Re measured using the iNMR software package (Mestrelab Research).shows amide proton intensity decay data for four representative residues. The amide proton of residue C2, which is in the unstructured N-terminus of amylin, exchanges with 
a fast rate. Residue G33, in strand b2 of the amylin fibril model exchanges with an intermediate rate. Amide protons that exchange with slow rates are represented by H18 and Y37, the C-terminal residues in strands b1 and b2. The observed differences in exchange rates between residues within the same strand (e.g. G33 and Y37 from strand b2), suggests that structural stability varies within a given element of secondary structure, 12926553 as is often found in folded globular proteins [17,34].Gaussian Network Model Calculations using the ssNMR Model of Amylin FibrilsTwo models of the amylin fibril structure satisfy the ssNMR data: Ebselen 4eql24930x2 and 78919-13-8 site 4eql5432x2 [10]. The models differ with respect to the b-strand two-residue periodicity that determines which residues face the interior and exterior of the amylin bhairpin fold [10]. Except where noted, the 4eql5432x2 model was analyzed, since this model is supported by EPR spin-label mobility data on amylin fibrils [11]. Theoretical B-factors based on the Gaussian Network Model (GNM) algorithm were calculated from the amylin fibril coordinate files with the oGNM online server ?[32], using a Ca-Ca cutoff distance of 10 A.Interpretation of Protection in Terms of the Amylin Fibril StructureFigure 3 shows time constants for exchange, determined for each residue from least-squares fits of amide proton decay data to an exponential model (Fig. 2). The largest time constants between 300 and 600 h are found for amide protons within, or immediately adjacent to the two b-strands (Fig 3). At the next level of protection, time constants between 50 and 150 h occur in the turn between the two b-strands but also for residues T9-N14 in the Nterminal part of strand b1 and for residues G33-N35 in strand b2. The fastest exchange is seen for residues K1-C7 at the N-terminus of the peptide, which are disordered in the amylin fibril structure [10?2]. The b-strand limits reported for the ssNMR [10] and EPR [11] models of amylin fibrils, together with those inferred from the HX results in this work are indicated at the top of Fig. 3. The ssNMR model [10] of the amylin 
protofilament (Fig. 4) consists of ten amylin monomers, packed into two columns of five monomers that are related 1516647 by C2 rotational symmetry. Figure 4A illustrates the intermolecular b-sheet hydrogen bonding between two adjacent monomers stacked along the fibril axis. Figure 4B shows the packing of the two columns of b-hairpins. The Cterminal strands b2 are on the inside of the protofilament, while the N-terminal strands b1 are on the outside. The protection data obtained for amylin fibrils (Fig. 3) is in overall agreement with the ssNMR model (Fig. 4) but there are some important exceptions. First, H18 is protected even though it is just outside the 8?7 limits reported to form strand b1 [10]. Residue H18 was restrained to form b-sheet hydrogen bonds in the ssNMR structure calculations [10], its secondary chemical shift predicts that it is in a b-sheet conformation [10], and its amide protons serve as a hydrogenbond donors to V17 from adjacent monomers in 62 of the amylin monomers that constitute the amylin fibril ssNMR model. In the ssNMR model, H18 falls in the b-sheet region of Ramachandran space in 9 of the 10 monomers that make.Re measured using the iNMR software package (Mestrelab Research).shows amide proton intensity decay data for four representative residues. The amide proton of residue C2, which is in the unstructured N-terminus of amylin, exchanges with a fast rate. Residue G33, in strand b2 of the amylin fibril model exchanges with an intermediate rate. Amide protons that exchange with slow rates are represented by H18 and Y37, the C-terminal residues in strands b1 and b2. The observed differences in exchange rates between residues within the same strand (e.g. G33 and Y37 from strand b2), suggests that structural stability varies within a given element of secondary structure, 12926553 as is often found in folded globular proteins [17,34].Gaussian Network Model Calculations using the ssNMR Model of Amylin FibrilsTwo models of the amylin fibril structure satisfy the ssNMR data: 4eql24930x2 and 4eql5432x2 [10]. The models differ with respect to the b-strand two-residue periodicity that determines which residues face the interior and exterior of the amylin bhairpin fold [10]. Except where noted, the 4eql5432x2 model was analyzed, since this model is supported by EPR spin-label mobility data on amylin fibrils [11]. Theoretical B-factors based on the Gaussian Network Model (GNM) algorithm were calculated from the amylin fibril coordinate files with the oGNM online server ?[32], using a Ca-Ca cutoff distance of 10 A.Interpretation of Protection in Terms of the Amylin Fibril StructureFigure 3 shows time constants for exchange, determined for each residue from least-squares fits of amide proton decay data to an exponential model (Fig. 2). The largest time constants between 300 and 600 h are found for amide protons within, or immediately adjacent to the two b-strands (Fig 3). At the next level of protection, time constants between 50 and 150 h occur in the turn between the two b-strands but also for residues T9-N14 in the Nterminal part of strand b1 and for residues G33-N35 in strand b2. The fastest exchange is seen for residues K1-C7 at the N-terminus of the peptide, which are disordered in the amylin fibril structure [10?2]. The b-strand limits reported for the ssNMR [10] and EPR [11] models of amylin fibrils, together with those inferred from the HX results in this work are indicated at the top of Fig. 3. The ssNMR model [10] of the amylin protofilament (Fig. 4) consists of ten amylin monomers, packed into two columns of five monomers that are related 1516647 by C2 rotational symmetry. Figure 4A illustrates the intermolecular b-sheet hydrogen bonding between two adjacent monomers stacked along the fibril axis. Figure 4B shows the packing of the two columns of b-hairpins. The Cterminal strands b2 are on the inside of the protofilament, while the N-terminal strands b1 are on the outside. The protection data obtained for amylin fibrils (Fig. 3) is in overall agreement with the ssNMR model (Fig. 4) but there are some important exceptions. First, H18 is protected even though it is just outside the 8?7 limits reported to form strand b1 [10]. Residue H18 was restrained to form b-sheet hydrogen bonds in the ssNMR structure calculations [10], its secondary chemical shift predicts that it is in a b-sheet conformation [10], and its amide protons serve as a hydrogenbond donors to V17 from adjacent monomers in 62 of the amylin monomers that constitute the amylin fibril ssNMR model. In the ssNMR model, H18 falls in the b-sheet region of Ramachandran space in 9 of the 10 monomers that make.