The FLOPSY (FLip-flOP SpectoscopY) scheme is a composite pulse sequences specifically designed for isotropic mixing purpose in TOCSY experiments (see The TOCSY block). The excellent efficiency over large bandwidths allows to be applied for homonuclear cross polarization in ¹³C NMR of labeled biomolecules.

Easy implementation on AVANCE spectrometers.

The FLOPSY-8 scheme ( 91JMR437-91 ) consists of the basic R(0)R(180)R(180)R(0) -R(0)R(0)R(180)R(180) mixing sequence where R=46(0)96(45)164(67.5)159(315)130(22.5)159(315)164(67.5)96(45)46(0). All flip angles and phases are specified in degrees (e.g., 46(0) denotes a 46 degree pulse with a relative phase of zero), and R(180) indicates a phase shift of all constituent pulses by 180 degrees.

It offers and excellent global offset quality factor (Q) ensuring that coherence transfer is efficiently achieved over the chemical-shift range of interest:
 

Sequence	Qx	Qy	Qz
IICT-1	0.73	0.52	0.57
IICT-2	0.71	0.51	0.5
IICT-3	0.72	0.52	0.52
IICT-4	0.59	0.58	0.63
MLEV-16	-0.31	-0.31	0.16
DIPSI-2	0.68	0.44	0.46
FLOPSY-2	0.62	0.55	0.61

 
FLOPSY scheme is somewhat shorter than WALTZ and DIPSI schemes and it can be expanded to FLOPSY-16 by inverting the basic R elements.

The FLOPSY sequence can be converted to a selective cross-polarization sequence by interleaving the basic  mixing sequence with delays ( 91JMR204-94 ).

Calibration of 90º and 180º pulses must be made at the required power. In general, for TOCSY applications, the power is adjusted to give a 90º pulse about 30-35 microseconds. A 10% variation in such calibrations could be tolerated.

For further experimental details, see the following tutorials:

• Tutorial: 90º ¹H observe pulse calibration.
• Tutorial: 2D TOCSY experiment.