Figure 12.18

lines and the Ha shifts are shown as horizontal dotted lines, and in crowded regions the Hn assignments are shown above or below the crosspeaks with the Ha assignments to the right or left. The protein is mostly ^-sheet, as shown in Figure 12.19, with four antiparallel ^-strands forming the "bread" and two a-helices the "sausages" of an open-faced sandwich. The NOE crosspeaks shown in Figure 12.18 are from the A, D, and B strands (1-10, 60-70 and 30-40, respectively) of the ^-sheet. Figure 12.20 shows an expansion of this region of the NOESY spectrum, with a portion of the ^-sheet shown at the right. The three sequential

Figure 12.20

a,N connectivities indicated on the structure (arrows) correspond to the NOE crosspeaks identified in the spectrum. Of course, when you first obtain a NOESY spectrum, the cross-peaks do not have names, so before we can interpret the data we must first assign all of the protein resonances to specific protons in the covalent structure.

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