Bieri Marco, Bürgi Thomas
Institut de Chimie, Université de Neuchâtel, Rue Emile-Argand 11, 2007- Neuchâtel, Switzerland.
J Phys Chem B. 2005 Dec 1;109(47):22476-85. doi: 10.1021/jp052409m.
The adsorption of N-acetyl-L-cysteine from ethanol solution on gold has been studied by in situ attenuated total reflection infrared (ATR-IR) spectroscopy, polarization modulation infrared reflection absorption spectroscopy, and a quartz crystal microbalance. After an initial fast adsorption, in situ ATR-IR revealed two considerably slower processes, besides further adsorption. The appearance of carboxylate bands and the partial disappearance of the carboxylic acid bands demonstrated that part of the molecules on the surface underwent deprotonation. In addition, the C=O stretching vibration of the carboxylic acid group shifted to lower and the amide II band to higher wavenumbers, indicating hydrogen-bonding interactions within the adsorbate layer. Based on the initial ATR-IR spectrum, which did not reveal deprotonation, the orientation of the molecule within the adsorbate layer was determined. For this, density functional theory was used to calculate the transition dipole moment vectors of the vibrational modes of N-acetyl-l-cysteine. The projections of the latter onto the z-axis of the fixed surface coordinate system were used to determine relative band intensities for different orientations of the molecule. The analysis revealed that the amide group is tilted with respect to and points away from the surface, whereas the carboxylic acid is in proximity to the surface, which is also supported by a shift of the C-O-H bending mode. This position of the acid group favors its deprotonation assisted by the gold surface and easily enables intermolecular interactions. Periodic acid stimuli revealed reversible protonation/deprotonation of part of the adsorbed molecules. However, only non-hydrogen-bonded carboxylic acid groups showed a response toward the acid stimuli.
通过原位衰减全反射红外光谱（ATR-IR）、偏振调制红外反射吸收光谱和石英晶体微天平研究了乙醇溶液中N-乙酰-L-半胱氨酸在金表面的吸附情况。在初始快速吸附之后，原位ATR-IR显示除了进一步吸附外，还有两个相当缓慢的过程。羧酸盐谱带的出现和羧酸谱带的部分消失表明表面上的部分分子发生了去质子化。此外，羧酸基团的C=O伸缩振动向低波数移动，酰胺II谱带向高波数移动，表明吸附层内存在氢键相互作用。基于未显示去质子化的初始ATR-IR光谱，确定了吸附层内分子的取向。为此，使用密度泛函理论计算N-乙酰-L-半胱氨酸振动模式的跃迁偶极矩矢量。将后者投影到固定表面坐标系的z轴上，用于确定分子不同取向的相对谱带强度。分析表明，酰胺基团相对于表面倾斜并远离表面，而羧酸靠近表面，这也得到了C-O-H弯曲模式位移的支持。酸基团的这种位置有利于其在金表面辅助下的去质子化，并易于实现分子间相互作用。高碘酸刺激显示部分吸附分子发生可逆的质子化/去质子化。然而，只有非氢键结合的羧酸基团对酸刺激有响应。
