Purine bases absorption spectra

The close correspondence of the DNA absorption spectrum with that of a mixture of mononucleotides of the same composition illustrates the weak nature of the interactions between neighboring purine and pyrimidine bases guanine (G), cytosine (C), adenine (A), and thymine (T) at an interplanar separation of 3.36 A in the unexcited double-helical configuration. On the other hand the structureless fluorescence band of (calf-thymus) DNA is red-shifted by 3500 cm-1 from the fluorescence spectral origin of the mononucleotides it closely resembles the fluorescence spectrum of the dinocleotide ApT (and of poly dAT) and is accordingly identified131 with the fluorescence... 

While the importance of IR and Raman spectroscopy for the structural elucidation of purine bases has diminished over the last few decades, its advantages for the study of oligo- and polynucleotides in particular with respect to base pairing is apparent. The IR absorption spectrum of a single-stranded polynucleotide is very similar to that of its component nucleotides, but drastic changes occur on formation of hydrogen-bonded helical structures. 

A nucleic acid gives more than 40 well-defined absorption bands in the IR spectrum (300-4000 cm region). For elucidating the nature of the normal vibrations of a nucleic acid, it is helpful to examine the effects of base composition on the vibrational frequencies, as well as the intensity and anisotropy of the IR absorption. Three of the four natural bases of a DNA (adenine, guanine, and cytosine) have an amino group w hich is considered to be nearly coplanar, because the purine or pyrimidine rings have aromatic character and the C—N bonds have some double-bond character. 

The purine and pyrimidine bases, as a result of their 7c-electrons, absorb strongly in the UV region of the spectrum. The approximate absorption maximum is 260 nm (8260nm = 10" ), which is in contrast to that of proteins at 280 nm. The will be dependent upon the structure of the base (hence the pH of the solution, as different tautomeric structures will predominate at different pH values), and derivatization of the base, but not markedly... 

---