50 D. SHUGAR 



1 watt == 10 6 /i watts == 1 joule/sec. == 10 7 ergs/sec. 



-16 



1 quantum = — - — - — r X 10 lb cal. 

 A (m mfjL) 



1992 



X 10" 12 ergs 



X(in nixx 



At X = 253.7 imz, 1 quantum = 7.8 X 10" 12 ergs. 



For a G30T8 germicidal lamp, the effective emitting length of which is 

 81 cm. and the circumference 8 cm., the total ultraviolet emission is about 

 7 watts, or 10,801 /xwatts/cm. 2 at the surface, or 108,000 ergs/cm. 2 /sec. 

 Since most of this is at 253.7 mix, the emission at the surface is 108,000 X 

 (10 12 /7.8) quanta/cm. 2 /sec. = 1.4 X 10 16 quanta/cm. 2 /sec. at 253.7 m/x- 



IV. Optical Properties and Techniques 



A knowledge of the absorption spectrum of a substance is a prerequisite 

 for photochemical studies. In the case of nucleoproteins, nucleic acids, and 

 their constituents this is all the more important in that spectral changes 

 provide a convenient tool for following the effects of irradiation and in 

 many instances for identification of some of the photoproducts. Interpreta- 

 tions of action spectra are also based on absorption spectra under the par- 

 ticular conditions prevailing. 



The optical properties of nucleic acids have already been dealt with in 

 Chapter 14 of Volume I and we shall confine ourselves here to some addi- 

 tional material which has appeared since the publication of this review 

 and which is of relevance to the photochemistry of polynucleotides. 



1. Far Ultraviolet Spectra 



The absorption spectra of deoxyribonucleic acid (DNA) and some nucleic 

 acid derivatives have been extended into the vacuum ultraviolet 46 in con- 

 nection with photochemical studies on nucleic acids, and necessarily involv- 

 ing the use of dry films. The spectra of DNA and cytosine, extended into 

 the near ultraviolet, were in reasonably good accord with those for the same 

 substances in solution, but marked differences were noted for adenine. Aside 

 from the undesirability of the use of films as compared to aqueous solutions, 

 vacuum spectrophotometers are neither accessible nor applicable readily 

 to routine measurements. 



A significant technical advance of the past three years has been the ex- 

 tension of the available wavelength range to almost 186 mix without the 

 use of vacuum spectrophotometers. The conversion of a Beckman model 

 DU for measurements to 192 m/x has been described by Taylor and Jones 47 ; 



46 J W. Preiss and R. Setlow, J. Chem. Phys. 25, 138 (1956). 



47 L. W. Taylor and L. C. Jones, Anal. Chem. 28, 1706 (1956). 



