Primitive and Present-day Photobiological Processes 193 



In contrast to this finding it is also well established that enzymes, the func- 

 tional groups of which are thiol groups, are much more readily inactivated by 

 radiation than are nonthiol enzymes [9]. A combination of the two observations 

 suggests that a large proportion of the radiation is trapped by sulphur compounds. 

 If they are essential groups, their efficient trapping power results in severe 

 damage to the system under consideration, but if they are non-essential, as is the 

 case for added protectors, they, instead of more important groups, are ruptured. 



A very interesting indication of the role of sulphur compounds has come from 

 the work of Gordy et al. [10]. Their technique was to subject various materials 

 to X-irradiation, and then examine the electron paramagnetic resonance spectra 

 resulting from any electron unpairing brought about by the X-irradiation. 



They found the surprising result that all keratin-type proteins (present 

 principally in outer 'protective' layers such as hair, feathers or scales and there- 

 fore subject to much radiation normally) showed a spectrum characteristic of 

 only the cystine molecule, in spite of the fact that this amino acid represents 

 only a few per cent of the total protein and therefore is responsible for only a 

 few per cent of the total absorption of X-rays. Here is a clear case of trapping, 

 this time probably via a 'hole' migration mechanism whereby electron loss from 

 anywhere in the protein ends up as electron loss from the S — S group of cystine. 



From all of the above considerations it seems reasonable to draw the conclu- 

 sion that, although many different radiation-absorbing and utihzing molecules 

 have been developed as life evolved the close connection of each of these with 

 the cystine molecule or other reactive sulphur-containing system points to the 

 latter as a component of the primitive photochemical process. 



EXPERIMENTAL 



Experiments were carried out using a General Electric looo-w high-pressure arc 

 (AH6 or BH6) as source of illumination. This arc emits about 5% of its energy as the 

 (strongly self-reversed) 1 849 A line of Hg. Selection of a good arc is important because 

 of absorption in the quartz capillary in some cases. For the same reason the 1849 A line 

 intensity falls in old lamps faster than does the general intensity. New arcs were used 

 after about 40 hours of operation. 



Two systems were employed. If material absorbing at 2537 A or at longer wavelengths 

 was present, a wide-angle grating monochromator was used with 2 mm slit width to iso- 

 late the 1849 A line. Using this system only about 0-0005 w of 1849 A Ught reached the 

 sample. This was sufficient to detect, after 300 hours irradiation, amino acid formation 

 from inorganic material, giving weak ninhydrin tests for alanine and glycine. 



For most experiments there were no absorbers of 2537 A present. In these cases direct 

 irradiation of the surface of the liquid with the fuU arc was employed with an aperture 

 /= 1-5. This gave about o-i w of 1849 A Ught absorbed by the sample itself. This 

 system had the advantage of giving more products in shorter times, but the disadvantage 

 that absorption of infrared Ught by the glass vessel used resulted in warming up of the 

 solutions. In these essentially preliminary investigations no attempt was made to control 

 this factor. 



Experiments were carried out on two kinds of system. In the first inorganic materials, 

 of the kind expected to be present in the sea in the pre-oxygen era were irradiated in 

 aqueous solutions in the presence of various homogeneous and heterogeneous catalysts 

 which it was expected might significantly affect the amino acid yield. The aqueous solu- 

 tions were desalted in ion-exchange columns (using A4 DuoUte resin for anions and 

 Amberlite IR 120 for cations), taken to dryness, the products dissolved in alcohol and 

 chromatographed on Whatman No. i paper, using butanol, acetic acid and water 

 (4:1 : 5) as the usual eluting agent. Phenol-water and pyridine-methanol were used for 

 confirmation in some cases. 



