Protection factor of nitrate ions. The fractional progression of the damage suffered by a solution of 25 mM SeMet in 25% (v/v) glycerol with either 1 M acetic acid (red error bars) or 1 M nitric acid (blue error bars) and the best-fit exponential curves to those data (solid grey and dotted black lines, respectively). The dose at which half of the SeMet has been damaged is called the `half-dose' (D½) for that sample. The difference between the D½ of the nitric and acetic acid solutions divided by the D½ of the acetic acid sample is called the `protection factor', and reflects the degree of the positive impact nitrate ions have on the D½ of SeMet. Errors in the D½ determinations are propagated to evaluate whether the protection factor is significant.