| OCR Text |
Show 109 eatures a lone par orbital S truc t ure III now f· capable of n excitation and it is possible that the Cotton effect of such ition is the cause evidence may be of the the ORD spectrao 78 have a trans- conclusive reported the pK for cytosine nucleoside The structure of the neutral form is V 4020 More n given by CD measuremento Levene and Simms to be change in '¢ as determined by X-ray diffraction.79 the ultraviolet absorption spectrum,80 and . the infrared is VI as spectrum. determined 81 and the structure of the r V 77 . infratred c, I work of Shimanouch5 ., by the recent protonated form N/H ) 0 VI .to Note that the Figure 608 is not protonated form VI capable of n :*. 1f" excitationo displays the ORD and absorption spectra at pH 7 and pH 10 Unfortunately, ,we have been able to measure the CD spectra only at pH 70 we Guided by our previous sU9cess with uridine and thymidine tntatively propose that the absorption spectra at pH 7 can be resolved into four band (Figure 609)0 1f The 1f* absorption bands and a n 1f* absorption placment of the n 1f* band was indicated about 250=m0 by the shoulder in the CD -spectra at A tentative explanation of the pH effect on the ORD curve of cytosine nucleoside is that the ,', n 1f transition gives a significant Cotton effect and that this Cotton effect is removed from the spectrum upon protonationQ |
