Como os metais trabalham juntos para enfraquecer as ligações resistentes de nitrogaªnio-nitrogaªnio
O nitrogaªnio, um elemento essencial para todas as células vivas, constitui cerca de 78% da atmosfera da Terra. No entanto, a maioria dos organismos não pode fazer uso desse nitrogaªnio atéque seja convertido em ama´nia.
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Domanio paºblico
O nitrogaªnio, um elemento essencial para todas as células vivas, constitui cerca de 78% da atmosfera da Terra. No entanto, a maioria dos organismos não pode fazer uso desse nitrogaªnio atéque seja convertido em ama´nia. Atéque os humanos inventassem os processos industriais para a santese de ama´nia, quase toda a ama´nia no planeta era gerada por micróbios usando nitrogenases, as únicas enzimas que podem quebrar a ligação nitrogaªnio-nitrogaªnio encontrada no dinitrogaªnio gasoso, ou N2.
Essas enzimas contem aglomerados de a¡tomos de metal e enxofre que ajudam a realizar essa reação cratica, mas o mecanismo de como o fazem não bem compreendido. Pela primeira vez, os químicos MIT agora determinada a estrutura de um complexo que se forma quando a N2 liga-se a esses grupos, e descobriram que os aglomerados sejam capazes de enfraquecer o azoto -azoto ligação a uma medida surpreendente.
"Este estudo nos permite obter insights sobre o mecanismo que permite ativar essa molanãcula realmente inerte, que tem uma ligação muito forte que édifacil de quebrar", diz Daniel Suess, professor assistente de Química da classe de desenvolvimento de carreira de 48 em MIT e o autor saªnior do estudo.
Alex McSkimming, um ex-pa³s-doutorado do MIT que agora éprofessor assistente na Tulane University, éo principal autor do artigo, que aparece hoje na Nature Chemistry .
Fixação de nitrogaªnio
O nitrogaªnio éum componente crítico das proteanas, DNA e outras moléculas biológicas. Para extrair nitrogaªnio da atmosfera, os primeiros micróbios desenvolveram nitrogenases, que convertem o gás nitrogaªnio em ama´nia (NH3) por meio de um processo denominado fixação de nitrogaªnio. As células podem então usar essa ama´nia para construir compostos contendo nitrogaªnio mais complexos.
"A capacidade de acessar o nitrogaªnio fixo em grandes escalas foi fundamental para permitir a proliferação da vida", diz Suess. "O dinitrogaªnio tem uma ligação muito forte e não éreativo, então os químicos basicamente o consideram uma molanãcula inerte. a‰ um quebra-cabea§a que a vida teve que descobrir: como converter essa molanãcula inerte em espanãcies químicas aºteis."
Todas as nitrogenases contem um aglomerado de a¡tomos de ferro e enxofre, e alguns deles também incluem molibdaªnio. Acredita-se que o dinitrogaªnio se ligue a esses aglomerados para iniciar a conversão em ama´nia. No entanto, a natureza dessa interação não éclara e, atéagora, os cientistas não foram capazes de caracterizar a ligação do N2 a um aglomerado de ferro-enxofre.
Para esclarecer como as nitrogenases se ligam ao N2, os químicos desenvolveram versaµes mais simples de aglomerados de ferro-enxofre que podem ser usados ​​para modelar os aglomerados que ocorrem naturalmente. A nitrogenase mais ativa usa um aglomerado de ferro-enxofre com sete a¡tomos de ferro, nove a¡tomos de enxofre, um a¡tomo de molibdaªnio e um a¡tomo de carbono. Para este estudo, a equipe do MIT criou um que tem três a¡tomos de ferro, quatro a¡tomos de enxofre , um a¡tomo de molibdaªnio e nenhum carbono.
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Um desafio em tentar imitar a ligação natural do dinitrogaªnio ao aglomerado de ferro-enxofre éque, quando os aglomerados estãoem uma solução, eles podem reagir com eles mesmos em vez de se ligar a substratos como o dinitrogaªnio. Para superar isso, Suess e seus alunos criaram um ambiente protetor ao redor do cluster, anexando grupos químicos chamados ligantes.
Os pesquisadores anexaram um ligante a cada um dos a¡tomos de metal, exceto um a¡tomo de ferro, que éonde o N2 se liga ao cluster. Esses ligantes evitam reações indesejadas e permitem que o dinitrogaªnio entre no aglomerado e se ligue a um dos a¡tomos de ferro. Uma vez que essa ligação ocorreu, os pesquisadores foram capazes de determinar a estrutura do complexo usando cristalografia de raios-X e outras técnicas.
Eles também descobriram que a ligação tripla entre os dois a¡tomos de nitrogaªnio do N2 éenfraquecida de forma surpreendente. Esse enfraquecimento ocorre quando os a¡tomos de ferro transferem grande parte de sua densidade de elanãtrons para a ligação nitrogaªnio-nitrogaªnio, o que torna a ligação muito menos esta¡vel.
Cooperação de cluster
Outra descoberta surpreendente foi que todos os a¡tomos de metal no aglomerado contribuem para essa transferaªncia de elanãtrons, não apenas o a¡tomo de ferro ao qual o dinitrogaªnio estãoligado.
"Isso sugere que esses clusters podem cooperar eletronicamente para ativar essa ligação inerte", diz Suess. "A ligação nitrogaªnio-nitrogaªnio pode ser enfraquecida por a¡tomos de ferro que, de outra forma, não a enfraqueceriam. Por estarem em um cluster, eles podem fazer isso cooperativamente."
As descobertas dos pesquisadores também confirmaram que versaµes mais simples do cluster ferro-enxofre , como as que eles criaram para este estudo, podem enfraquecer efetivamente a ligação nitrogaªnio-nitrogaªnio. Os primeiros micróbios a desenvolver a capacidade de fixar nitrogaªnio podem ter desenvolvido tipos semelhantes de aglomerados simples, diz Suess.
Suess e seus alunos estãoagora trabalhando em maneiras de estudar como as versaµes mais complexas e naturais de aglomerados de ferro- enxofre interagem com o de dinitrogaªnio.