Fast iZQC for In Vivo Spectroscopy
In an intermolecular multiple quantum experiment, the signal is largely limited by sample inhomogeneity, which limits the data acquisition time and produces line broadening. Resolution enhancement can be obtained in principle by spinning the sample at the magic angle (MAS) .
When a sample is spun at the magic angle, the chemical shift anisotropy is partially reduced since each spin is subjected to the same average magnetic field. However, when we spin a sample at the magic angle, we also average out the dipolar field interaction . Since the iMQC signal is directly created by the dipolar field, this means that no iMQC signal will be produced by MAS. Therefore, in order to be able to detect the iMQC signal, and still remove the chemical shift anisotropy, we need to combine the MAS with a dipolar recoupling sequence. Many different sequences have been developed for dipolar recoupling. In our experiments, we used the DRAMA (Dipolar recoupling under magic angle ) sequence, implemented by Tycko. The sequence consists of a series of 90 and 180 pulses, synchronized with the sample spinning. Since the purpose of the dipolar interaction is to convert antiphase magnetization into in phase magnetization, the recoupling sequence needs to be introduced during the detection time, t2. A scheme of the sequence is in figure 1. In this figure, a basic double quantum sequence in the presence of magic angle spinning is paired with a recoupling DRAMA sequence.
Since the iMQC signal grows linearly with the dipolar field, a large number of DRAMA modules need to be employed. However, the detection time is limited by the transverse relaxation of the system, which will limit the number of DRAMA modules, n, that one can use in the sequence. The gradients to create the modulation of the magnetization also need to be carefully chosen. The gradient pair needs to be rotor-synchronized in order to refocus the iMQC signal.
During magic angle spinning, a gradient applied along the z axis with a duration equal to the rotor period will create a modulation of the magnetization similar to that created by a gradient oriented along the magic angle, with an amplitude reduced by cos (54.7) .
This will not create any iMQC signal either. One possibility is to have a very short gradient pulse compared to the spinning frequency, imposing a limit on the spinning speed. Another possibility is to create shaped gradient pulses synchronized with the sample rotation, which, in the spinning sample frame, will lead to a modulated magnetization along the z axis.
A demonstration of the technique is reported in figure 2. In this case, a very short gradient pulse, 0.5 ms, was used, while the sample was spun at 122 Hz. A variable number of DRAMA modules were also used to detect the growth of the iDQC peak with the dipolar field evolution time.
Figure 1. The 2-Quantum CRAZED sequences with MAS and DRAMA modules used to investigate the effect of DRAMA on the iDQC signal intensity.
Figure 2. Experimental Results on an off resonance water sample, spinned at 122Hz, and with a corelation gradient of G = 12.6 Gauss/cm and 0.5 ms duration. a) Standard CRAZED experiment , no spinning, no DRAMA (n=0). b) CRAZED experiments with n=0. c) CRAZED experiment with n=1. d) CRAZED experiment with n=2.
