The β-Sheet Structure pH Dependence of the Core Fragments of β2-Microglobulin Amyloid Fibrils

Amyloid fibril structures of peptides having the sequence of the #20-41 region of beta(2)-microglobulin (beta(2)m(20-41) and beta(2)m(21-29)) were investigated with FTIR microscope spectroscopy. pH dependence of the amide I band of the IR absorption spectra was analyzed. For beta(2)m(20-41), the beta-sheet content at pH 6.0 was 55% (12.1 residues) and the beta-sheet was located in the N21-G29 and I35-V37 regions, but at pH 2.5 it was 60% (13.1 residues) and located in the F22-V27 and P32-V37 regions. The two structures with different amounts of beta-sheet were switched at pH 4.5, which was close to the pI of this sequence (ca. 4.3) rather than the pK(a) of the carboxyl groups (5.8 +/- 0.3 and 3.3 +/- 0.4), suggesting the importance of the total number of charges rather than protonation of specific residue(s). The spectral characteristics of the IR linear dichroism and C-13 isotope labeling indicated the formation of parallel beta-sheet structure in both regions, and the stacking direction was unaltered by pH change. Unexpectedly however, the IR spectra of the beta(2)m(21-29) fibril demonstrated the simultaneous presence of parallel/anti-parallel beta-sheets. The relevance of the pH dependence of the m beta(2)m(20-41) fibril structure to that of beta(2)m fibrils is discussed.