Band-selective 13C homonuclear 3D spectroscopy for solid proteins at high field with rotor-synchronized soft pulses

We demonstrate improved 3D 13C-13C-13C chemical shift correlation experiments for solid proteins, utilizing band-selective coherence transfer, scalar decoupling and homonuclear zero-quantum polarization transfer. Judicious use of selective pulses and a z-filter period suppress artifacts with a two-step phase cycle, allowing higher digital resolution in a fixed measurement time. The novel correlation of C(ali)-C(ali)-CX (C(ali) for aliphatic carbons, CX for any carbon) reduces measurement time by an order of magnitude without sacrificing digital resolution. The experiment retains intensity from side-chain carbon resonances whose chemical shift dispersion is critical to minimize spectral degeneracy for large proteins with a predominance of secondary structure, such as beta-sheet rich fibrillar proteins and alpha-helical membrane proteins. We demonstrate the experiment for the beta1 immunoglobulin binding domain of protein G (GB1) and fibrils of the A30P mutant of alpha-synuclein, which is implicated in Parkinson's disease. Selective pulses of duration comparable the rotor period give optimal performance, but must be synchronized with the spinning in non-trivial ways to minimize chemical shift anisotropy recoupling effects. Soft pulses with a small bandwidth-duration product are best for exciting the approximately 70 ppm bandwidth required for aliphatic-only dimensions.