PHOTOSY?;THESIS I S TKOPIC',IL S U S L I G H T BY
s.R .
DHAR h S D R . P. RANTAL
During recent years Balyl and his coworkers have published a series of important paperson photo-chemical transformations analogous to those occurring in plant life. Their researches indicate the possibility of the process first suggested by Baeyer on assimilation of carbon in plants. They have first tried to prove the synthesis of formaldehyde frcm carbon dioxide and water, and its further pnlymerisation into reducing sugars on exposing to ultraviglet light. I n the subsequent paper they have proceeded to synthesise nitrogenous compounds of carbon by starting from cartion dioxide in water or formaldehyde and a nitrate or nitrite and arrive at results which lead to the possibility of further formation of complex substance?. I n the third paper they start from formaldehyde and aniinoniuin hydroxide for the formation of higher nitrogenous substances such as proteins and alkaloids. Their results in the first two papers are in line with similar observations of Noore and Xebster2 on the one hand and those of Baudisch? on the other: consequently their results agree but not always. Raly and his workers have, however, given little details of their experimental procedure in the first part of their work. In fact, Spoehr's criticism4 doubting the accuracy of some of the published results, elicited the details5 of their m o d u s o p e i n n d i . In all their work on the photo-synthesis and photo-catalyqis both ?\1oore and Baly have used the mercury vapour lamp as their source of ultraviolet light and have used quartz vessels as the receptacle of their reactants. Moore, who worked extensively on the formation of formaldehyde and reducing sugars hy photosynthesis, found that no formaldehyde was formed when water alone, saturated by a stream of carbon dioxide wa? exposed to bright wnlight in quartz tubes. In the presence of I$ Feel?, carbon dioxide and n-ater combined in sunlight to give distinct test of formaldehyde with dchryver's reagent. while a similar solution kept in the dark gave negative results and hence Moore concluded that an inorganic transformer is necessary. and that formaldehyde is actually built up by the inorganic colloid ahsorbing the energy of the sunlight and becoming activated. Baly, Heilbron and Barker. have succeeded in condensing water and carbon dioxide into formaldehyde by ultraviolet light without the use of any catalyst. Instead of taking a saturated solution of carbon dioxide in water, they have passed carbon dioxide into water in the presence of light from mercury vapour lamp. They are of the opinion that Moore's failure to obtain formaldehyde from solutions of carhon dioxide exp3sed to sunlight or to ultraviolet light without the presence of an inorganic catalyst is due to the polyJ. Chem. S O C119, , 1025 i 1 9 2 1 ) , 121, 1078
* Proc. ROY.SOC87B,
( 1 9 2 2 1 ; 123, 185
165, jjh (1914). 90B, 168 (1918).
3 B e r 46, 113 (1913). J Am Chem SOC.45, 183 (1923). S a t u r e , dept I (1923)
(1923).
PHOTOGYPTHESIS I N TROPICAL SKYLIGHT
927
merisation of formaldehyde into carbohydrate as fast as it is formed. M-hile in an agitated solution, some of the formaldehyde formed is carried to the back of the vessel and thus protected from the further action of light. This explanation is open to objection. Baly and his coworkers further state that' the effective light for the formation cf formaldehyde has very short wa\-e length, i. e., 2 0 0 p p and for the formation of sugars. 290pp. They have heen definite in their assertion that there are present in sunlight no rays of the very short, wave lengths necessary for the direct phctcsynthesis of formaldehyde and in their photocatalysts' they state that 110 reaction takes place if ammonia sat,urated wit>h carhen dioxide is espcsed to ultraviolet light in glass tubes behind a screen of plate $1 a spite of these assertions. it is idle to dismiss sunlight off hand, as incapable of photosynthesising reactions which are generally occurring in plant life where sunlight is the only source cf illumination. T h e decomposition of ozone by light gives us a case in support of our c3iitention. Il-eigert' eniployed ultraviolet light and found that the effective deozonising regim for t'he photochemical decomposition of ozone was in the neighhourhood of 2 jqpp. I t n-as known thzt in the presence of chlorine light of longer ITave length, e. g.. that of the blue and violet portims cf the spectrum would cause the decomposition. Griffith and Shutt3 shon-ed that hy the light of the carbon arc projection lamp consuming 30-3 ; amp. at room temperature ozonised oxygen is deozonised by the action of visilile light of wave lengths between 760 t o 6 7 0 p p and 61 j to ; r o u p . Thus the older theory that ultraviolet light alone was resp.znsible for deozmisation falls flat. To our mind sirnilar doubts exist almut the correctness of t h tatemelit of Baly and his coworkers, nljout the fnilure 3f simlipht in formaldehyde Pynthc , -1 number of experinients were therefore carried 3ut by us during the last two years throughout in glass vessels and tropicel sunlight. In some cases the iezcting mixtures were sealed in thin glass tubes and exposed to sunlight. In other cases the solutions under esaniimtion were placed in the sun in glass heakers or evaporating dishes, or else in n-ide-nicuth h i l i n g tukes. carbon dioxide was passed in a numher of solutisns for a n-hole de.!- in the sun. The rapidity with which the sdutions in open vessels evaporated made it difficult to expose the same solution t o the sun for a very great length of t8ime. The \T-ell-known chemical tests (given in Tognoli's "Heagents and Keaction;") were applied in all cases. Benedict's solution was used for detecting sugar and not Fehling's solution as formaldehyde itself reduces Fehling's but not Benedict's. Control blank tests were carried a n side by side to ensure correctness of results. Wherever possible the solutions were distilled before testing for formaldehyde. The melting point determination and the microscope were also utilised to examine the various nitrcgenecus products. In Table I are given the results of the experiments carried on in the sunlight. J. Chem. SOC. 123, 185 (1923).
Z.physik. Chem. 80, 78 (1912). J. Chem. SOC. 123, 2752 (1923).
TABLE I Experiments ? n Szin1ight:(a') formation of formaldehyde. Esperimpnt
Tlrnc of E ~ p o . u ~
COZ passed in water
I
Eo
hour
6 hours
+ methyl orange (catalyst) + methylene blue + chromium salt + uranyl nitrate + capper sulphate + FeC13 Do + colloidal Fe(OH)3
Do Do Do Do Eo Do
Do Do Do Eo Co Eo
JYater saturated with CO, in a sealed tube Eo colloidal (Fe(OH)3in a sealed tube A sealed tube containing COZ saturated in water placed into another tube containing chlorophyll solution. Paraldehyde solution C'OZ in a sealed tube. CO2 passed in chlorophyll extract in a beaker. XaHCO3 solution in a beaker Glucose solution
+
+
+
+
DO
Do, ferric salt reduced. DO
90 hours
traces of formaldehyde obtained. Do. no sugar
7 5 hours
formaldehyde, no sugar.
3 6 hours
Formaldehyde, no sugar.
90 hours
6 hours I8
hour9
30 hours
DO Cane sugar solution Do KzCz04 solution Do HzC204 solution Do H2C204 ferrous ammonium sulphate solution. Eo uranium salt DO Sodium tartrate solution Do Sodium citrate solution Do Methyl alcohol in water 6 hours Glycerol in water 30 hours Acetone in water in a sealed 90 hours tube Chlorophyll in water 8 hours Chlorophyll in alcohol FeCI3 8 hours
+
I~1-snit 8
traces of formaldehyde obtained. formaldehyde obtained. no sugar. Do Do Do Do
Formaldehyde in the distillate. E o , in the distillate. S o formaldehyde in the distillate. no sugar. Do
Do Do Formaldehyde formed.
Do Do Do Do Do Formaldehyde and sugar obtained. Formaldehyde formed. Do
929
PHOTOSYSTHESIS I S TROPICAL S U S L I G H T
TABLEI (continued) Exp~rimcnt
Time of Expo,-urr
R e w 1t s
hour
Do
Oxygen passed in methyl alcohol and water (b) formation of sugar. HCOH in water in a basin
I
5 hours 2 j hours j hours I 2 hours 1 2 hours each
no
H('OH in water and COZ HCOH FeC13 in water HCOH Si. Co. Cr or Cu salts. I 2 hours HCOH aluminum salt. K l I n O + H20:!or cerium salt. HCOH in water in sealed tube 90 hours HCOH oxalic acid Do HCOH Fe(OH)3colloid I 2 hours (e) Formation of Sitrogenous Compounds. BSO, HCOH solution in 50 hours a beaker.
+ + +
+ + +
Do, in a sealed tube
KSOa
+ CO, + water
I 00
hours
IIO
hours
j
hours
20
hours
+ H 2 0 + C'O, in a beaker. T\",OH + C O , + HCOH
SH,OH
in a beaker
+
KH40H HCOH in a beaker.
+ CuCOa
(d) Other Reactions. K H , gas 0,in sealed tube S H 4 0 H solution 02 DO
+
+
H103 solid in a sealed tube HI03 solution K N 0 3 solution KC103 solution
80 hours
I
j o hours
6 hours 6 hours in
diffused light. I O hours I O hours 2 hours 30 hours
Yo sugar, no formic acid. Do, only paraformaldehyde KO sugar. Sugar obtained. S o sugar in any case. Xo sugar.
KO sugar. Do Do A yellowish colour developed. S o form-hydrosamic acid. Oxygen gas, traces of KKO3 formic acid, reducing sugar obtained. Sitrite formed. No sugar, no formaldehyde, no formhydroxaniic acid. Traces of nitrate and nitrite. S o amine, no urea. Methylamine. 11. P. of hydrochloride 19 j o . No pyridine. An amine, an alkaloid obtained. KO nitrite, nitrate or sugar. Kitrogen formed. Nitrite formed. Do Iodine formed. S o iodine formed. KXO, formed. Traces of KC1 formed.
N. R. DIIXR .4SD R . P. S A S Y A L
930
TABLE I (continued) Experiment
CS2 in a sealed tube
Chloral hydrate. Oxygen passed through aqueous alcohol.
Time of E s p o m c 3 hours
IO
I
hours hour
Results
Slightly decomposes and becomes orange-red; longer exposure makes it black due to the separation of carbon. Partly decomposes into a soluble chloride. Formation of acetaldehyde.
B. Experiments in Cltrapiolet Light frorti a Qunrtx Jlerczrry L a m p . COZpassed in water in a I hour Traces of formaldehyde. beaker. K a t e r saturated with COS in I hour Traces of formaldehyde (greater than the previous beaker. one) Dilute HCOH in a basin Do 50sugar. Do, in a sealed tube. Do Do HCOH containing Fe(OH)3 2 or 3 hours S o formation of sugar. colloid. HCOH in water Do Do Do FeC13 Do Do Do SaHC03 Do DO Glycerol H20 130 Appreciable quantity of sugar and formaldehyde formed. Air freed from nitrites by passing through F e S O I solution was bubbled through conductivity water. Do Traces of nitrite obtained.
+ +
+
Xhile these experiments were being carried on the solar spectrum was examined with a Rowland’s grating on a specially sensitised film. It was found that there existed no line in the spectrum of wave length less than z g o p p . Thus from the results of our experiments where we have definite proof of the synthesis of formaldehyde from carbon dioxide and water with or without the use of any organic or inorganic catalyst, we can safely say that Baly’s statement that there exists in sunlight no rays of very short wave length capable of carrying out this particular synthesis is not quite correct. M-e are of t h e opinion that the intensity and prolonged csposure have much to do with this photosynthesis of formaldehyde in sunlight.
PHOTOSYNTHESIS I N TROPICAL SUNLIGHT
93 1
Keigertl in a theoretical paper has shown that supposing according to Thienberg2 the following scheme represents the photosynthesis of formaldehyde, 2 H2O = H2 HzOz - 92000 cal. C O z H2 H 2 0 2 = HCOH H20 O2 - 18000 cal.
+
+
+
+
+
then in this reaction the light energy z Nhv is necessary. Calculation shows that in this case light rays of wave-length 5 1 7 p p are effective and that the radiation is transformed completely into chemical energy, in the presence of chlorophyll. This chlorophyll in this case is supposed to transfer the light energy in effecting the decomposition of water into Hzand Hz02. Were this correct, evidence would have been found of the decomposition of HZO in sunlight alone in presence of chlorophyll. This result has not yet been observed but seems to be likely. K e have frequently observed that many of the experiments giving negative results show that a positive result can only be qbtained under very favourable circumstances when the intensity of light is very great and the exposure has been sufficiently long. Formaldehyde which could be detected in a solution of carbon dioxide in water when exposed to the sun for a little more than an hour on an intensely lighted day, could not be obtained when a similar solution was exposed to six hours on a dusty or cloudy day. The experiments giving positive results in March or April are difficult to reproduce in Sovember or December. Though we have got definite evidence of the formation of formaldehyde from carbon dioxide and water, in the synthesis of sugar Iye have not yet been so successful. But here too n-e are compelled to doubt Baly's statement that sugar is spthesised by radiation of ware length 290pp. For in this case wherever formaldehyde is formed according to Baly. sugar ought to be also formed. Heilbron3 has rather modified the theory first put forward b y them in 1921, that in the process of carbon assimilation there are two definite stages i. e. an activated formaldehyde is first formed and then it polymerises into reducing sugar. They believe that this two-step theory is not quite tenable. But what they put forward now is that as soon as formaldehyde is formed, it is transformed into sugar and this redecoinposes into formaldehyde of which we get the tests. This modification of their views is quite in accordance with Moore's statement that all compounds of biological origin decompose into formaldehyde when exposed to light. Without doubting Moore's results, we are inclined to view the previous theory of two stages as put forward by Baly as more probable. That formaldehyde is not at once polymerised into sugar we can safely say. Only twice did we get any trace of sugar when formaldehyde was exposed to the sun in presence of FeC13 and methyl orange in dilute solution. Oxidatzon 212 Sunlight. Sunlight is very effective in helping oxidation. Thus we have found that ethyl and methyl alcohols are oxidised to the respective Z. physik. Chem. 106, 313 (1923).
* 2 . phvsik.
Chem. 106, 326 (1923). S a t u r e , April 1 4 (1923).
93 2
S . R . DHAR A S D R . P. S A S Y A L
aldehydes. When air or oxygen is passed into the solution, ammonia gives easily tests for nitrites even in diffused light. S H , and O2 when exposed in sealed tube for a long time are completely decomposed into S z . Glycerin gives a coloration with Schiff's reagent if oxygen is passed into it for a day in bright light. Theze results in oxidation are interesting in view of the previous results obtained by l i i t t r a and Dhar' in the induced oxidation of methyl alcohol, ethyl alcohol, glycerin, etc., in presence of sodium sulphate or ferrous hydroxide by passing air or oxygen through the mixture. I n the above reactions light behaves as an inductor or a positive catalyst ,* The reducing nature of sunlight is no less apparent, HI03 solid gives I*; KC103 gives traces of KC1; KK03 solution easily breaks into K S O s ; FeCL solution is reduced by passing carbon dioxide, dilute formaldehyde reduces FeCL solution. The influence of catalyst such as colloidal ferric hydroxide or uranium hydroxide or methyl orange is not very clear. Though formaldehyde formation is in general helped by organic and inorganic catalysts. the best of which is chlorophyll, their action on the formation of sugar is not so marked. I t seems the intensity of light is of more importance in such cases than the presence of the catalyst. Even when we have used ultraviolet light, the influence of the catalyst in sugar synthesis has not been marked. Hence it is difficult to assert with Moore, whether the catalytic change is a surface action in which the light energy is converted into chemical energy at the surface of the colloidal aggregate or whether each catalyst absorbs selectively those rays which polymerise forinaldehyde into sugars. Ferric chloride which Moore has found to catalyse the reaction between water and carbon dioxide to form formaldehyde, was found by us to help in the production of reducing sugar when formaldehyde and ferric chloride were exposed in solution together. Ultraviolet light is undoubtedly more effective in certain photosyntheses. But tropical sunlight, particularly from April to July when its intensity is at its highest, contains a number of rays which under ordinary circumstances. can effect synthesis of complex compounds from very simple compounds. Especially in the case of alkaloids, the experiments show conclusively that there is a tendency in the photochemical transformation to pass from simple and lower order of combinations to the higher and complex organic compounds. Methylamine. which is formed in about 1 2 hours in tropical sunlight when ammonia and formaldehyde solution are exposed, passes into a number of complex nitrogenous substances akin to alkaloids, one of which Baly claims to have identified as coniine but not confirmed by our experiments.
A further conclusion may be drawn that in the synthesis of natural nitrogenous compounds, the origin of nitrogen may either be ammonia or a nitrate. Potassium nitrate gives potassium nitrite and so does ammonium hydroxide Z. anorg. Chem. 122, 146 (1922). *Compare Dhar: Proc. .Ikad. Wet. Amsterdam, 18, 1097 (1916).
933
PHOTOSYSTHEBIS IS TROPICAL SUNLIGHT
give ammonium nitrite on exposure. Ammonium salts, amines, and even carbanide break u p into ammonia and then into a nitrite. This transformation of ammonium hydroxide into amines and also nitrites complicate the final products which necessarily depend on the amount of exposure. The failure H\ in our experiments to detect form-hydrosamic acid OH/
c
=
s //o \OH
in
nitrite and formaldehyde reaction can perhaps be explained by the fact of Baudisch’s observation, that formhydroxamic acid on further exposure to light is reduced to ammonia. Thus we can start from potassium nitrite to get ammonia, either of which substance can be utilised as a source of nitrogen for plant life. Our experiments with ultraviolet light show t h a t nitrogen and oxygen can directly combine in its presence to form higher oxides or nitrogen. We are of the opinion that the same change can also take place in tropical sunlight. The experiments are likely t o throw a flood of light on the nitrogen assimilation by plants. dnother series of photochemical transformation was first indicated by Moore, that all substances of biological origin give formaldehyde on exposure to light. Or as Baly suggests that an equilibrium is set up between ”COH CO,
‘F\ Sugars
J
Stewart’ has indicated a line which is perhaps more analogous to what actually occurs in photosynthesis in the plant life. Starting from sugar and ammonia, CHzOH
CHO
CHZOH
CHZOH
CHO
CHO
we can get a pyrrole derivative by the simple process of dehydration. By a similar process we can theoretically arrive at alkaloids where the light would be helping by dehydrating and rehydrating the intermediate compounds. Form-hydroxamic acid, which according to Baly is the first step to complex nitrogenous photosynthesis, is perhaps the origin of ammonia which acting on the carbohydrates leads to the formation of further highly complex substances. Ammonia and formaldehyde can form aci-nitromethane (according to Baudisch) and this is a very reactive substance which a t once undergoes “Recent Advances in Organic Chemistry,” 258 (1918).
93 4
S . R. DHAR A S D R. P. BAKTAL
further transformation specially in the presence of many catalysts which are present in plants, the nature of which is difficult to elucidate. I t is yet too premature t o declare with any degree of certainty what lines the actual photosynthesis in nature follow to produce the simplest of vital products. But from our experimental results we conclude that the difference between sunlight and artificial ultraviolet light is one of degree and not of kind. Chemical Laboratory, Unuersity of Allahabad, -4 Elahabad, India.
