Interactions among β-sheets are critical in driving the aggregation of the β-amyloid peptide (Aβ) to form oligomers and fibrils in Alzheimer’s disease. Two main regions of this 40- or 42-residue peptide are known to adopt β-sheet structure and promote aggregation: the central region and the C-terminal region. The central region comprises the hydrophobic pentapeptide LVFFA (Aβ17–23), and the C-terminal region comprises the hydrophobic undecapeptide AIIGLMVGGVV (Aβ30–40) or the hydrophobic tridecapeptide AIIGLMVGGVVIA (Aβ30–42). This dissertation is devoted to studies of the assembly and coassembly of peptides derived from the central and C-terminal regions of Aβ, which are critical in the formation of oligomers and fibrils in Alzheimer’s disease pathology.
Chapters 2 and 3 of this dissertation describe a two-part investigation of peptides derived from the central and C-terminal regions of Aβ. The first part, called “Assembly of Peptides Derived from β-Sheet Regions of β- Amyloid”, introduces two macrocyclic β-sheet peptides that contain residues 17–23 (LVFFAED) from the central region and residues 30–36 from the C-terminal region, and elucidates how each peptide self-assembles in aqueous solution. 1H NMR spectroscopy shows that the peptides assemble to form tetramers. Incorporation of a single isotopic label into each peptide and 15N-edited NMR spectroscopy facilitated the identification and quantification of the monomers and tetramers. Molecular modeling further elucidates the structures of the tetramers. These studies provide insights into the peptide interactions and supramolecular assembly of an important peptide in an important amyloid disease.
The second part, called “Coassembly of Peptides Derived from β-Sheet Regions of β-Amyloid”, asks how the macrocyclic β-sheet peptides coassemble upon mixing. This question is important because the two regions generally segregate in most fibril structures of Aβ but coassemble in the oligomers. The two macrocyclic β-sheet peptides form a complex mixture homotetramers and heterotetramers upon mixing. 15N-Editted NMR spectroscopy shows that three heterotetramers form in addition to the homotetramers. Job’s method of continuous variation and nonlinear least-squares fitting to help establish the identity of each heterotetramer, and also reveal a surprising preference for one particular heterotetramer. These studies illustrate the role of molecular recognition in amyloid assembly and push the boundaries for experimental studies involving complex supramolecular equilibria.
For the remainder of this dissertation, the focus of the research shifts to a study on macrocyclic β-sheets that contain N-methyl blocking groups, rather than Hao. These studies were intended to conclude ongoing efforts to characterize analogues of peptides derived from Aβ17–36. These analogues were studied by X-ray crystallography and 1H NMR spectroscopy, and also with biophysical and biological techniques. DOSY NMR studies revealed these peptides oligomerize in solution, which have not been observed previously by other biophysical techniques. Additional NMR experiments, and corroboratory analytical ultracentrifugation studies, are needed to further establish this finding. These studies show that evaluating the oligomers N-methyl peptides is possible by 1H NMR spectroscopy. These efforts to correlate solution-phase biophysical techniques with 1H NMR spectroscopy may prove useful to other members the Nowick lab.
