We are seeing some very bizarre behavior with the BioPack pulse program for measuring 1H-15N NOEs in proteins. We're running VNMRJ 2.1B on an Inova-600 with a cold probe, the pulse program is gNnoeA.c, and we are using a 5 sec. relaxT. The first problem is that water suppression is horrible. For comparison we did a 15N HSQC and removed the flip-back pulse and the “spoiler” gradient midway through H to N transfer, so that only the coherence pathway selection was contributing to water suppression. We still got excellent water suppression.
We noticed that the lock level meter on the remote status unit was erratic. On some scans it dips a lot when the gradients fire, down to 38 (from 80). On other scans it only dips to 54. Still others there is little or no dip. So we began to suspect the gradients. We hooked a scope up to the BNC test point on the front of the gradient amplifier and, sure enough, we see bizarre and weird gradient patterns that correlate with what we were seeing on the remote status display. Sometimes a gradient sticks on for a long time, sometimes we get one very large gradient, sometimes some of the weaker gradients are missing.
The weird thing is that if we do the NOE experiment with no irradiation of 1H (the no NOE version), we still have all of these problems, but if we set relaxT to zero and d1 to 1.0 there are no gradient problems. We also have no gradient problems with any other experiment, including experiments with equal numbers of gradients and more or less equal intensities and durations. Only this one experiment, with long relaxT value, has the problem. So somehow the loop for irradiation of 1H is causing the problem.
It looks like a hardware problem, but I suspect the pulse sequence code because only this one pulse sequence causes the problem.
Does anyone have experience with this pulse sequence in BioPack? Any suggestions would be greatly appreciated.
Neil
Received on Wed Nov 09 2011 - 13:17:08 MST