FTIR (a) and parallel (b) and perpendicular (c) 2D IR spectra of hIAPP isotopically labeled at Ala-13. Solid boxes designate features due to β-sheet (box A), turn (box B), isotope label (box C), and random coil (box D). Dashed boxes indicate cross-peaks due to coupling between the turn and β-sheet (box xAB and xAB′), the label and β-sheet (box xAC), and the label and turn (box xBC). Contours are evenly spaced from 50% of the minimum to 50% of the maximum, with zero contours removed to reduce noise.