phyrin C3iH 36 N 4 when decarbonylated; in the one, an atom of the
metal iron is included, in the other an atom of the metal mag-
nesium; these metals, however, are not in the state in which they
occur in their ordinary salts. When completely degraded, both
compounds give rise to a mixture of the three simple pyrroles:
CH 3 .C C.C 2 H 6 CH 3 .C C.C 2 H 6 CH 3 .C C.C 2 H S
II II II II II II
CH,.C C.CH, CH 3 .C CH HC CH
\/ \/ \/
NH NH NH
The character and complexity of their structure will be ap- parent on consideration of the following formula assigned pro- visionally by Willstiitter to athioporphyrin, the derivative com- mon to both compounds:
CH = CH
I I CHr C CH C C
N
C 2 H 6 C C
CjHs C C \ I NH
CH 3 C = C
CH 3
N II
\
C CH
C \ C = C C 2 H S
HN | \ C C = CH,
CH 3
Athioporphyrin is convertible into a magnesium derivative, aethiophyllin, CsiH^^Mg, which is probably formed from it by the displacement of the two atoms of hydrogen in the two NH groups shown in the above formula. Perhaps the iron occupies a like position in haemoglobin.
In haemoglobin, the coloured system is loosely coupled with a peculiar protein, globin, present to the extent of 94% in the com- plex molecule; in the less weighty molecule of chlorophyll, the coloured system is coupled with the wax alcohol, phytol, CjoHsaOH. Both appear to be derivatives of dicarboxylic acids: the disposition of the CO 2 H groups in haemoglobin is not clear but probably they are in connexion with the globin; in chloro- phyll, one is neutralized by methyl, the other by the phytol radicle. Chlorophyll, unlike haemoglobin, is associated, in most plants, with an enzyme, by which it is hydrolyzed into phytol and the carboxylic acid, chlorophyllid ; not only may the action be reversed (to the extent of 65%) but if hydrolysis be affected, in presence of either methylic or ethylic alcohol, methyl or ethyl takes the place of the phytol radicle. The be- haviour of the enzyme is precisely that of the enzyme lipase towards fats and towards mixtures of fatty acids and alcohols. Alkalies convert chlorophyll into the corresponding dicarboxylic acid, from which the magnesium is easily displaced by hydrogen by means of acid :
(MgN^HMOKCOz.CwH,,) + 2 OH 2 = [MgN < C 32 H 3 oO](CO 2 H) 2 -|-CH 3 . OH+C 20 H 39 .OH
The special activities of haemoglobin and chlorophyll are in no way accounted for, at present, by what is known of their structure: colour apparently is of no consequence in the former but it is held to be the prime factor in the functional activity of the latter. Presumably both act as participate agents, in virtue of their high molecular weights, not in solution. The oxygen-hold- ing power of haemoglobin is commonly ascribed to the iron and it is supposed that the gas enters actually into combination with the molecule; whilst the former is mere matter of opinion, the latter view is supported by evidence, i.e. by the fact that the formation of oxyhaemoglobin involves the addition of a definite proportion of oxygen. Chlorophyll is not known to behave in a similar way towards carbon dioxide. Willstiitter has shown, how- ever, that when the gas is passed into water in which chlorophyll is suspended, this is converted into phaephylin, the magnesium being wholly displaced, as indicated by the equation
The action may be stopped halfway, when apparently the mag-
nesium is only half dissected out of the molecule and is retained, perhaps, together with an added molecule of carbon dioxide, thus .C.C.N.C. .C.C.N.C.
Mg. C.C.N.C.
+H 2 C0 3 =
I MgO.C.O.OH
.C.C.NH.C
On the assumption that such a mechanism is operative, it is pos- sible to understand how the carbonic acid is brought into the circuit of change and under the direct influence of the pigment. The acid would be at a maximum concentration at the surface of the particles. The acid radicle MgO.CO.OH would necessarily be a terminal point from which electrolysis could proceed: so that if, on exposure to light, a photoelectric wave were propa- gated from this point, throughout a circuit in which acid-water was included, the water would be electrolysed and the carbonic radicle might well be subjected to the attack of hydrogen ions and reduced, ultimately to formaldehydrol, chlorophyll being regenerated in the process. The correlative product of electrol- ysis would be hydrogen peroxide (2OH 2 =2H+H 2 O 2 ).
The evolution of oxygen from the plant in such case would be the consequence of the decomposition of hydrogen peroxide, per- haps by a" catalase." That evolution of oxygen and reduction of carbonic acid are coincident phenomena can scarcely be .doubted, as the gas is only produced in presence of the acid and the volume liberated is proportional to that of the carbon dioxide absorbed. It is conceivable that one of the chlorophyll compo- nents may play the part of a catalyst, even that the more oxi- dised may act as platinum black, in the manner Willstatter has suggested: but these are all matters of mere surmise at present. Maybe a more complex circuit is formed than that postulated, one in which perhaps a depolarizer is included; these are all points, however, which must be left for further inquiry.
It is conceivable that the function of iron in haemoglobin is similar to that pictured of magnesium in chlorophyll: that the iron atom becomes partially separated from the molecule, owing to the formation of a perhydrol radicle, similar to that postulated by Willstatter as the active agent in platinum black and as oper- ative in ferrous sulphate perhydrol.
Whatever the process it is to be supposed that formaldehydrol [CH 2 (OH)2] is the initial product of the assimilation of at- mospheric carbon dioxide by the plant; no other explanation that will meet the facts has been advanced. No laboratory proof that carbonic acid can be reduced and " sugar " produced in minute amount, however, is of the least value in enabling us to understand the origin of life. We have to account not for the formation of sugar but of one of the several not to say many possible isomeric forms as a fundamental structural unit: to ex- plain why of the two glucoses of like structure but enantiomor- phic i.e. related to one another as an object is to its image or one hand to the other both of which are produced simultaneously in equal amounts when the synthesis is effected under artificial conditions, only the one is formed within the plant. What act or accident determined such selection, it is impossible to say; what- ever happened, the future course of natural action was limited thereby to one type of symmetric material to the one-handed forms, in genetic relationship with that first selected. Innate peculiarities, only dimly perceptible at present, are also operative in restricting the number of the primary constructive units. It is a remarkable fact that formaldehydrol gives rise to a hexose almost directly, yet it is to be supposed that condensation takes place gradually. In presence of weak alkali it rapidly gives rise to both fructose and sorbose; other products are formed which have not yet been identified; no intermediate products have been reported, such as are formed forthwith undergo further change. The product of the interaction of two molecules would be glycol- lic aldehydrol:
CH 2 (OH) J +CH 2 (OH), ! -CH 2 (OH).CH(OH) 2 -|-OH 2
This is a known substance: it has been shown to give rise to the same hexose as formaldehydrol under the influence of alkali. If three molecules of formaldehydrol were to interact directly or glycollic aldehydrol were to be attacked by formaldehydrol, two
