Conformational Preferences of Aß25-35 and Aß35-25 in Membrane Mimicking Environments.

BACKGROUND: The structural transition of aggregating Abeta peptides is the key event in the progression of Alzheimer's Disease (AD). OBJECTIVE: In the present work, the structural modifications of toxic Aß25-35 and the scrambled Aß35-25 were studied in Trifluoroethanol (TFE) and in aqueous SDS micelles. METHODS: Using CD spectroscopic investigations, the conformational transition of Aß25-35 and Aß35-25 peptides were determined in different membrane mimicking environments such as TFE and SDS. An interval scan CD of the peptides on evaporation of TFE was performed. TFE titrations were carried out to investigate the intrinsic ability of the structural conformations of peptides. RESULTS: We show by spectroscopic evidence that Aß25-35 prefers beta sheet structures upon increasing TFE concentrations. On the other hand, the non-toxic scrambled Aß35-25 peptide only undergoes a transition from random coil to α-helix conformation with increasing TFE. In the interval scan studies, Aß25-35 did not show any structural transitions, whereas Aß35-25 showed transition from α-helix to ß-sheet conformation. In membrane simulating aqueous SDS micelles, Aß25-35 showed a transition from random coil to α-helix while Aß35-25 underwent transition from random coil to ß-sheet conformation. CONCLUSION: Overall, the current results seek new insights into the structural properties of amyloidogenic and the truncated sequence in membrane mimicking solvents.

Structural preferences of Aß fragments in different micellar environments.

Amyloid diseases occur due to conformational change in the native protein. Understanding the amyloid peptide structural stability and conformational preference at the molecular level in membranous environment may lead to advancement in drug design and therapy. The conformational preferences of amyloid peptide fragments, Aß1₋11, Aß12₋22, Aß23₋33 and Aß34₋42 was studied in buffers, trifluoroethanol (TFE) and sodium dodecyl sulfate (SDS) micelles using circular dichroism spectroscopy. The fragment, Aß1₋11 in TFE adopts a mixture of random coil and turn conformations. Aß12₋22 and Aß23₋33 underwent transition from random coil to helix conformation, while Aß34₋42 exhibited ß-sheet conformation in initial stage which was unaltered on complete evaporation of TFE. Addition of SDS to Aß12₋22 and Aß34₋42 favors ß-sheet structure, which was predominant in the case of Aß34₋42. However, in Aß1₋11 and Aß23₋33, no secondary structural change was noticed even at high SDS concentrations. On aging, all the peptide fragments showed ß-sheet conformational transition. The C-terminal fragment has the ability to adopt predominant ß-sheet conformation even in the presence of detergent and membrane mimicking environment. Altogether, the structural information gained from the short fragments could be further used for determining their role in the organization of Aß peptide in stable fibril form.

Aggregation and conformational studies on a pentapeptide derivative.

Most of the disease causing proteins such as beta amyloid, amylin, and huntingtin protein, which are natively disordered, readily form fibrils consisting of beta-sheet polymers. Though all amyloid fibrils are made up of beta-sheet polymers, not all peptides with predominant beta-sheet content in the native state develop into amyloid fibrils. We hypothesize that stable amyloid like fibril formation may require mixture of different conformational states in the peptide. We have tested this hypothesis on amyloid forming peptide namely HCl(Ile)(5)NH(CH(2)CH(2)O)(3)CH(3) (I). We show peptide I, has propensity to form self-assembled structures of beta-sheets in aqueous solutions. When incubated over a period of time in aqueous buffer, I self assembled into beta sheet like structures with diameters ranging from 30 to 60 A that bind with amyloidophilic dyes like Congo red and Thioflavin T. Interestingly peptide I developed into unstable fibrils after prolonged aging at higher concentration in contrast with the general mature fibril-forming propensity of various amyloid petides known to date.