Protein Folding In Membrane Investigated By Sum Frequency Generation Spectroscopy

The interaction mechanism between proteins and membranes is crucial to the realization of proteins' biological functions in organisms. Study of the interaction between proteins and membrane will help promote our understanding to the essence of life. The structure of protein is primarily designed by the genetic information from DNA, and finally functionalized in specific local environment. Therefore, it is helpful for us to figure out how protein structure changes when contact with membrane and how protein works in membrane. There are many biological and chemical factors that may affect proteins' activities in biological process, which include the specific molecule structure of protein, salting effects, charge effects, hydration and dehydration, PH value, concentration of peptide, etc.Considering the complexity of peptide-membrane interaction mechanism which surficial specificities can't be ignored, we apply nonlinear vibrational sum frequency spectrum to study vary kinds of factors that have influence on membrane-protein interaction. The investigations focus on the molecular structure of phospholipid, membrane protein secondary structure transition, and concentration effects. Antibacterial peptide pardaxin is chosen as model molecule for the study on phospholipid molecules charge effects and protein concentration effects. We found that increase of negative change on the head group of lipid molecules promote the transition degree of antibacterial peptide pardaxin from random coil structure to the Alfa helix structure, which means charge on lipid may result in different dynamic pathway during the insertion of peptide into membrane. The secondary structure transition in the negatively charged phospholipid membrane implies Alfa helix structure is supported in membrane. In the mixed phospholipid membrane with partial charge, the proportion of protein structure conversion decreased compared to the negative charged phospholipid membrane. For the amphiphilic lipid molecule with no charge on head group, there is no structure transition. In the experiment of peptide concentration effect, we found that the proportion of different secondary structure in the amphiphilic phospholipid membrane was affected by the peptide concentration. Higher proportion of the irregular coiled coil structure was found, compared to more helix structure at low peptide concentration.Throughout the whole investigation, signals at several wavenumber region are collected for the structure analysis, include the amide I band, amide ? band, amide A band, etc. The amide ? signals are capable of separating the spectral profiles of the random-coil and ?-helical structures at the interface. The intensity ratio of coil and helix peaks becomes a prime indicator that allows one to directly capture the dynamical change of the coil-helix transition. With this approach, using pardaxin as a model, the influence of lipid charge on the peptide folding degree at the cell membrane surface has been nicely elucidated. It is evident that the negative charge of the lipid increases the folding degree of pardaxin upon interfacial adsorption and promotes the formation of a-helical structure during the insertion of peptide into the lipid bilayer. This robust spectral approach can thus greatly enhance our ability to monitor the dynamics of membrane proteins in a real cell environment in situ.