Base nitrogenada

Purinas e pirimidinas e seus derivadas

Base nitrogenada (português brasileiro) ou base azotada (português europeu) é um composto heterocíclico contendo um heteroátomo de nitrogênio.

As bases desse tipo são classificadas em cinco:

As bases nitrogenadas se ligam covalentemente com uma pentose (um açúcar) e com um grupo fosfato. No caso das pirimidinas é o N-1; e N-9 das purinas essa ligação com a pentose é feita por uma ligação N-b-glicosídica ao carbono 1 da pentose, e o fosfato é esterificado no carbono 5. Essas estruturas se organizam em pares, formando assim o DNA, caso a pentose seja a desoxirribose. As bases organizam-se aos pares (sempre Adenina-Timina, citosina-Guanina). No caso do RNA, o açúcar com que as bases se ligarão será a ribose e no lugar da Timina entrará o Uracilo (A=U e C≡G).

Ligacao N-beta-glicosidica
Ligacao N-beta-glicosidica

Propriedades Químicas

As bases nitrogenadas são compostos fracamente básicas e por isso são chamados de base. O deslocamento dos elétrons entre os átomos no anel confere а maioria das ligações caráter parcial de ligação dupla, o que, por sua vez, confere as pirimidinas e as purinas aromaticidade. Essa característica confere às bases nitrogenadas algumas formas tautoméricas, dependendo do pH do meio.[2]

Formas tautomericas da uracila

Outras propriedades das bases são[2]:

  • Apresentam comportamento hidrofóbico próximo ao pH neutro da célula;
  • Em meio ácido e alcalino as bases ficam carregadas;
  • A sua solubilidade aumenta em meio ácido ou alcalino;
  • As interações que ocorrem entre as bases na estrutura de ADN é via pontes de hidrogênio.

Emparelhamento das bases nitrogenadas

Interacao das bases nitrogenadas

Encontrando-se o ADN em dupla hélice, às bases, se estáveis, emparelham-se com as respectivas bases complementares: Adenina (A) com Timina (T), Citosina (C) com Guanina (G). No ADN só aparecem as bases ATCG, onde (A) pareia com (T) através de duas ligações de hidrogênio (A=T) e (G) pareia com (C) através de três ligações hidrogênio (C≡G): regra de Chargaff. A base Uracila (U) só aparece no RNA, substituindo a Timina e ligando-se com a Adenina.

Referências

  1. ↑ a b Soukup, Garrett A. (2003), «Nucleic Acids: General Properties», eLS, ISBN 9780470015902 (em inglês), American Cancer Society, doi:10.1038/npg.els.0001335 
  2. ↑ a b Nelson, David L. and Michael M Cox (2008). Lehninger Principles of Biochemistry, ed. 5, W.H. Freeman and Company. p. 272. ISBN 071677108X.
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