Fig. 1.

Pulse sequence of the experiment for the ${\text{H}}_i^{\text{N}}{\text{H}}_{i-1}^{\alpha }$dd – C${{}^{\prime }}_{i-1}$ CSA CCR rate measurement. ${\text{C}}^{\alpha }$ evolution is in real-time mode, C${}^{\prime }$ evolution is in constant-time mode and N evolution is in semi-constant time mode: $\text{a}=(\text{t}3+{\Delta }_{\text{N-CO}})/2$, $\text{b}=\text{t}3(1-{\Delta }_{\text{N-CO}}/\text{t}{3}^{\text{max}})/2$, $\text{c}={\Delta }_{\text{N-CO}}(1-\text{t}3/\text{t}{3}^{\text{max}})$. The delays were set as follows: ${\Delta }_{\text{N-H}}=5.4\text{ms}$, ${\Delta }_{\text{N-CO}}=28\text{ms}$, ${\Delta }_{\text{CO-CA}}=9.1\text{ms}$, ${\Delta }_{\text{CA-HA}}=3.2\text{ms}$, $\text{Tc}=28\text{ms}$. Unless noted explicitly, pulse phases are set to x. Phase $\varphi a/x$ depends on the version of experiment; for reference experiment x, for cross experiment y. 16-step phase cycle was used: $\varphi 1$ = x, -x, $\varphi 2$ = 2(x), 2(-x), $\varphi 4$ = 4(x), 4(y), 4(-x), 4(-y). Receiver phase $\varphi rec=\varphi 1+\varphi 2+2·\varphi 4$. Selective pulses affecting only ${\text{H}}^{\text{N}}$ nuclei were pc9 (Kupce and Freeman 1994) for $\pi /2$ pulses and reburp (Geen and Freeman 1991) for $\pi$ pulses. Simultaneous inversion of ${\text{C}}^{\alpha }$ and C${}^{\prime }$ spins was achieved using 6-element composite pulse (Shaka 1985). Selective pulses for C${}^{\prime }$ and ${\text{C}}^{\alpha }$ nuclei were q5 (for $\pi /2$ pulses) and q3 (for $\pi$ pulses) (Emsley 1969). The pulses labelled ”present in ’a’/’x’ only” were executed only in reference/cross version of the experiment, respectively