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T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 14, No. 12
Progress toward t h e Artificial Synthesis of Carbohydrates and Proteins' By R. W. Thatcher NEWYORKAGRICULTURAL EXPERIMENT STATION,GENEVA,N. Y.
W
HILE it is not within the lifetime of the present generation that the practices of civilized men will result in critical depletion of the stored energy of the plants of the Carboniferous Era and of the mighty primeval forests of the newly discovered countries of the earth, already there are indications of the approach of that unfortunate time. Before long, as time is counted in history, necessity will compel serious consideration of the problem of replenishment or substitution for the energy stored in plants t o supply the needs of mankind. Even now there are signs of approaching difficulties. The demand for foodstuffs for increasing populations, while tempoarily halted by the depopulation of the World War and subsequent starvation, will begin anew within a very few years. The inroads of modern necessities upon our rapidly diminishing supplies of timber, and the ravages of insect pests and plant diseases upon economic food and fiber crops, stimulate serious, sober thought in the minds of those who are following the evolutionary development of the earth's inhabitants. The contemplation of the approach of the time when the last ton of coal shall be mined and the last gallon of mineral oil consumed gives to the thoughtful student the same blood-chilling sensation that affects him when he sees through his powerful telescope the cold and barren wastes of the moon. While these present indications of future dire necessities are not sufficient to cause general gloomy forebodings, they furnish an adequate reason for the utmost interest by chemists in the possibility of a better understanding of how nature has through countless ages stored up the vast reservoirs of solar energy which modern civilization is now so rapidly dissipating. Photosynthesis has come to have an intensely practical, as well as a fascinating theoretical, interest.
RECENTDISCOVERIES During very recent years certain biochemists have devoted much study to this problem, and, while it is far from being solved, some of its aspects are better understood than they were even five years ago. Exactly how the plant cell accomplishes its total constructive function is as yet a mystery, but some of the steps in the process are now understood and have been artificially duplicated. Several decades ago Emil Fischer and his students accomplished the artificial synthesis of many soluble sugars and of certain polypeptides which closely resemble the natural proteins, but by roundabout processes, and by means of violent chemical reagents, such as could not possibly be conceived to be in any way analogous to those which a plant cell utilizes in its synthetic work. For this plant process it has long been recognized that the gases of the atmosphere are the real initial raw products, and certain fairly simple empirical chemical equations have been cited as representing the steps through which the photosynthetic process goes on; but until very recently attempts t o duplicate experimentally and under artificial conditions any of these simple transformations have always failed. Furthermore, while it has been accepted as a fact that chlorophyll-containing plant cells are able to synthesize carbohydrates of all degrees of complexity from the atmospheric gases, until within the past five years it has been supposed that, although the gaseous nitrogen of the air is undoubtedly the ultimate 1 Received
July 24, 1922.
source of supply of this element, the synthesis of proteins by plants requires the fixation of combined nitrogen in the soil prior to its absorption by the roots of higher plants. Very recently, however, Benjamin Moore and his associates2 have demonstrated that unicellular algae, in the absence of all nitrogen except that in the atmosphere and in the presence of carbon dioxide, can fix nitrogen and grow and form proteins by a process which derives its energy solely from light and is, therefore, photosynthetic. They found also that the rate of this unicellular growth may be accelerated by the presence of oxides of nitrogen such as are present in the atmosphere. Further, Moore has concluded from other experiments3 that green seaweed can and does grow and synthesize both carbohydrates and protein, using only the carbon from bicarbonates of calcium and magnesium present in sea-water and nitrogen from the atmosphere. He explains the failure of fresh water and terrestrial plants to accomplish the same transformatibns as being due to their inability to establish a satisfactory exchange of alkali material between themselves and their enveloping medium, as do the sea plants. PHOTOSYNTHESIS OF
CARBOHYDRATES
As to the mechanism by which photosynthesis of carbohydrates is accomplished, recent investigations4 indicate that under , the stimulus of short wave length of light (200 ~ p )aqueous carbon dioxide can be fairly easily converted into formaldehyde, without the presence of chlorophyll or any other energy-absorbing agent; and that this photosynthesis can be actively photocatalyzed by certain basic colored substances, such as colloidal uranium hydroxide, ferric hydroxide, methyl orange, or malachite green, in the presence of visible light. They also point out that the polymerization of formaldehyde into reducing sugars takes place easily in the presence of light of wave length 290 p ~ yand that this polymerization can likewise be photocatalyzed by deeply colored alkaline copper solutions. These reactions have been shown to be reversible, depending upon the type of light used and the character of the photocatalyst which is present in the reacting mixtures. 'Thus, in a system containing reducing sugars, formaldehyde, carbon dioxide, and water, equilibrium is set up between these various components at various points depending on the wave length of the light which falls on the mixture and the nature of the photocatalyst which is present. If ultraviolet light of very short wave length is employed, equilibrium lies far over on the carbon dioxide side; in the presence of a suitable photocatalyst to absorb light of slightly greater wave lengths, formaldehyde is produced in considerable quantities; while in the presence ol photocatalysts which absorbs light of wave length 290 pp, active polymerization of formaldehyde to reducing sugar goes on, and, in the presence of a photocatalyst which is capable of catalyzing both stages of the reaction, the equilibrium is shifted entirely over to the reducing. sugar side. It has been suggested, although not yet experimentally demonstrated, that chlorophyll is an ideal photocafalyst for both stages of the synthesis of soluble sugars from carbon dioxide and water, and the formation of Proc. Roy. Soc. (London),913 (1Q20),201. J. Chem. SOC.(London), 119 (1921), 1555. 4 Zbid., 119 (1921),1025.
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carbohydrates in the growing leaf from very small concentrations of carbon dioxide without the existence of free formaldehyde as an intermediate product is thus explained. If this explanation is finally given successful experimental demonstration, the next step seems fairly obvious-namely, that the reversible changes of reducing sugars t o more complex carbohydrates may be likewise photocatalyzed, presumably by light of still greater wave lengths. If this should be found t o be true, the entire mechanism of the production of carbohydrates from carbon dioxide and water will be understood, and an ample supply of the photocatalytic chlorophyll, or other plant pigments for artificial synthesis of these important food substances, will be easily available in the noneconomic plants of the country. It would, therefore, seem that the possibility of economic artificial synthesis of one of the important constituents of human food is at least brought much nearer to realizati6n by these recent biochemical researches. As t o the actual part which chlorophyll plays in the photosynthesis of carbohydrates, three possible modes of action are now receiving experimental study. The first of these, which has naturally been longest considered as the probable function of chlorophyll, is that it acts chemically as a catalyst by forming unstable intermediate products with formaldehyde or possibly other simple intermediate compounds of the photosynthetic process. Indeed, several investigators6 have shown that chlorophyll actually does form a compound with formaldehyde from which complex the formaldehyde may be either given off or absorbed in order to establish a proper equilibrium in the photosynthetic process, and to afford in the plant a mechanism by which the quantity of free formaldehyde is regulated. In this way the amount present in free form at no time reaches that which would be toxic to the cell protoplasm. The second of these conceptions is that, regardless of the color and of the general chemical reactivity of chlorophyll, the mineral constituent (magnesium) which it contains is held in proper colloidal form to exert a definite catalytic effect upon the photosynthetic process. The third, and most recent, explanation of the mechanism of chlorophyll action is that the pigment acts as a photocatalyst, or light screen, to absorb and transmit the energy from light rays of the proper wave length to accomplish the several steps in the photosynthesis of carbohydrates. As has been pointed out above, this last conception has very recently been given experimental confirmation, and the artificial synthesis of simple carbohydrates from atmospheric gases under the influence of proper photocatalysts has already been accomplished. PHOTOSYNTHESIS OF PROTEINS
Not as much progress toward an understanding of the mechanism of the synthesis of proteins by plants, or toward the successful artificial duplication of the process, has yet been made as has in the case of the carbohydrates. It has been shown that the ordinary synthesis of proteins by plants supplied with nitrogen in some oxidized (preferably nitrate) form is not necessarily a photosynthetic process, as it can occur in the dark and in the absence of chlorophyll or any other light-absorbing pigment. However, atmospheric nitrogen cannot be used by plants for this purpose, except in the case of certain bacteria, notably those which live in symbiosis with the legumes in the nodules on the roots of the host plants and other low plants, particularly marine algae. As has been pointed out, it has recently been suggested by Moore that the ability of green algae to synthesize proteins from atmospheric nitrogen, by a process which seems t o be photosynthetic-i. e., deriving its energy from'solar light--is apparently due to their favorable environment for free exchange of alkali material between their tissues and the surrounding medium, sea water. This indicates that the synthesis of proteins through the nitrogen--,ammonia+amino acid+ 6
j'roc. ROY.SOC.(London),SOB (1908),30;8a (1910),226.
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protein course may yet be understood and possibly artificially duplicated in the near future. Heretofore, it has been supposed that the oxidation of atmospheric nitrogen to nitrate form is a necessary preliminary step to its utilization in the synthesis of proteins, a t least by ordinary cell protoplasm. Presumably, the nitrate nitrogen must then be reduced in the plant to nitrite, and then t o the ammonia form, in order to enter the amino arrangement which is required for the greater proportion of the protein nitrogen. These preliminary oxidation and reduction changes have heretofore been little understood, but progress toward a satisfactory knowledge of the mechanism of their control is now being made. Artificial syntheses of amino acids by the action of ammonia upon glyoxylic acid and sorbic acid, both of which may be obtained by the oxidation of simple sugars, have been accomplished. The reversible condensation of these amino acids into proteins is readily accomplished in all living protoplasm under the influence of special enzymes which are almost universally present in the cytoplasm. We are still in the dark as to the mechanism by which this condensation is brought about, but recent investigations have thrown so much light upon the whole photosynthetic process that it seems reasonable t o expect that we may soon reach a working understanding of the condensation process, both of the sugar starch and of the amino acid 75protein transformations. When this result is reached, a long step will have been taken toward the solution of the problem of artificial synthesis of food products, with which to supplement the stored energy of bygone ages that is being so rapidly dissipated by the demands of our modern civilization.
Use of Oxygen in Metallurgical Operations Use of oxygen in connection with the enrichment of the blast in the blast furnace and in practically all phases of pyro-metallurgical work will furnish the key t o success in the further development of such metallurgical operations, according to Dr. F. G. Cottrell, formerly director and now consulting metallurgist of the United States Bureau of Mines, who first directed the Bureau's attention to this subject. Through this enrichment process it is hoped to increase the efficiency of metallurgical operation with a resultant production of metals at lower cost and possibly the use of lower grade ores. The Bureau of Mines now has outlined plans for two studies which will be carried on simultaneously. The first will cover the present-day processes for the production of oxygen, in order t o determine the feasibility of attempting to produce oxygen, or oxygenated air, in such amounts and a t such a cost as t o permit of its use in metallurgical operations. The second study will be devoted t o the feasibility of using oxygen, or oxygenated air, in metallurgical operations. Because of his interest in this investigation, M. H. Roberts, vice president of the Franklin Railway Supply Co., was asked t o select an advisory committee to work with the Bureau of Mines and to act as chairman of this committee. The committee will consist of F. G. Cottrell, director of the Fixed Nitrogen Research Laboratory; W. L. DeBaufre, chairman of the mechanical engineering department of the University of Nebraska; D. A. Lyon, chief metallurgist of the Bureau of Mines; R. B. Moore, chief chemist of the Bureau of Mines; R. C. Tolman, professor of physical chemistry and mathematical physics, California Institute of Technology; J. W. Davis, mechanical engineer of the Bureau of Mines; F. W. Davis, metallurgist of the Bureau of Mines; Frank Hodson, president of the Electric Furnace Construction Company; and P. H. Royster, assistant metallurgist of the Bureau of Mines, Previous to the war, some work was done in Belgium on the enrichment of the blast with oxygen in connection with the smelting of iron ores in the blast furnace. I n the United States, the late J. E. Johnson, Jr., was interested in the possible use of oxygen in metallurgical operations and carried on some experimental work along these lines previous t o his death. On November 3, 1922, Dr. Edgar F. Smith delivered a lecture on Joseph Priestley before the Priestley Club of the University of Pennsylvania. On this occasion various Priestleyana were. for the first time exhibited to the public.
