FIG. 1.

Labeling schemes employed in the model tripeptides used to illustrate the effect of dipolar truncation. AGG-2 $\left(\text{Ala-}\left[2\text{-}{}^{13}\text{C}\right]\text{Gly-}\left[1\text{-}{}^{13}\text{C}\right]\text{Gly}\right)$ is a two-spin system with a dipolar coupling of 66 Hz, corresponding to a distance of 4.86 Å between nuclei 1 and 2. AGG-3 $\left(\text{Ala-}\left[1,2\text{-}{{}^{13}\text{C}}_2\right]\text{Gly-}\left[1\text{-}{}^{13}\text{C}\right]\text{Gly}\right)$ is a three-spin system formed by adding a third labeled nucleus (spin 3) to AGG-2, forming a strong dipolar coupling of 2.15 kHz with spin 2. The coupling between spins 1 and 3 is 150 Hz. GGV-3 $\left(\left[1\text{-}{}^{13}\text{C}\right]\text{Val-}\left[2\text{-}{}^{13}\text{C}\right]\text{Gly-}\left[1\text{-}{}^{13}\text{C}\right]\text{Gly}\right)$ is a three-spin system similar to AGG-3, but with the third labeled nucleus (spin 3) two bonds away from spin 2. GGV-3 has a weak coupling of 80 Hz corresponding to a distance of 4.56 Å between spins 1 and 2, and a medium coupling of 550 Hz corresponding to a distance of 2.43 Å between spins 2 and 3.