Conformation change of α-synuclein(61-95) at the air-water interface and quantitative measurement of the tilt angle of the axis of its α-helix by multiple angle incidence resolution spectroscopy

Various techniques have been developed to determine protein's structure to understand how proteins work. Compared with X-ray crystallography requiring proteins to form single crystal structure and NMR which usually needs long time measurement, surface FT-IR techniques are able to quickly determine the tilt angle (the key information to determine whether the alpha-helix is transmembrane) of peptides/proteins in a monolayer at the interface (e.g. membranes). Specifically, for alpha-helical peptides/proteins in membrane, the tilt angle of the axis is one of the key information. In this paper, Multiple Angle Incidence Resolution Spectroscopy (MAIRS), a recently developed surface FTIR technique, was applied for the first time to quantitatively determine the tilt angle of the axis of alpha-helical model peptide related to alpha-synuclein (alpha-syn). alpha-Syn is a 140-amino-acid presynaptic protein whose aggregation is the hallmark of Parkinson's disease (PD). It is difficult for alpha-syn to form a single crystal structure and the primary structure of alpha-syn constitutes three domains: the N-terminus containing residues 1-60; the nonamyloid component (NAC) which spans residues 61-95 and is highly prone to aggregation; and C-terminus with residues 96-140. Here, the NAC part (i.e., alpha-syn(61-95)) responsible for the aggregation was found to change its unstructured conformation in aqueous solution to alpha-helix at the air-water interface by circular dichroism and MAIRS. In addition, the instinct power of MAIRS to quantitatively measure the tilt angle of the axis of alpha-helical alpha-syn(61-95) in monolayer was fully exhibited. Therefore, MAIRS is a potential supplemental technique to X-ray crystallography and NMR to determine the structure of membrane peptides/proteins.