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.

=Phytol.=--This alcohol, which furnishes the characteristic ester group in the chlorophyll of plants, is a compound of very unusual composition, which has never been found in any other form or in any other type of compound which is present in either plant or animal tissues. Careful studies of its addition and oxidation products prove that it has the following structural arrangement:

H H H H H H H H H H | | | | | | | | | | H-C-----C-----C-----C-----C-----C-----C-----C-----C=====C-----C--CH_{2}OH | | | | | | | | | | | CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}CH_{3}

As this formula indicates, the compound contains one unsaturated, double-bond linkage, one primary alcohol group, and eleven methyl groups. As has been said, this alcohol occurs nowhere else in nature, and its presence and function in the chlorophyll molecule are, as yet, wholly unexplainable. Phytol itself is a colorless, oily liquid, with a high boiling point (145° in vacuo, 204° at 10 mm. pressure).

THE CONSTITUTION OF THE CHLOROPHYLLS

As has been mentioned, chlorophyll _a_ differs from chlorophyll _b_ by having one more oxygen and two less hydrogen atoms in the molecule, and in having one of its nitrogen atoms in the "lactam" arrangement. These differences in structure are represented by the following formulas which are commonly used to represent the two compounds, but which do not show the arrangements of the major groups of the complex molecules:

COO·C_{20}H_{39} COO·C_{20}H_{39} / / MgC_{31}H_{29}N_{3}-COO·CH_{3} MgC_{32}H_{28}O_{2}N_{4}-COO·CH_{3} |\ Chlorophyll _b_ NH-CO Chlorophyll _a_

The chlorophylls are unstable compounds, readily acted upon by acids or alkalies, and by the enzyme chlorophyllase, which splits off the phytyl alcohol group. The progressive action of acids and of alkalies in breaking down the molecule, and the products of its oxidation and reduction, have served to establish the chemical composition of the compound in each case. Because of the importance of these pigments in the whole metabolic processes of the plant, it seems to be desirable to consider the nature of these reactions in some detail, as follows:

=Decomposition of the Chlorophylls by Alkalies.=--The first action of dilute alkalies on the chlorophylls is to split off, by hydrolysis, the alcoholic groups of the esters, producing the crystalline tri-basic acids, or _chlorophyllins a_ and _b_. Each of these chlorophyllins exists in two forms, the normal and the iso, in which the attachment of the COOH groups to the other groups in the molecule is in different positions. Hence, chlorophyll _a_ yields chlorophyllin _a_ and isochlorophyllin _a_, and chlorophyll _b_ yields chlorophyllin _b_ and isochlorophyllin _b_, all four of which are tri-basic acids.

These compounds, when heated with alkalies, split off carbon dioxide in successive stages, losing one COOH group at each step, thus yielding a series of simpler compounds of the following types: First, di-basic acids; second, monobasic acids; and finally, _ætiophyllin_, a compound in which no COOH group is present. In all of these compounds, derived from chlorophylls by the action of alkalies, the Mg remains in the molecule, and all the Mg-containing derivatives from the chlorophylls are known as "phyllins." At the stage at which only one COOH group remains in the molecule, only one group arrangement is possible, and the derivatives from chlorophyllin _a_ and isochlorophyllin _b_, and those from chlorophyllin _b_ and isochlorophyllin _a_, are identical. At the final stage, the derivatives from all four forms are identical. This may be graphically illustrated by the following diagram indicating the progressive decomposition of the two chlorophylls under the action of alkalies:

[Illustration: Decomposition of Chlorophyll a and Chlorophyll b]

=Decomposition of Chlorophylls by Acids.=--The first action of dilute acids upon chlorophylls is to remove the magnesium, without otherwise changing the molecule. Two hydrogens go in in the place of the magnesium. Dilute acids act in precisely the same way upon each of the "phyllins" shown in the above scheme. In this way, a whole series of compounds, corresponding to each of the chlorophylls and their alkali-decomposition products, but with the magnesium lacking in each case, has been prepared. Thus,

COO·C_{20}H_{39} / Chlorophyll _a_, MgC_{31}H_{29}N_{3}-COO·CH_{3}, \\ (NHCO)

COO·C_{20}H_{39} / becomes Phæophytin _a_, C_{31}H_{31}N_{3}-COO·CH_{3}, \\ (NHCO)

COO·C_{20}H_{39} / Chlorophyll _b_, MgC_{31}H_{29}O_{2}N_{4} \ COO·CH_{3}

COO·C_{20}H_{39} / becomes Phæophytin _b_, C_{32}H_{30}O_{2}N_{4} \ COO·CH_{3}

Similarly,

Isochlorophyllin _a_, becomes Phytochlorin _e_,

Chlorophyllin _a_, becomes Phytochlorin _f_, and _g_, COOH / C_{32}H_{32}ON_{4} \ COOH

Isochlorophyllin _b_, becomes Phytorhodin _g_

Chlorophyllin _b_, becomes Phytorhodin _i_ and _k_, COOH / C_{32}H_{30}O_{2}N_{4} \ COOH

And bodies known as "porphyrins" are similarly derived from all the other known phyllins.

For example: cyanophyllin, MgC_{31}H_{32}N_{4}(COOH)_{2}, becomes cyanoporphyrin, C_{31}H_{34}N_{4}(COOH)_{2}; ætiophyllin, MgC_{31}H_{34}N_{4}, becomes ætioporphyrin, C_{31}H_{36}N_{4}, etc.

Phytochlorin _e_ and phytorhodin _g_ are the chief products of the decomposition by acids of the chlorophylls. Indeed, it was the production of these compounds which led to the discovery of the existence of the two chlorophylls. When treated with alkalies, they lose their carboxyl groups and become ætioporphyrin.

=Decomposition of the Chlorophylls by Oxidation and Reduction.=--When acted upon by oxidizing agents, such as chromic acid, the porphyrins yield two chief oxidation products, which are pyrrole derivatives having the following formulas,

CH_{3}-C-CO CH_{3}-C-CO ¦ \ ¦ \ ¦ NH ¦ NH ¦ / ¦ / CH_{3}-CH_{2}-C-CO HOOC-CH_{2}-CH_{2}-C-CO

Methylethylmalein imide Hæmatinic acid imide

By reduction, there have been obtained from the chlorophylls and the various porphyrins, three isomeric pyrrole derivatives having the following formulas,

CH_{3} H CH_{3} | | | C_{2}H_{5}-C=C C_{2}H_{5}-C=C C_{2}H_{5}-C=C | \ | \ | \ | NH | NH | NH | / | / | / CH_{3}-C=C CH_{3}-C=C CH_{3}-C=C | | | CH_{3} CH_{3} H

Phyllopyrrole Hæmopyrrole Isohæmopyrrole

As a result of the study of these decomposition units, Willstätter has suggested the following formulas for the structural arrangement of ætiophyllin and ætioporphyrin, the compounds which result from the removal of all of the acid groups and finally of the magnesium from the chlorophylls,

H HC===CH | | | CH_{3}-C--C C---C ¦ \\ // ¦ ¦ N N ¦ ¦ / · · \ ¦ C_{2}H_{5}-C--C · · C---CH \ · · // C------------C / · · \ C_{2}H_{5}-C==C · · C==C-C_{2}H_{5} | \ ·· / | | N-----Mg-----N | | / \ | CH_{3}-C==C C==C-C_{2}H_{5} | | CH_{3} CH_{3}

Ætiophyllin

H HC===CH | | | CH_{3}-C--C C---C ¦ \\ // ¦ ¦ N N ¦ ¦ / \ ¦ C_{2}H_{5}-C--C C---CH \ // C------------C / \ C_{2}H_{5}-C==C C==C-C_{2}H_{5} | \ / | | N N | | / \ | CH_{3}-C==C C==C-CH_{3} | | CH_{3} CH_{3}

Ætioporphyrin

The COOH groups which are attached to these compounds to form the various phyllins and porphyrins, as well as the original chlorophylls, are supposed to be attached to the C_{2}H_{5} groups in the above formulas, the different modifications, or compounds, depending upon the position in which one or more of these attachments are made.

SIMILARITY OF CHLOROPHYLL AND HÆMOGLOBIN

It seems to be desirable, at this point, to call attention to the remarkable similarity in the chemical composition of chlorophyll, the most important pigment of plants, and hæmoglobin, the all-important respiration-regulating pigment in the blood of animals. Hæmoglobin is a complex compound, consisting of about 96 per cent of albumin (a protein, see