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The chiral flip mechanism of S- a -alanine (S- a -Ala) and tetravalent selenium ( Se (IV )) complexes(S-a-Ala Se(IV)) was studied by means of M06 and MN15 methods based on density functional theory and SMD model method of polarized continuum. The study of reaction channels shows that S-a-Ala- Se(lV) can be transferred when the carbonyl O atom as the only medium of a-H proton; the proton transfers to a-C from amino group N after the a-H proton is transferred to carbonyl O atom; after the proton of protonated amino transfers to carbonyl O atom α-H proton achieves chiral flip in three channels with amino group N as the medium of protolysis. Investigation on the potential energy surface shows that the third reaction channel has the most advantage, the free energy barrier is 227.6 kJ/ mol under the effect of recessive solvent, it comes from the transient state of the proton moving from amino group N to α-C, Under the dominant solvent effect, the free energy barrier is 155.6 kJ/mol,it comes from the transient state of the proton moving from protonated amino group N to carbonyl O atom. The results show that chiral α -Ala·Se is difficult to racemize in water-liquid phase, and thus it is relatively safe to be used to supply α-Ala and Se(IV) for life simultaneously. [ABSTRACT FROM AUTHOR]

The chiral flip mechanism of S- α-alanine (S- α-Ala) and tetravalent selenium (Se (IV)) complexes (S-α-Ala⋅Se (IV)) was studied by means of M06 and MN15 methods based on density functional theory and SMD model method of polarized continuum. The study of reaction channels shows that S-α-Ala⋅Se (IV) can be transferred when the carbonyl O atom as the only medium of α-H proton; the proton transfers to α-C from amino group N after the α-H proton is transferred to carbonyl O atom; after the proton of protonated amino transfers to carbonyl O atom, α-H proton achieves chiral flip in three channels with amino group N as the medium of protolysis. Investigation on the potential energy surface shows that the third reaction channel has the most advantage, the free energy barrier is 227.6 kJ/mol under the effect of recessive solvent, it comes from the transient state of the proton moving from amino group N to α-C; Under the dominant solvent effect, the free energy barrier is 155.6 kJ/mol, it comes from the transient state of the proton moving from protonated amino group N to carbonyl O atom. The results show that chiral α-Ala⋅Se (IV) is difficult to racemize in water-liquid phase, and thus it is relatively safe to be used to supply α-Ala and Se (IV) for life simultaneously. [ABSTRACT FROM AUTHOR]

The study investigated the effect of water molecule(cluster) and aqueous solvent on S-proline( Pro)divalent copper complex(S-Pro·Cu(II))enantiomerization under the gas phase using the methods of M06 and MN15 based on density functional theory combining with self-consistent reaction field of SMD model. The results showed that gas phase Cu2+ and carbonyl O monodentate ligand is the most stable. The enantiomerization of S-Pro•Cu(II)can be realized in 5 channels a, b, c, d and e. In channel a, b and c α -H transfers directly with O, Cu and N as a bridge respectively. After Cu and carbonyl O are bidentately coordinated α-H transfers with O and O and N unite as a bridge in channel c and d(α-H transfers with Cu as a bridge, it is hydrogen anion migration. The other is hydrogen proton migration). The potential energy surface calculation shows that the dominant pathways of the enantiomerization of isolated S-Pro•Cu(II) are pathways b and c, and the activation energy of the reaction is 153. 4 and 160. 3 kJ·mol, 1 respectively. The dominant pathway is channel c in the water vapor phase environment and the activation energy of the reaction is 143. 7 kJ·mol-1. Channel b is the most disadvantaged, and the enantiomerization in this channel cannot be achieved. The activation energy of channel c drops to 89. 9 kJ·mol-1 under the effect of aqueous solvent. The results show that Pro•Cu) can maintain its chiral characteristics well in the gas phase, and is easy to be enantiomerized in the water vapor phase. [ABSTRACT FROM AUTHOR]

The optical isomerization of amphoteric S-α-Ala·Na+ in water-liquid phase environment was investigated by using the M06-2X method based on density functional theory and the SMD model method based on self consistent reaction field (SCRF) theory. The results showed that the optical isomerization of S-α-Ala·Na+ has three channels a,b and c. In channel a, the proton transfers with amino nitrogen as a bridge after the isomerization of amphoteric S-α-Ala·Na+ to neutral S-α-Ala·Na+. And the proton is transferred merely using carbonyl oxygen as a bridge in channel b. In channel c, the proton transfers to α-C from protonation amino after α-H is transferred to carbonyl oxygen. The potential energy surface showed that the channel a has the most advantage under the effect of recessive solvent and the energy barrier of the rate-determining step is 234. 8 kJ·mol-1; While channel b and c had a disadvantage, they both have the common energy barrier of the rate-determining step about 267. 9 kJ·mol-1. Under the effect of explicit solvent, channel a turned to an inferior channel, and its energy barrier of the rate-determining step is between 155. 9 and 156. 5 kJ·mol-1; channel b and c have a slight advantage and the energy barrier of the rate-determining step is between 132.2 and 138.6 kJ·mol-1. The results showed that α-alanineNa (I) complex could only racemize at a extremely slow speed in the water-liquid environment. [ABSTRACT FROM AUTHOR]