Role of bilayer characteristics on the structural fate of aß(1-40) and aß(25-40).

The ß-amyloid (Aß) peptide is derived from the transmembrane (TM) helix of the amyloid precursor protein (APP) and has been shown to interact with membrane surfaces. To understand better the role of peptide-membrane interactions in cell death and ultimately in Alzheimer's disease, a better understanding of how membrane characteristics affect the binding, solvation, and secondary structure of Aß is needed. Employing a combination of circular dichroism and deep-UV resonance Raman spectroscopies, Aß(25-40) was found to fold spontaneously upon association with anionic lipid bilayers. The hydrophobic portion of the disease-related Aß(1-40) peptide, Aß(25-40), has often been used as a model for how its legacy TM region may behave structurally in aqueous solvents and during membrane encounters. The structure of the membrane-associated Aß(25-40) peptide was found to depend on both the hydrophobic thickness of the bilayer and the duration of incubation. Similarly, the disease-related Aß(1-40) peptide also spontaneously associates with anionic liposomes, where it initially adopts mixtures of disordered and helical structures. The partially disordered helical structures then convert to ß-sheet structures over longer time frames. ß-Sheet structure is formed prior to helical unwinding, implying a model in which ß-sheet structure, formed initially from disordered regions, prompts the unwinding and destabilization of membrane-stabilized helical structure. A model is proposed to describe the mechanism of escape of Aß(1-40) from the membrane surfaces following its formation by cleavage of APP within the membrane.

Bilayer surface association of the pHLIP peptide promotes extensive backbone desolvation and helically-constrained structures.

Despite their presence in many aspects of biology, the study of membrane proteins lags behind that of their soluble counterparts. Improving structural analysis of membrane proteins is essential. Deep-UV resonance Raman (DUVRR) spectroscopy is an emerging technique in this area and has demonstrated sensitivity to subtle structural transitions and changes in protein environment. The pH low insertion peptide (pHLIP) has three distinct structural states: disordered in an aqueous environment, partially folded and associated with a lipid membrane, and inserted into a lipid bilayer as a transmembrane helix. While the soluble and membrane-inserted forms are well characterized, the partially folded membrane-associated state has not yet been clearly described. The amide I mode, known to be sensitive to protein environment, is the same in spectra of membrane-associated and membrane-inserted pHLIP, indicating comparable levels of backbone dehydration. The amide S mode, sensitive to helical structure, indicates less helical character in the membrane-associated form compared to the membrane-inserted state, consistent with previous findings. However, the structurally sensitive amide III region is very similar in both membrane-associated and membrane-inserted pHLIP, suggesting that the membrane-associated form has a large amount of ordered structure. Where before the membrane-associated state was thought to contain mostly unordered structure and reside in a predominantly aqueous environment, we have shown that it contains a significant amount of ordered structure and rests deeper within the lipid membrane.