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12-4 solvation and substitution

Nucleic acids, an important group of heterocyclic compounds are the

bases which form the nucleic acids. Because of their function as building blocks

of the living cell, nucleic acids are etremely important. !here are two types of

nucleic acids, deoyribonucleic acid "#N$% and ribonucleic acid "&N$%. Both typesconsist of two long chains of alternating sugar and phosphate residues. !he

chains are linked at the sugar units by nitrogenous bases which are either

substituted purines or substituted pyrimidines. 'urine and pyrimidine are

heteroaromatic compounds with the structures (

!he substituted purine and pyrimidines are the unit which absorb ultraviolet

radiation in the nucleic acids and their many derivatives. !he sugar and

phosphate residues are transparent above 2)) nm.

#N$ and &N$ contain the two substituted purines, adenine and guanine,

and the substituted pyrimidine, cytosine. &N$ also contains the pyrimidine, uracil

whereas #N$ contains thymine. !he absorption bands of these various bases in

water are listed in teble 12-1)

12-4.*olvation and substitution

$s already eplained, the energy re+uired to bring a about an electronic

transition in the ultraviolet region