The Electron Transport Of Histidine Molecular Junctions

Biological activities in bio-systems,for example,photosynthesis and cell respiration all require chemical reactions involving long range charge transfer across proteins inside the cell membrane or across the cell membrane.Researches on how charge transfer across complex proteins may help us understand reactions in lives.On the other hand,it could provide possibilities for us to utilize the efficient charge transport across proteins to prepare bio-electronic devices.Peptides as part of protein show similar charge transport properties with less complex structure,serving as an easier tool to answer questions about how to build functional bio-electronic devices.We used self-assembled monolayers of hepta-histidine peptides and histidine doped alanine peptides to study the charge transport properties:1.The conformation of the self-assembled monolayer(SAM)and charge transport of tailor-made oligopeptide hepta-histidine derivative(7-His)was modulated through p H control of the assembly environment.Histidine is found to be an efficient tunneling mediator in monolayer junctions with attenuation factor comparable with conjugated molecule.Successful theoretical model fitting indicates a linear increase of number of tunneling sites as the 7-His SAM thickness increase following deprotonation of histidine.Combined with ultraviolet photoelectron spectroscopy(UPS)measurement,modulable charge transport pathway through 7-His with imidazole groups of histidine as tunneling foot stones is revealed.2.We studied charge transport of a model peptide junction,where oligo-alanine peptide was doped by histidine at different position,and the series of peptides were selfassembled into a monolayer on gold electrode with soft EGa In as top electrode to form molecular junction.It was found that histidine increased the overall conductance of the peptide,meanwhile,its position modulated the conductance as well.Quantitative analysis by transport model and ultraviolet photoelectron spectroscopy(UPS)indicated a sequence dependent energy landscape of the tunneling barrier of the junction.Density-functional theory(DFT)calculation on the electronic structure of histidine doped oligo-alanine peptides revealed localized highest occupied molecular orbital(HOMO)on imidazole group of the histidine,which decreased charge transport barrier.3.We have successfully coupled copper ions into H-1、H-4、H-7 and 7-Ala peptide monolayers.With ARXPS experimental analysis the position of copper ions in the peptide monolayers has been determined.In monolayers of H-1 and H-4 copper ions are believed to couple at a position near the gold substrate,while in monolayers of peptide H-7 and 7-Ala copper ions are found in the middle of the monolayers.From EGa In measured I-V analysis of these copper ions coupled peptide junctions a position dependent modification of the junction energy landscape is revealed.With copper ions coupled to the middle position of the monolayers,as in H-7 and 7-Ala,the tunneling barrier is lowered following by an increase in the junction current,while in H-1 and H-4 copper ions reside at the bottom of the monolayers next to the gold substrate,resulting a minor increase in the junction current,which may originate from the increase in coupling strength between peptide and gold electrode.