The most prominent radiation-induced specific changes in two crystal forms of lysozyme. Red: radiation damage map aFo (where a is a slope coefficient obtained from the linear interpolation of radiation-induced changes of intensities, see §2.3) contoured at −4σ. Green: radiation damage map contoured at +4σ. Blue: 2Fo − Fc map contoured at 1σ. Orange: Fo − Fc map contoured at +3σ. (a) Tetragonal lysozyme. The highest peaks are concentrated around three disulphide bridges; the positions clearly indicate that disulphide bridge Cys30–Cys115 is reduced. (b) Monoclinic lysozyme crystallized with sodium nitrate. The highest peaks are concentrated around nitrate ions, the changes around water 326 combined with the presence of positive Fo − Fc map features and the 2Fo − Fc map indicates a nitrate ion rather than a water molecule. In the monoclinic form, both the C-termini are decarboxylated. There are no changes to disulphide bridges.