Letter pubs.acs.org/JPCL
Broad-Band DREAM Recoupling Sequence Kong Ooi Tan, Anders B. Nielsen, Beat H. Meier,* and Matthias Ernst* Physical Chemistry, ETH Zürich, Vladimir-Prelog-Weg 2, 8093 Zürich, Switzerland S Supporting Information *
ABSTRACT: We describe a more broad-banded version of the DREAM double-quantum dipolar-recoupling sequence, which is devised by superimposing a phase-alternating RF irradiation scheme, that is, XiX pulses, on top of the original DREAM sequence. We call this sequence XiXCW DREAM. The recoupling conditions and the corresponding first-order effective Hamiltonian are analyzed using triple-mode Floquet theory. The performance of the XiXCW DREAM sequence is compared to the original DREAM sequence by numerical simulations and experiments on small model substances and the model protein ubiquitin. The results confirm that XiXCW DREAM shows a wider recoupling bandwidth compared to that of DREAM, therefore making the choice for the position of the carrier frequency less critical.
SECTION: Biophysical Chemistry and Biomolecules
H
omonuclear polarization transfer schemes1,2 are an important building block in multidimensional NMR experiments because they allow the identification of pairs of nuclei coupled by spin−spin interactions. In solid-state NMR, the polarization transfer is often mediated via dipolar couplings that are distance-dependent and hence allow distance measurements and structure determination.3,4 Dipolar interactions are averaged out by magic angle spinning (MAS) in a first-order average Hamiltonian (AHT) picture5 but can be reintroduced via recoupling sequences. A large number of recoupling sequences have been developed over the years based on different concepts.1,2 The homonuclear rotary resonance (HORROR)6 sequence is an example of such recoupling sequences where the rf field amplitude of continuous wave (CW) irradiation is matched to half of the MAS frequency in order to reintroduce the homonuclear dipolar coupling. The HORROR sequence is a simple sequence, but it has limited practical importance due to its sensitivity to chemical shift offsets and rf field inhomogeneity. The sensitivity to the chemical shift offsets is a consequence of the low rf field amplitude, in particular, at slower MAS frequencies. On the other hand, this feature makes it an attractive sequence for applications in biological samples at faster MAS spinning rates exceeding the carbon chemical shift range. The adiabatic version of HORROR, called DREAM,7 mitigates the problems due to offset and rf inhomogeneity and increases the theoretical transfer efficiency to 100%. These advantages have made the DREAM sequence a popular sequence in multidimensional correlation spectroscopy, which is used for resonance assignments in proteins.8−12 However, the bandwidth of the DREAM polarization transfer is still limited to roughly the spinning frequency. In addition, the cross-peak intensities depend strongly on the position of the rf carrier frequency. A detailed discussion is provided by Westfeld et al.13 © 2014 American Chemical Society
Here, we propose a modified DREAM sequence that reduces the sensitivity to the carrier position to a greater extent by superimposing a phase-alternating rf irradiation scheme,14−16 that is, XiX pulses,17 on top of the CW field. Figure 1a illustrates the basic idea of superimposing the two sequences where the individual rf field amplitudes are added if the two sequences have the same phase or subtracted if they have opposite phase. The possible recoupling conditions of the new sequence are analyzed using a triple-mode Floquet framework.18 In principle, these schemes allow us to considerably increase the rf amplitude while maintaining a HORROR-type recoupling. We will focus here on one specific resonance condition, where the average rf power is still low and yet high transfer efficiency can be achieved. We consider a system of NS dipolar-coupled spins. The full time-dependent Hamiltonian of the system under MAS and rf irradiation in spherical tensor notation5 is given by /̂ (t ) =
NS
2
∑ ∑
2 (S )
ωS(pn)e inωrt T1,0p +
p = 1 n =−2
+
∑ ∑ p < q n =−2 n≠0
(S S ) ∑ ωS(0) T p q + /̂ rf (t ) pSq 0,0 p
