30. PHOTOCHEMISTRY OF XUCLEIC ACIDS 67 



destroyed gives four main degradation products: ADP, adenosine mono- 

 phosphate (AMP), adenine and nicotinamide, but no adenosine 126, 127 (see 

 above); while about 50% of the total ribose, practically all of it from the 

 nicotinamide moiety, no longer reacts to orcinol. 126 



The behavior of triphosphopyridine nucleotide (TPN) is analogous to 

 that of DPX, with the exception that only about 30% of the ribose (again 

 mainly from the nicotinamide moiety) is destroyed following total loss of 

 coenzyme activity. However, in this case the splitting of the ribose phos- 

 phate ester bond of the nicotinamide nucleotide moiety liberates, instead 

 of ADP, a compound similar to the latter but with an additional phosphate 



Nicotinamide ribose — ! — P — P — ribose — adenine 



group at position 2' (or 3') of adenosine. The identification of this product 

 on the basis of enzymic tests was regarded only as tentative. 128 It is, how- 

 ever, fully in agreement with the structure of TPX proposed by a number 

 of observers since 1950 (see Volume I, Chapter 4) and may, in fact, be re- 

 garded as additional supporting evidence for such a structure. 



In contrast to the above Shigemoto 129 reports the products of photochem- 

 ical degradation of DPN at 260 nux to include nicotinamide-ribose-S'-phos- 

 phate, AMP, and adenosine while for ATP the corresponding products are 

 AMP and adenosine. In the absence of the original publication it is not 

 possible to comment on this discrepancy. 



In line with what has been pointed out regarding the transparency of 

 carbohydrates at 253.7 m/z (Section V, 1), the destruction of the nicotin- 

 amide-bound ribose must be due either to energy transfer or to strain rup- 

 ture of the ribose ring as a result of interaction between the nicotinamide 

 and adenine rings. That such interaction does indeed exist has been shown 

 by Weber 130 who found that DPXH exhibits a hyperchromicity of about 

 22% at 260 nux, in line with an earlier observation of Whitby 131 on flavin 

 adenine nucleotide which was found to exhibit an extinction coefficient 30 % 

 less than that of its components. It was also demonstrated by Weber 130 

 that energy absorbed by the adenine portion of DPNH can appear as 

 fluorescence in the nicotinamide ring with a 30% transfer efficiency. 



The resistance of the adenine-bound ribose to destruction is in accord 

 with the fact that considerably higher doses are necessary for destruction 

 of ribose in nucleic acid chains (Section VI, 1). 



127 B. Ekert and R. Monier, Bull. soc. chim. biol. 40, 793 (1958). 



128 M. W. Seraydarian, Am. J. Physiol. 181, 291 (1955). 



129 T. Shigemoto, Osaka Daigaku Igaku Zassi 10, 513 (1958); Chem. Abstr. 52, 13832i 

 (1958). 



130 G. Weber, J. chim. phys. 65, 878 (1958). 



131 L. G. Whitby, Biochem. ./. 54, 437 (1953). 



