DYEING AND OTHER PROCESSES OF COLOURING 299 



show, (b) The lysochrome is much less soluble in the 

 solvent than in these substances. 



Lipid-soluble lysochromes that fulfil all these conditions are 

 necessarily insoluble in water. 



A single Japanese worker ^^^ claims to have tested about 3,000 

 substances to find which of them could be used as lysochromes. 

 Less than a dozen in all, however, have found favour with histo- 

 chemists in general, and none of these is among the 3,000. The 

 chosen few are diverse in chemical composition. Some of them are 

 not related to any group of dyes. Coloured lipids are themselves 

 suitable, and it may thus be said that a lipid is set to catch a lipid. 

 The carotene of the fruit of red pepper (Capsicum sp., Solanaceae), 

 for instance, is easily extractable and gives good results.^"^ The 

 related substance, annatto, from the fruit of a small tree {Bixa 

 orellana, Bixaceae), could also be used; it is a straight-chain 

 compound, one of the very few wholly aliphatic compounds that 

 are strongly coloured. It is used for tinting butter. Alkannin, 

 another plant-product (from the root of Alkanna tinctoria, Bora- 

 ginaceae), was formerly much used as a lysochrome, especially in 

 botanical studies. The molecule somewhat resembles alizarine, but 

 one of the lateral rings has been broken and spread out as a straight 

 chain. 



Nowadays most workers prefer the synthetic lysochromes, 

 which rely on the same chromophores that give colour to dyes, but 

 lack the typical auxochromes. The latter, if present, would cause 

 ionization, and as a consequence the substance would be unusable 

 as a lysochrome; for it would be soluble in aqueous media, 

 insoluble in lipids, and capable of dyeing proteins and other tissue 

 constituents. It is characteristic of lysochromes that they are non- 

 ionic compounds. 



The first reasonably satisfactory synthetic lysochrome was 

 Sudan III, introduced in Italy towards the end of the last cen- 



O 



-N=N- ^~^ 



\_/ I 



tury.i^^ It is a^ red compound, related to the disazo dyes. The 

 absence of typical auxochromes will be noticed. Sudan IV ^** 



