qTROSY--a novel scheme for recovery of the anti-TROSY magnetisation.

In TROSY experiments, spin state selection (S3) retains only the single HSQC sub-spectrum with minimal T2 relaxation and maximal resolution, yet at the cost of eliminating half of the available polarisation as undesired anti-TROSY component. We here introduce queued TROSY (qTROSY) as a novel scheme to partially recover and exploit this anti-TROSY polarisation in two concatenated scans. After initial orthogonal spin state separation (oS3), anti-TROSY polarisation is explicitly stored while its TROSY counterpart follows the desired coherence pathway recorded in a first scan A. The immediately appended scan B then quantitatively converts the recovered anti-TROSY polarisation into a second TROSY spectrum, skipping the time-limiting long reequilibration delay. Both concatenated qTROSY scans thus ideally exploit the full initial polarisation within almost the same measurement time. In practice, T2 relaxation losses accruing during the coupling evolution delays reduced anti-TROSY polarisation recovery below 40%, obviating sensitivity enhancement through addition of both qTROSY scans; yet, scan B retained a complete scan A spectrum with up to 75% intensity. We therefore propose to employ qTROSY asymmetrically, compacting two separate conventional into one queued TROSY-type experiment with significantly reduced measurement time, implying primarily the concatenation of different three- or higher-dimensional experiments. Both anti-TROSY polarisation recoveries and possible time savings are largest for deuterated and smaller non-deuterated proteins, extending the rentability limit of the TROSY principle towards smaller molecular weights.