气相中亮氨酸单体构象的理论研究

Theoretical study of the Leucine monomer in gas phase

在密度泛函(DFT)B3LYP/6-311++g(2d,p)水平上,优化了气相中亮氨酸单体的稳定构象,在密度泛函和多体微扰2种理论水平上,分别采用6-311++g(2d,p)和cc-pVQZ基组对单体的稳定构象进行了单点能计算.结果表明:密度泛函理论计算出的2个能量最低构象Ⅰb1和Ⅱb1与实验测出的2种构象一致.用QM/MM方法计算了第1种氢键类型下9种构象的构象能,能量最高的Ⅰa3构象比能量最低的Ⅰb1构象的能量高出大约20 kJ/mol,这种能量差异主要是由空间位阻决定的.对Ⅰb1和Ⅱb1构象,采用B3LYP和MP2方法,多种基组进行重新优化,通过比照实验的旋转常数,可以发现Ⅰb1构象用MP2方法优化得到的旋转常数与实验值较接近,而Ⅱb1构象用B3LYP方法优化得到的旋转常数与实验值更接近.

In the density functional theory（DFT） B3LYP/6-311＋＋g（2d,p） level,we have optimized the stable conformations of leucine monomer in gas phase,and in the density functional and multi-body perturbation theory level,we computed the single point energy of the leucine monomer with 6-311＋＋g（2d,p） and cc-pVQZ basis sets,respectively.The result indicates that density functional theory for calculating the two lowest energy conformations Ⅰb1 and Ⅱb1 is consistent with the experimental measure results.We studied the nine conformations which contain the first type of hydrogen bond with the QM/MM method,and found that the conformation energy difference between Ⅰa3 and Ⅰb1 is about 20 kJ/mol.This energy difference is mainly determined by steric hindrance.We reoptimized the conformationⅠb1 and the conformation Ⅱb1 using a variety of basis sets in B3LYP and MP2 methods,and compared the rotational constants with the experimental results.We found that when using the MP2 method,the rotational constants of Ⅰb1 is more close to the experimental results,but when using the B3LYP method,the rotational constants of Ⅱb1 is more close to the experimental results.