 250 300 350 400 Se cross-section at 12680 eV 20 40 60 80 100 120 140 dose (MGy) half-dose

12. “protection factor” of nitrate ion Absorbed Dose (MGy) Fraction unconverted 25mM SeMet in 25% glycerol 0.0 0.2 0.4 0.6 0.8 1.0 0 20 40 60 80 100 Half-dose = 27.4 ± 1 MGy 1 M nitric acid Half-dose = 16.2 ± 0.8 MGy 1 M acetic acid Protection factor: 69% ± 10%