Labeling schemes employed in the model tripeptides used to illustrate the effect of dipolar truncation. AGG-2 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 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 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.
Simulated buildup curves of magnetization transfer from spin 2 (blue) to spin 1 (red) and spin 3 (green) in model spin systems AGG-2, AGG-3, and GGV-3 with various recoupling sequences. ZQ sequences: RFDR, , ; , , . DQ sequence: , , . Mixed sequence: DRAWS, , .
Cross sections of 2D RFDR correlation spectra through the diagonal peak of spin 2 for three different mixing times. The arrow indicates the cross peak between spin 2 (a carbon at 44 ppm) and the distant spin 1 (a carboxyl carbon at 175 ppm). The intensity of this cross peak is diminished in AGG-3 and GGV-3 compared to AGG-2, demonstrating the attenuation of polarization transfer due to the dipolar truncation effect.
Top: Experimental RFDR buildup curves for AGG-2, AGG-3, and GGV-3. Each data point is the integrated peak intensity of 2D correlation peaks normalized to the diagonal peak at zero mixing. RFDR experiments employed 50 kHz pulses and 115 and 90 kHz decoupling fields during the mixing period and were performed at a MAS frequency of 8.929 kHz. Bottom: Numerical simulations of polarization transfer in AGG-2, AGG-3, and GGV-3 showing long-term behavior of RFDR mixing. These simulations included CSA parameters, eight neighboring spins, and experimental parameters identical to those performed in our experiments.
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