78 D. SHUGAR 



versibility can be observed for 4-methyluracil as reported, 142 a very high 

 degree of reversibility prevails for 1 ,3,4-trimethyluracil; experiments with 

 5-hydroxymethyluracil have been negative, but 5-hydroxymethylcytosine 

 gives some indication of formation of reversible photoproducts. 



An extremely interesting observation in this connection has been made 

 by Wang, 154 who found that the final product of irradiation of 1 ,3-dimethyl- 

 thymine is A^,A^'-dimethylmethylmalonamide; by analogy with the situa- 

 tion for dimethyluracil (Section V, 4, a) the most logical explanation of this 

 is the initial uptake of a water molecule at the 4 , 5 double bond ; the inability 

 to observe this intermediate is due to the greater ease with which it under- 

 goes dehydration as compared to uracil analogs. The possibility is envisaged 

 that in nucleic acid chains the hydroxyhydro intermediate of thymine may 

 be more stable than in the free state. It remains only to add that such a 

 possibility can readily be tested by studying the photochemical behavior 

 of a synthetic polynucleotide containing only thymine, using a procedure 

 similar to that for oligo- and poly-U 69 (Section VII, 1, b). 



From all that has been said above, it is clear that the behavior of a given 

 derivative is difficult to predict and must be determined experimentally. 

 The significance of the substitution of position 5 in relationship to the photo- 

 chemistry of the T-even phages will be pointed out in Section X. 



d. Kinetics, Concentration, and Solvent Effects 



Most of the observations reported in the previous sections were obtained 

 from studies on dilute solutions (about 10~ 4 M) transmitting some of the 

 incident energy; under these conditions, the decrease in concentration re- 

 sulting from irradiation will be exponential, i.e., the reaction will be first- 

 order (assuming that adequate correction is made for any absorption by 

 the photoproduct). If the optical density of the solution is so high that all 

 the incident energy is absorbed for an appreciable part of the reaction, then 

 the reaction will be zero-order. The quantum yield should also be constant 

 for both cases. 



Moore and Thomson 142 actually found that <t> for Up was 0.019 at 10 -8 M 

 and 0.037 at 10~ 2 M. A detailed examination of the kinetics of photolysis 

 of a variety of uracil derivatives, conducted in relation to the photo- 

 chemistry of synthetic oligonucleotides, has now shown that the kinetic 

 behavior of concentrated solutions is not as simple as appears from the 

 behavior of dilute solutions, and that even the rate of the reverse reaction 

 may be markedly affected. 69 For a compound such as 2-methoxycytosine 

 even the nature of the photoproduct is altered in concentrated solution. 39 



Also of considerable interest, mainly to the physical chemist and synthetic organic 

 164 S. Y. Wang, Nature 184, B.A. 59 (1959). 



