Figure 3. Analysis of the F1-coupled HSQC experiment.

(A) Pulse sequence diagram of the F1-coupled ¹H,¹³C HSQC experiment used in this work. The delay τ is 4 ms (1/2Jch), phase ϕ₁ = x, -x; receiver phase ϕ_rx = x, -x. States-TPPI quadrature detection is achieved by incrementing the phase of the pulse marked ϕ. Alternatively, for measurements in the time domain, the phase ϕ can be set to any value from 0 to 180°. Gradients are applied as 1 ms smoothed squares, with strengths g0 = 0.0935 T m^-1, g1 = 0.0385 T m^-1, g2 = 0.275 T m^-1, g3 = 0.19 T m^-1 and g4 = 0.07 T m^-1. (B) F1-coupled ¹H,¹³C HSQC spectrum of [²H,¹³CH₃-Ile]-labelled FLN5 (950 MHz ¹H Larmor frequency) with cross-sections illustrating the differential relaxation of ¹H spin states, as indicated, within the ¹³C multiplet. (C) Ratio of the initial amplitudes of inner and outer transitions, I_in/I_out = (3 – Δ)/(3 + 3Δ) (Eq. 3), plotted as a function of S²τ_c with Δ calculated according to Eq. 2. (D) Variation in ¹³C single quantum relaxation rates as a function of S²τ_c, calculated at 800 MHz according to Fig. 2, with a ¹³C csa of 25 ppm. The ¹³C R₂ has been estimated in the absence of interactions with external protons. (E) Variation in the relative intensities of transitions in the ¹³C multiplet as a function of S²τ_c, calculated from the rates in panel D by Fourier transformation of Eq. 3, including 20 Hz exponential line broadening.