=Amorphous and Crystalline Chlorophyll.=--When the chlorophyll of plants is extracted by alcohol and the alcoholic extract evaporated nearly to dryness, beautiful dark green crystals are obtained. Willstätter has shown, however, that in these crystallized forms the ethyl group (from the ethyl alcohol used) has replaced the phytyl group (see below) which is present in the pigments as they exist in the plant tissues; and that, when extracted by other solvents than alcohol, the pigments may be obtained in the amorphous forms in which they exist in the plant. This change from amorphous to crystalline compounds may be understood from the preliminary statement that the chlorophylls are esters of tri-basic acids, in which one acid hydrogen is replaced by the methyl (CH_{3}) group and a second by the phytyl (C_{20}H_{39}, from phytol, or phytyl alcohol, C_{20}H_{39}OH) group. When treated with ethyl alcohol (C_{2}H_{5}OH) for the purpose of extracting the pigments, the ethyl (C_{2}H_{5}) group replaces the phytyl group, thus yielding a methyl-ethyl ester, and these esters are the crystalline forms of the chlorophylls. This replacement is made possible through the action on the original pigment in the tissues of an enzyme, _chlorophyllase_, which is also present in the tissues, which splits off the phytyl group, forming phytyl alcohol, and leaving a free COOH group in the pigment, with which the alcohol used in the extraction forms the ethyl ester (see Chapter IX for a discussion of the formation and hydrolysis of esters). While the chlorophylls are tri-basic acids, only two of the acid COOH groups actually function in ester-formation. The third acid group seems not to exist as a free acid group; but in chlorophyll _a_, it is in what is known as the "lactam" arrangement, represented by the --CONH-- group, and in chlorophyll _b_, it is probably in the "lactone" arrangement, represented by the --COO-- group; the two bonds in each case being attached to different structural units in the molecule (see page 106). The change from amorphous to crystalline forms may be represented by the following formulas, in which the R represents the whole of the complex group to which the acid ester groups are united: COO·CH_{3} COO·CH_{3} COOH / / / R R R \ \ \ COO·C_{20}H_{39} COO·C_{2}H_{5} COOH Amorphous chlorophyll Crystalline chlorophyll Chlorophyllin or or methyl-phytyl methyl-ethyl chlorophyllide chlorophyllide "Chlorophyllin," the compound in which the ester groups have been converted into free acid groups, as indicated above, may be obtained from either amorphous or crystalline chlorophyll by treatment with caustic potash dissolved in methyl alcohol.