We apply our novel multi-frame method1 to design homonuclear mixing pulses that are broadband, narrowband, or multi-band, to suit various TOCSY experiments in protein spectroscopy. Our pulses have higher bandwidth (for the same RF power) than commonly used broadband pulses such as FLOPSY. We have designed the pulses analytically, rather than by numerical optimization, by constructing a series of nutating frames. We choose pulse parameters in successive frames to effectively compress a large chemical shift bandwidth arbitrarily many times, while largely maintaining couplings between spins. Pulse parameters can be chosen analytically to either maximize bandwidth, create multi-band structures, or select a narrow spectral region.
As an example application we apply a broadband pulse followed immediately by a narrowband pulse to the aliphatic region of the carbon spectrum. The broadband pulse correlates side chain carbons with their respective carbon alphas. The narrowband pulse is selective for the alpha region, and therefore resolves weak inter-residue couplings between neighboring alpha carbons. This two-stage approach generates cross peaks between side chain carbons and the alpha carbons of adjacent residues.
Takeuchi et al proposed a CaCa TOCSY experiment for alternate 13C labeled samples, providing correlation between alpha carbons2 . Our method is for uniformly labeled samples, and correlates the entire aliphatic region to neighboring residues' alpha carbons. Inter-residue cross peaks are distributed over a large spectral width, and in general there are several cross peaks between any adjacent residues. This helps alleviate ambiguity and overcrowding in CaCa spectra. These correlations cannot be observed using only broadband mixing, as the strong CaCb couplings prevent transfer across weak CaCa couplings.
Figure 1: A. Broadband TOCSY of a PIFHA [U-13C,15N] labeled pentapeptide resolving 35 Hz intra-residue couplings. B. Two stage broad-then-narrow TOCSY further correlates each peak to the neighboring carbon alphas (all peaks are in phase; colors are a visual aid only).