The Theoretical Research Of The Neutral Leucine And Leucine Interacts With Waters In The Gas Phase

Density functional theory calculations of structures and vibrational modes are reported for the leucine in the gas phase, using B3LYP gradient-corrected hybrid density functionals, standard 6-311++g(2d,p) basis sets on all atoms.A single point energy calculations for the leucine stable conformers at B3LYP and MP2 level, using 6-311++g(2d,p) and cc-pVQZ basis sets.The result indicates:Density functional theory for calculating the two lowest energy conformationsâ… b1 andâ…¡b1 is consistent with the experimental measure results.The nine conformations which contain the first type of hydrogen bond using the QM/MM method have been studied, and found that the conformational energy difference betweenâ… a3 andâ… b1 is about 20kJ/mol. This energy difference is mainly determined by steric hindrance.We reoptimize the conformationâ… b1 and the conformationâ…¡b1,using a variety of basis sets in B3LYP and MP2 methods, and compare the rotational constants with the experimental results, we found when using the MP2 method, the rotational constants ofâ… b1 is more close to the experimental results, but when using the B3LYP method, the rotational constants ofâ…¡b1 is more close to the experimental results.Leucine interacts with waters through hydrogen bonds.The stable structures of neutral or zwitterionic leucine combined with one to two waters and their interactions energy have been calculated, the relative ability of different pattern of hydrogen bond have been determined, the IR spectrums have been presented. When the leucine involve different pattern of intramolecular hydrogen bond, water in the most stable compounds occupy different regions. When the leucine monomer includes the first type of hydrogen bond, water interacts with carboxyl in the leucine. Whichever conformers include the first type of hydrogen bond, the binding energy is close to each other, the hydrophobia effect is negligible.When water appears in other regions, the differences of binding energy is remarkable, in these conformers the hydrophobia effect is significant. The conformers involve the second type hydrogen bond appear stronger hydrogen bond inside themselves, if water locates off the intramolecular hydrogen bond is more stable.It is not found the complex of zwitterionic leucine combine one water. There are the complexes of zwitterionic leucine with two waters,but the binding energy is higher than nonionized leucine combine two water. The most stable structure of nonionized leucine combines two water is quadrangle-like,in this structure the effect of electronic conjugate is reinforced, the electron can run in larger area, resulted in the lower molecular energy.The strongest hydrogen bond type is that hydroxyl in the leucine interacts with oxygen in the water, in this structure the frequency of the O-H stretching vibration drop most.