Theoretical Study Of The Identification And Separation Of Cysteine Enantiomers On Metal Surfaces

Chiral molecules are mirror-symmetrical but not completely coincident.They are commonly used in medicine,cosmetics,food additives and other fields.Especially in the pharmaceutical industry,most drugs are composed of chiral molecules.However the left and right chiral molecules usually match and recognize each other with the macromolecules in the living body,and usually exhibit different or even opposite physiological effects.One of them is beneficial to the human body,however the other is harmful.Therefore,the accurate and efficient identification and separation of chiral molecules are of great significance for medical synthesis.In this paper,we selected the cysteine enantiomers with thiol group as the research object,and simulated the adsorption behaviors of cysteine enantiomers on Au(532)step surface through the first-principles-based density functional theory method.By analyzing the adsorption behavior difference of cysteine enantiomers on the substrate,it was proposed to apply external strain and step doping at the substrate to explore ways for enhancing the separation ability of cysteine enantiomers.It is found that applying 2% tensile strain to the Ag-alloyed Au(532)surface leads to a dramatic increase(by 89%)in cysteine enantioselectivity as compared to that of pristine Au(532).In addition,considering the actual needs of industrial production,the effect of temperature on the separation ability of cysteine enantiomers was also explored.The main conclutions of this paper are as follows:(1)When the cysteine adsorbed on Au(532),its thiol group would undergo dehydrogenation reaction,and the effect of van der Waals force on the adsorption system cannot be ignored.The adsorption energy difference of the cysteine enantiomers on Au(532)was 0.18 eV,indicating that the surface has good separation ability for the cysteine enantiomers.By studying the adsorption configuration of the left and right chiral cysteine molecules on Au(532),it is found that the difference in the adsorption energy is mainly due to the different covalent bonds and the difference of adsorption structures between L-and D-cysteine on Au(532).(2)The compressive and tensile strain of 2% and 1% were applied to the substrate.It was found that the cysteine enantiomers have the same stable adsorption configurations under different strain conditions,but their separation abilities were significantly different.When compressive strain was applied to the substrate,the separation ability of cysteine enantiomers on Au(532)is weakened;however when subjected to tensile strain,its separation ability is enhanced.The main reason is that the compressive strain limits the adsorption difference of cysteine enantiomers on Au(532).In order to explore the universality of the method that the external strain can regulate the separation ability of the cysteine enantiomers on Au(532),we researched the separation additions of cysteine enantiomers on other metal surfaces(Ir,Pd,Pt,Cu and Ag).It is found that the application of tensile(compressive)strain enhances(weaks)the separation ability of the chiral cysteine molecules on any metal surface.(3)The separation ability of cysteine enantiomers on different alloy surfaces is significantly different.When the Pd or Pt atom was doped at the step,the separation ability of cysteine enantiomers was weakened;however when the Ag atom was doped,the separation ability was enhanced.In addition,the external strain can also regulate the separation ability of the cysteine enantiomers on alloy surfaces.It is found that the application of tensile(compressive)strain enhances(weaks)the separation ability of cysteine enantiomers on any alloy surface.(4)It is found that the adsorption energy difference is essentially due to the electronic interaction difference between cysteine enantiomers and substrate.The greater the electronic energy difference is,and the stronger the separation ability is no matter what under the strain conditions or on alloy surface.In addition,considering the actual temperature requirements in industrial production,we investigated the effect of temperature on separation ability.It is found that the separation ability of cysteine enantiomers on alloy surface gradually increases with increasing temperature,while the separation ability slightly reduces under strain conditions.However,whether it is under strain conditions or step doping,the metal step surface has good identification and separation ability for cysteine enantiomers at room temperature.