They are anthraquinone derivatives and dimeric glycosides. These glycosides contain an aglycone group that is a derivative of anthraquinone. It was believed that rhein along with other anthraquinone glycosides imparted this activity. It is estimated that 85% of disperse dyes are azos or anthraquinone dyes. Senna derivatives are a type of stimulant laxative and are of the anthraquinone type. The chemical compounds responsible for this laxative effect are anthraquinone and emodin. Anthracene is converted mainly to anthraquinone, a precursor to dyes. Natural anthraquinone derivatives tend to have laxative effects. Hydrogen peroxide is produced industrially by the anthraquinone process which involves using 2-alkyl-9,10-anthraquinones for hydrogenation. A type II polyketide synthase is responsible for anthraquinone biosynthesis in the bacterium "Photorhabdus luminescens". The anthraquinone is reduced to 9,10-dihydroxyanthracene which then can react with lignin. Oxygen and the organic phase react under formation of the anthraquinone and hydrogen peroxide. Several other isomers of anthraquinone are possible, including the 1,2-, 1,4-, and 2,6-anthraquinones. Many derivatives of anthraquinone are used but 2-ethylanthraquinone is common because of its high selectivity. Its laxative effect, due to its anthraquinone content, is also well proven.