【作者】 李平；

【导师】 步宇翔；

【作者基本信息】 山东大学 ， 物理化学， 2006， 博士

【摘要】 随着X-ray和NMR等实验手段分辨率的提高，含顺式肽键单元的蛋白质数目也随之增加。作为一种反常的存在形式，顺式肽键单元的出现总是具有功能或结构上的特定意义，但对其存在形式和稳定性尚无明确的认识。此外，低垒氢键在酶催化中的存在性及其意义也是人们目前争论的焦点问题之一。鉴于此，本论文就重点围绕顺式肽键和低垒氢键的相关问题，利用密度泛函方法（DFT），系统考察了顺式肽键单元模型化合物——甘氨酰胺及其衍生物的相关性质和低垒氢键在丝氨酸蛋白酶催化进程中的存在性及其意义，并取得了一些有意义的研究成果，具体如下： （1） 首先，我们系统地考察了甘氨酰胺在气相和液相中的构象行为。结果表明：在气相中，存在七种稳定的构象，分别对应于三对镜像异构体和一个具有Cs平面对称性的构象（分别记作构象Ⅰ、Ⅱ、Ⅲ和Ⅳ），其中的镜像异构体可以进一步通过圆二色光谱（VCD）得到证实。通过与相关实验和理论数据的比较，澄清了各种构象之间的相对稳定性，确认了甘氨酰胺的最稳定构象，从而为后续研究其相关性质建立了一个统一的基准。进而探讨了其相关的光谱性质，如红外光谱、拉曼光谱和核磁共振等光谱，为实验上各种构象的检测和光谱指认提供了理论依据。在溶液中，各种构象的相对稳定性由气相中的Ⅰ＞Ⅱ＞Ⅲ＞Ⅳ＞变为Ⅰ＞Ⅱ＞Ⅳ＞Ⅲ，其中构象Ⅰ仍为最稳定构象。相对于气相结构，除构象Ⅱ中的二面角发生较大变化以外，其余构象的结构参数变化不大。在红外光谱特征上，主要表现为红外吸收强度的增加和特征谱羰基振动频率的明显红移。 （2） 鉴于甘氨酰胺相关性质的缺乏，在上述气相构象的基础上，我们进一步系统考察了其酸碱性和氧化还原性。结果表明，在气相中，甘氨酰胺中三个碱性位点的相对活性为：氨基N＞羰基O＞酰胺N；三个酸性位点的相对活性为：酰胺N＞烃基C＞氨基N。因此，甘氨酰胺在气相中主要表现为氨基N碱和酰胺N酸。与去质子化相比，在气相中，甘氨酰胺更容易表现为以烃基C原子为中心的自由基。值得一提的是，计算的质子亲合势与质谱实验中得到的结果非常吻合，充分证实了所选模型和计算方法的可靠性。在氧化过程中，失去单电子后的甘氨酰胺构象以两个C原子之间键的显著伸长为结构特征，并从理论上预测了-NH₂取代-OH会使得体系更容易被氧化。计算的价键型电子亲合势均为负值，表明在气相中甘氨酰胺不能使得一个额外的电子填充到更多还原

【Abstract】 More and more proteins containing cis peptide bonds have been found with the increasing resolution of X-ray and NMR experiments. Being an abnormal occurrence of peptide bonds, the cis peptide bonds are always associated with the certain structures and functional performances. However, the structural forms and stabilities of them have not yet been clarified systematically to date. In addition, there has still been a hot debate on the existence and significance of low-barrier H-bond (LBHB) in catalytic triad of serine proteases. Thus, in the present doctoral dissertation, relevant studies on the model compound glycinamide possessing the cis peptide bond unit and the existence and significance of the LBHB in the catalytic triad of serine proteases have been systematically and extensively investigated employing the Density Functional Theory (DFT). As a result, some significant progresses have been made, which can be described as follows.(1) Firstly, to pave the way for the future study, we have investigated the conformational behaviors of glycinamide in the gas phase and in solution systematically. A total of seven conformers have been located on the potential energy surface (PES) of glycinamide in the gas phase, corresponding to three pairs of mirror-image conformers and one conformer possessing the Cs symmetry. Analyses of relative energies and comparisons with relevant experimental results suggest that the global minimum of glycinamide should be characterized by the intramolecular H-bond formed between amide and amino group, which can be further confirmed by the interatomic distances, charge distributions, and vibrational analyses. Furthermore, relevant spectroscopic properties, such as IR, Raman, NMR, and VCD spectra, have been discussed to provide some helpful information for the identification of the available conformers in the gas phase. In solution, the relative stabilities among the available conformers remain the same as those of the gas phase except for the reverse order between two minor conformers due to their larger differences in dipole moments. Some significant structural changes in dihedral angles have been observed for the second minimum upon solvation. Additionally, all the IR intensities are much stronger than those in the gas phase and the characteristic bands of carbonyl group have been red-shifted.更多还原