Fig. 5.

The 2D proton-detected ¹³C/¹H HSQC NMR spectrum of (a) natural abundant L-alanine and (b) [U-¹³C/¹⁵N]-L-isoleucine. The MAS frequency was 60 kHz, the sweep width in the indirect dimension was 30 kHz and no proton decoupling was applied. For the L-alanine spectrum the delays τ and τ′ were set equal to 500 μs and 48 scans were collected for each of the 48 complex t₁ points. An additional 48 t₁ points were produced with forward linear prediction prior to processing the L-alanine 2D spectrum. For [U-¹³C/¹⁵N]-L-isoleucine, the delays τ and τ′ were set equal to 150 μs and 16 scans were collected for each of the 128 complex t₁ points.