Departamento de Química-CCE, Universidade Estadual de Londrina, Londrina, PR, 86051-990, Brazil.
Amino Acids. 2010 Apr;38(4):1089-99. doi: 10.1007/s00726-009-0318-8. Epub 2009 Jul 5.
The present study examined the adsorption of cysteine, thiourea and thiocyanate on bentonite and montmorillonite at two different pHs (3.00, 8.00). The conditions used here are closer to those of prebiotic earth. As shown by FT-IR, Mössbauer and EPR spectroscopy and X-ray diffractometry, the most important finding of this work is that cysteine and thiourea penetrate into the interlayer of the clays and reduce Fe(3+) to Fe(2+), and as consequence, cystine and c,c'-dithiodiformamidinium ion are formed. This mechanism resembles that which occurs with aconitase. This is a very important result for prebiotic chemistry; we should think about clays not just sink of molecules, but as primitive vessels of production of biomolecules. At pH 8.00, an increasing expansion was observed in the following order for both minerals: thiourea > thiocyanate > cysteine. At pH 3.00, the same order was not observed and thiourea had an opposite behavior, being the compound producing the lowest expansion. Mössbauer spectroscopy showed that at pH 8.00, the proportion of Fe(2+) ions in bentonite increased, doubling for thiourea, or more than doubling for cysteine, in both clays. However, at pH 3.00, cysteine and thiourea did not change significantly the relative amount of Fe(2+) and Fe(3+) ions, when compared to clays without adsorption. For thiocyanate, the amount of Fe(2+) produced was independent of the pH or clay used, probably because the interlayers of clays are very acidic and HSCN formed does not reduce Fe(3+) to Fe(2+). For the interaction of thiocyanate with the clays, it was not possible to identify any potential compound formed. For the samples of bentonite and montmorillonite at pH 8.00 with cysteine, EPR spectroscopy showed that intensity of the lines due to Fe(3+) decreased because the reaction of Fe(3+)/cysteine. Intensity of EPR lines did not change when the samples of bentonite at pH 3.00 with and without cysteine were compared. These results are in accordance with those obtained using Mössbauer and FT-IR spectroscopy.
本研究考察了半胱氨酸、硫脲和硫氰酸根在两种不同 pH 值（3.00、8.00）下在膨润土和蒙脱石上的吸附。这里使用的条件更接近前生物地球的条件。傅里叶变换红外光谱、穆斯堡尔和电子顺磁共振以及 X 射线衍射表明，这项工作最重要的发现是，半胱氨酸和硫脲渗透到粘土的层间，并将 Fe(3+)还原为 Fe(2+)，从而形成胱氨酸和 c,c'-二硫代二氨基甲酸离子。这种机制类似于 aconitase 发生的机制。这对于前生物化学来说是一个非常重要的结果；我们不应该仅仅将粘土视为分子的沉降物，而应该将其视为生物分子原始生产的容器。在 pH 8.00 时，对于两种矿物，观察到以下顺序的膨胀增加：硫脲>硫氰酸根>半胱氨酸。在 pH 3.00 时，没有观察到相同的顺序，并且硫脲表现出相反的行为，是产生最低膨胀的化合物。穆斯堡尔光谱表明，在 pH 8.00 时，膨润土中 Fe(2+)离子的比例增加，对于硫脲增加一倍，对于半胱氨酸则增加一倍以上，对于两种粘土都是如此。然而，在 pH 3.00 时，与未吸附的粘土相比，半胱氨酸和硫脲并没有显著改变 Fe(2+)和 Fe(3+)离子的相对含量。对于硫氰酸根，生成的 Fe(2+)量与 pH 或粘土无关，可能是因为粘土的层间非常酸性，形成的 HSCN 不会将 Fe(3+)还原为 Fe(2+)。对于硫氰酸根与粘土的相互作用，无法确定形成的任何潜在化合物。对于 pH 8.00 时的膨润土和蒙脱石与半胱氨酸的样品，电子顺磁共振表明，由于 Fe(3+)/半胱氨酸反应，导致 Fe(3+)的线强度降低。当比较 pH 3.00 时膨润土与和不与半胱氨酸的样品时，EPR 线的强度没有变化。这些结果与穆斯堡尔和傅里叶变换红外光谱得到的结果一致。
