摘要
构成地球上的生命体蛋白质有20种氨基酸,唯有甘氨酸没有手性,并且甘氨酸是最简单的氨基酸。研究甘氨酸对蛋白质结构和生物体手性的研究有重大意义。它在结晶状态上,有α、β、γ三种形态,并且以两性离子的形式存在(NH3+CH2COO?)。α-甘氨酸具有单斜结构,空间点群分别是P21/n。我们用恒温慢蒸发法制得了高纯度的α-甘氨酸单晶。
1999年Chilcott等人发现,通常是绝缘体的α-甘氨酸,在304K左右有导电行为,但导电机理不清楚。为了了解其导电机理,本文中研究了α-甘氨酸单晶的变温拉曼光谱,发现晶体中NH3+扭曲振动模式分裂,表现为N?H(6)…O(1) (491 cm?1)和N?H(7)…O(2) (483cm?1)模式,以及CO2?摇摆模式(503 cm?1),在304K均有不连续性变化。由于α-甘氨酸晶胞中NH3+和CO2?基团构成的电偶极子在变温下重新定向,出现两性离子电荷重心变化致使晶体极化,引起晶体在304K左右发生了铁电相变。
我们也研究了α-甘氨酸多晶粉末的变温拉曼光谱,发现它和单晶有类似的行为,NH3+扭曲振动模式分裂,并且在304K有不连续性变化。在整个α-甘氨酸晶体结构中,极化是晶体本身内在的一种性质,所以α型-甘氨酸多晶粉末在大约在304K也可能发生了铁电相变。同时发现晶格振动(162cm-1)、CH2弯曲振动、CH2扭曲振动、CH2反对称伸缩振动模式和COO -1弯曲振动、COO -1反对称伸缩振动、CN伸缩振动以及NH3+扭曲振动、NH3+对称变形模式在90-170K和308-400K两个温度区域的分裂,它们分裂的不同原因被解释。
Glycine, a biologically important compound, is the simplest and uniquely achiral among the amino acids. Investigation of glycine could be far-reaching with respect to protein structure and the origins of biochirality. In the crystalline state, glycine hasα,βandγ-polymorphs, existing as zwitterionic form (NH3+CH2COO?).α-glycine has monoclinic structure (space group symmetry P21/n) with four formula units per unit cell. High-purity single crystals ofα-glycine were grown from saturated aqueous solution ofα-glycine by slow evaporation at 277K. In 1999, Chilcott found that electrical impedance measurements of single crystal glycine revealed anomalous temperature dependence of the conductance and capacitance around 304K. This unusual electric behavior was not explained by the conduction mechanism. The present paper investigates temperature-dependent Raman spectra ofα-glycine single crystal at different temperatures. We find a discontinuous frequency variation of the NH3+ torsional mode ofα-glycine crystal around 304K, which was assumed due to the NH3+ torsion mode splitting and deformations. The electrical dipoles of NH3+CH2COO? are reoriented and the crystal had to undergo a ferroelectric transition around 304K.
Raman spectra ofα-glycine polycrystalline powder are also studied from 90 to 400K. we found frequency variation of the NH3+ torsional mode in polycrystalline powder exhibits a similar discontinuous change around 304K. Because the polarization of a ferroelectric is an effect inherent in the entire structure of the crystal, polycrystalline powder could also undergo a ferroelectric transition around 304K. 162cm-1 ,CH2 twist ,CH2bend ,CH2 asym str, COO -1bend, COO -1asym str, CN str , NH3+ torsion and NH3+ sym def mode show splitting , the reasons of which are explained.
引文
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