Figure 3.

Simulated and experimental C_fast profiles for the binding of htt^NT and htt^NTQ₇ to SUVs. (A) Simulated profiles of C_fast as a function of ¹⁵N frequency offset for Lys⁵ of htt^NT (red) and htt^NTQ₇ (blue) in the presence of SUV vesicles at a molar ratio of 1:4 peptide to lipid. The curves were calculated using a variant of eq 7 that accounts for Δω ≠ 0¹⁵) with the following values for the exchange parameters (where the first value refers to htt^NT and the second to htt^NTQ₇): k_ex = 204.7/208.3 s⁻¹, p_B = 0.07/0.08, $R_1^{\text{A}}-R_1^{\text{B}}$ = 1.5/1.5 s^{− 1}, $R_2^{\text{A}}$= 2.4/3.6 s^{− 1}, $R_2^{\text{B}}$ = 2300/2300 s⁻¹, ω₁ field strength = 2000 Hz, number of spin locks N = 2, and Δω/(2π) = −211.8 Hz (−3.47 ppm at 600 MHz). (B) Experimental ¹⁵N-${\text{C}}_{\text{fast}}^{\max }$ profiles measured for htt^NT (red filled-in circles) and htt^NTQ₇ (blue filled circles) in the presence of SUVs. The open circles are the best-fit profiles obtained by including ${\text{C}}_{\text{fast}}^{\max }$ into the target function together with the DEST and ΔR₂ data (see Experimental Section). (C) Kinetic scheme used for modeling the binding of htt^NT and htt^NTQ₇ to the surface of SUVs. The data for residues 3−14 are fit to a two-state exchange between the free peptide $\left({\text{P}}_{\text{coil}}^{\text{A}}\right)$ and membrane-bound helical peptide $\left({\text{P}}_{\text{helix}}^{\text{B}}\right).$ For residues 15−16 of htt^NT and 15−23 of htt^NTQ₇, there is an additional process involving the interconversion between ${\text{P}}_{\text{helix}}^{\text{B}}$ and ${\text{P}}_{\text{tethered}}^{\text{B}}$, where the residues in the ${\text{P}}_{\text{tethered}}^{\text{B}}$ state are not attached to the membrane but tethered via the membrane-bound helical residues. The overall process for the conversion of ${\text{P}}_{\text{coil}}^{\text{A}}$ to ${\text{P}}_{\text{tethered}}^{\text{B}}$ (via ${\text{P}}_{\text{helix}}^{\text{B}}$ ) is shown in the inset.