1 Cristais Anidros das Bases do ADN s˜ ao Semicondutores de Gap Largo
1.3 Sum´ ario do Cap´ıtulo
que as bases nucleot´ıdicas nos cristais anidros (e possivelmente para cadeias lineares) comportam-se como um semicondutor de gap largo para el´etrons se movimentando ao longo da dire¸c˜ao de empilhamento, enquanto o transporte de buracos ´e limitado no empilhamento envolvendo a timina.
O empilhamento de bases nucleot´ıdicas do ADN idˆenticas nos cristais anidros ´e res- pons´avel diretamente por suas caracter´ısticas semicondutoras. Por outro lado, o empilha- mento de bases do ADN distintas n˜ao mudar´a radicalmente a caracter´ıstica de transporte de cargas de um cristal hipot´etico formado a partir de duas ou mais bases. Estendendo esse racioc´ınio para o empilhamento de trˆes, quatro ou mais bases de ADN, seguindo qual- quer arranjo, pode-se concluir que a caracter´ıstica semicondutora das cadeias de ADN ´e devido principalmente ao empilhamento das bases nucleot´ıdicas, um resultado de sig- nificativa relevˆancia. Desvios de sua caracter´ıstica semicondutora pode ser atribu´ıda `a influˆencia das cadeias a¸c´ucar-fosfato, de mol´eculas de ´agua assim como de ´ıons [9]. Um
gap de energia direto 2,73 eV foi obtido para o cristal de guanina monohidratado atrav´es
de c´alculos de primeiros princ´ıpios utilizando o funcional LDA , 50 meV maior do que o
gap direto de 2,68 eV encontrado nesta tese para o cristal anidro de guanina. Nesse caso,
n˜ao se pode sugerir que s˜ao as mol´eculas de ´agua respons´aveis por esse acr´escimo devido ao fato de que procedimentos, figuras e, inclusive, o pacote (programa) de c´alculo s˜ao diferentes entre o apresentando por Ortmann et al. [25, 26], uma vez que tais diferen¸cas metodol´ogicas tamb´em podem levar a uma mudan¸ca na estimativa do gap de energia do cristal monohidratado da guanina.
1.3
Sum´ario do Cap´ıtulo
Resumindo, neste cap´ıtulo realizamos c´alculos DFT na obten¸c˜ao da geometria otimi- zada para os cristais anidros das quatro bases nucleot´ıdica do ADN, guanina (G), adenina (A), citosina (C) e timina (T) usando a aproxima¸c˜ao da densidade local (LDA) para o funcional de troca e correla¸c˜ao e estimamos os gaps de energia desses cristais a partir de medidas de absor¸c˜ao ´opticas. Os gaps obtidos por meios de c´alculos LDA mostram valores menores do que as medidas experimentais, como esperado, e os valores estimados a partir da absor¸c˜ao ´optica mostradas aqui s˜ao, em geral, menores do que os dados experimen- tais dispon´ıveis na literatura (exceto a guanina). A ordem crescente dos gaps de energia te´oricos (LDA) ´e G < A < C < T, enquanto a sequˆencia experimental n˜ao ´e consensual: a sequˆencia obtida nesta tese (a partir de medidas de absor¸c˜ao ´optica) ´e A < G < C < T, em contraste com as medidas espectroc´opicas de raios-X realizadas por MacNaughton et
64 1 Cristais Anidros das Bases do ADN s˜ao Semicondutores de Gap Largo
al. [60] que obtiveram a ordem G < C < A < T nos valoes dos gaps de energia. Para os
el´etrons e buracos se movendo ao longo de determinadas liga¸c˜oes de hidrogˆenio (paralelas ao plano molecular do cristal de uma dada base), as massas efetivas s˜ao geralmente eleva- das, exce¸c˜ao feita `a timina. Entretanto, quando os mesmos el´etrons se movem ao longo do eixo do empilhamento π − π, as massas efetivas ficam entre os 4,0 e 6,3 vezes a massa do el´etron livre (m0), sugerindo que as bases nucleot´ıdicas empilhadas comportam-se como
um semicondutor de gap largo para os el´etrons. O transporte de buracos perpendicular ao plano molecular tamb´em e favorecido pelo empilhamento das bases, exceto a timina. Finalmente, a fun¸c˜ao diel´etrica foi calculada para cada cristal anidro, onde foi observado uma anisotropia muito acentuada para a luz incidente polarizada nos casos da guanina, adenina e timina, mas n˜ao para a citosina.
65
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