Departamento de Química-CCE, Universidade Estadual de Londrina , Brazil .
Astrobiology. 2011 Jun;11(5):409-18. doi: 10.1089/ast.2010.0521. Epub 2011 Jun 14.
Minerals adsorb more amino acids with charged R-groups than amino acids with uncharged R-groups. Thus, the peptides that form from the condensation of amino acids on the surface of minerals should be composed of amino acid residues that are more charged than uncharged. However, most of the amino acids (74%) in today's proteins have an uncharged R-group. One mechanism with which to solve this paradox is the use of organophilic minerals such as zeolites. Over the range of pH (pH 2.66-4.50) used in these experiments, the R-group of histidine (His) is positively charged and neutral for alanine (Ala), cysteine (Cys), and methionine (Met). In acidic hydrothermal environments, the pH could be even lower than those used in this study. For the pH range studied, the zeolites were negatively charged, and the overall charge of all amino acids was positive. The conditions used here approximate those of prebiotic Earth. The most important finding of this work is that the relative concentrations of each amino acid (X=His, Met, Cys) to alanine (X/Ala) are close to 1.00. This is an important result with regard to prebiotic chemistry because it could be a solution for the paradox stated above. Pore size did not affect the adsorption of Cys and Met on zeolites, and the Si/Al ratio did not affect the adsorption of Cys, His, and Met. ZSM-5 could be used for the purification of Cys from other amino acids (Student-Newman-Keuls test, p<0.05), and mordenite could be used for separation of amino acids from each other (Student-Newman-Keuls test, p<0.05). As shown by Fourier transform infrared (FT-IR) spectra, Ala interacts with zeolites through the [Formula: see text] group, and methionine-zeolite interactions involve the COO, [Formula: see text], and CH(3) groups. FT-IR spectra show that the interaction between the zeolites and His is weak. Cys showed higher adsorption on all zeolites; however, the hydrophobic Van der Waals interaction between zeolites and Cys is too weak to produce any structural changes in the Cys groups (amine, carboxylic, sulfhydryl, etc.); thus, the FT-IR and Raman spectra are the same as those of solid Cys.
矿物质对带电荷 R 基团的氨基酸的吸附能力大于不带电荷 R 基团的氨基酸。因此，在矿物质表面由氨基酸缩合形成的肽应该由带电荷的氨基酸残基组成。然而，在今天的蛋白质中，大多数氨基酸（74%）具有不带电荷的 R 基团。解决这一悖论的一种机制是使用亲有机物的矿物质，如沸石。在这些实验中使用的 pH 值范围内（pH2.66-4.50），组氨酸（His）的 R 基团带正电荷，丙氨酸（Ala）、半胱氨酸（Cys）和蛋氨酸（Met）的 R 基团呈中性。在酸性热液环境中，pH 值可能比本研究中使用的 pH 值还要低。对于研究的 pH 值范围，沸石带负电荷，所有氨基酸的总电荷为正。这里使用的条件近似于原始地球的条件。这项工作最重要的发现是，每种氨基酸（X=His、Met、Cys）与丙氨酸（X/Ala）的相对浓度接近 1.00。这是前生物化学中的一个重要发现，因为它可能是解决上述悖论的一种方法。孔径大小不影响半胱氨酸和蛋氨酸在沸石上的吸附，硅铝比不影响半胱氨酸、组氨酸和蛋氨酸的吸附。ZSM-5 可用于从其他氨基酸中纯化半胱氨酸（Student-Newman-Keuls 检验，p<0.05），丝光沸石可用于分离氨基酸（Student-Newman-Keuls 检验，p<0.05）。傅里叶变换红外（FT-IR）光谱表明，丙氨酸通过[Formula: see text]基团与沸石相互作用，蛋氨酸与沸石的相互作用涉及 COO、[Formula: see text]和 CH(3)基团。FT-IR 光谱表明，沸石与组氨酸的相互作用较弱。半胱氨酸在所有沸石上的吸附率都较高；然而，沸石与半胱氨酸之间的疏水范德华相互作用太弱，无法对半胱氨酸基团（胺、羧酸、巯基等）产生任何结构变化；因此，FT-IR 和拉曼光谱与固态半胱氨酸相同。
