TROSY

The chemical shift resolution and sensitivity of NMR generally benefit from an increase in magnetic field strength.  As a result, large sums of money are spent on magnets with higher and higher fields.  The boost in sensitivity means that smaller and smaller quantities of sample are needed and measurements can be completed in shorter periods of time.  There are particular cases however, where an increase in magnetic field can lead to a loss of sensitivity and resolution.  This is the case for ¹⁵N decoupled ¹H spectra of the ¹H-¹⁵N spin pairs in very large ¹⁵N labelled proteins.  The very long correlation times of the protein combined with the high resonance frequencies associated with high field strength lead to very short T₂ relaxation times and therefore broader lines.  The broad lines account for a significant loss in resolution and sensitivity.  One might then wonder why protein structural chemists spend so much money on very high field magnets.  What follows is one possible answer to this question.

The two main relaxation mechanisms for the ¹H and ¹⁵N in proteins are dipolar coupling and chemical shielding anisotropy.  These two mechanisms are also cross correlated with one another.  The cross correlation term is of different sign for each of the two peaks in a ¹H-¹⁵N doublet resulting in one of the peaks of the doublet having a shorter T₂ (and broader line) than the other one.  If ¹⁵N decoupling is applied, one sees a single resonance with a line width determined by the average of the two components of the doublet.  This is illustrated in the figure below for a ¹H-¹⁵N spin pair in a small and large molecule at high field .  The same is true in the ¹⁵N-¹H doublets in the ¹⁵N spectra of ¹⁵N-¹H spin pairs.

At very high fields, One of the lines in the ¹H-¹⁵N doublet is very sharp and the other very broad.  If, in the ¹H spectrum of a protein, we could eliminate all of the broad doublet components leaving only the sharp ones, we would have a high resolution ¹H spectrum.  Further, if we could combine such a measurement with an HSQC, we would have a high resolution ¹H-¹⁵N HSQC at high field.  The combination of these two measurements is called transverse relaxation optimized spectroscopy (TROSY).  TROSY data collection employs an HSQC measurement with neither ¹H nor ¹⁵N decoupling elements (as described in a previous post) as well as other elements which suppress the broad lines of the doublets and retain the sharp lines.  The results of this are illustrated in the figure below for small and large proteins at high field.

Clearly, it is not advantageous to use the TROSY technique on small proteins rather than the conventional HSQC.  For large proteins at high field however, there is a significant sensitivity and resolution advantage compared to a conventional HSQC.  It should be noted that the TROSY cross peaks are shifted by ½ ¹J_HN in both the F2 and F1 domains.  The figure below shows a superposition of a conventional HSQC (black) and a TROSY (blue) for a protein at 500 MHz.

One can clearly see the ½ ¹J_HN shift in the F2 and F1 domains of the TROSY compared to the HSQC.  In this case, the conventional HSQC gives higher sensitivity than the TROSY.

Thank you to Adam Damry of Professor Roberto Chica’s research group at the University of Ottawa for providing the sample of ¹⁵N labelled protein.