Comparison of Raman and Surface FT-IR to Determine the Orientation of α-Synuclein (61-95) in Monolayer

Abstract

Membrane proteins poses a lot of challenges for analytical techniques. Thus, it is not a surprise that they are reported to only account for 2.4 % of the solved structures in protein databank, despite being encoded by ~20-30 % of all genes in total genomes. Resolving membrane protein structure is critical because the malfunction of proteins causes many diseases. For instance, the misfolding and subsequent aggregation of alpha-synuclein (α-syn), a protein made up of 140 amino acids, is currently thought to be a main cause of dopaminergic degeneration in Parkinson's disease. An important segmental peptide spanning residues 61-95 also known as the nonamyloid component (NAC) has been detected in the brain of Parkinson disease patients. α-Syn accumulates in the presynaptic terminals where high concentrations of vesicles exist at the amphiphilic interface. However, the reason of the accumulation of α-syn in the presynaptic terminals has been unclear due to the complication of the membrane structure. Fortunately, the amphiphilic membrane structure has been mimicked by the air-water interface as a simple model by a Langmuir monolayer technique. When combined with surface FTIR techniques such as p-polarized multiple-angle incidence resolution spectroscopy (pMAIRS), the conformation and orientation of membrane proteins can be evaluated by deconvoluting the amide I band. In this study, 13C isotope was introduced into α-syn (61-95) at 93G to provide residue-level information. Then, the effect of varying the holding time on the orientation was also investigated. In addition, Raman spectroscopy was also employed in this study and pMAIRS prove to be effective in determining the orientation of α-syn (61-95) even in monolayer.