Viii. a Specific Colour Reaction for Ergosterol

THE unique function of ergosterol as the parent substance of vitamin D made it desirable to find a colour reaction for it, by means of which it could be detected in the presence of other sterols. In searching for such a reaction, the property of formaldehyde of shifting the colour from the red into the blue part of the spectrum in the usual colour reactions of sterols [Whitby, 1923; Rosenheim, 1927] suggested an investigation of the behaviour towards sterols of the aldehyde corresponding to trichloroacetic acid. This acid is known to give rise to colour reactions with cholesterol [Tschugajeff, 1900]. Although anhydrous chloral proved to be non-reactive, it was found that both chloral hydrate and trichloroacetic acid give a characteristic blue colour reaction with ergosterol, whilst all the other naturally occurring sterols investigated, when purified from ergosterol, remain colourless under the same conditions. The specimens of ergosterol used in these experiments had been fractionally recrystallised at 360 and possessed the highest optical activity, [a]D1320, so far recorded [Tanret, 1908; see also Bills and Honeywell, 1928]. The reactions to be described are therefore unlikely to be due to impurities or degradation products of ergosterol. In contradistinction to naturally occurring sterols it was found, when studying the reaction of sterol derivatives, that the production of an immediate red colour with either of these reagents is specific for the Al,2 (or Al, 13) linkage of the sterol ring system. These observations suggest an explanation of the mechanism of sterol colour reactions, which is applicable to all of them and will be discussed later.