Crystallisation special methods

Ergosine is prepared by boiling ergosinine with phosphoric acid in a mixtnre of alcohol and acetone, or with potassinm hydroxide in diluted alcohol, mixtures. of the two alkaloids being formed and requiring special methods of separation it crystallises from ethyl acetate in prisms, m.p. 228° (dec.), faJi - 161° [o]g> - 193° (c = 1 CHCl,) or + 24° (c = 1, acetone). The hydrochloride, B. HCl. COMcj, separates from acetone in diamond-shaped plates, m.p. 235° (dec.). The hydrobromide, B. HBr. COMcj, forms needles from acetone, darkens at 200° and decomposes at 230°. The nitrate, B. HNO3. COMeg, crystallises in needles from acetone, darkens at 185° and decomposes at 215°. 

The first group are considered methods of choice for the fractionation of crude extracts and the second group of methods are more valuable for carrying out finer sub-fractionation within a previously precipitated fraction. Fractionation by the second group of methods can be carried out by variation of pH or temperature or both. pH fractionation has been more widely used, but in special cases temperature variation at constant pH and ionic strength can be a valuable method for inducing crystallisation of proteins (e g. carboxyhaemoglobin and myosin). 

The main advantage of diffusion modification method to structure glasses is the relatively precise etch stop at the position of the threshold value of the silver concentration. The maximum permitted diffusion time is 20 h which allows for a maximum depth of structures of 400 pm. This hmit of diffusion time is due to the onset of undesired LMS crystallisation at the glass surface during the crystallisation process. The reason is the presence of too much silver in the glass surface which will be reduced also without special UV exposme and forms nuclei dming thermal treatment. If diffusion times in excess of 10 h are required a surface treatment should follow the diffusion process. 

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