The DIPSI (Decoupling In the Presence of Scalar Interactions) schemes are 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.

Several versions of the DIPSI scheme have been proposed ( 88JMR274-77 and 92JMR670-96 ):

DIPSI-2: R(0)R(0)R(180)R(180) where
R=320(0)-410(180)-290(0)-285(180)-30(0)-245(180)-375(0)-265(180)-370(0)

DIPSI-3: R(0)R(180)R(180)R(0) where
R=245(180)-395(0)-250(180)-275(0)-30(180)-230(0)-360(180)-245(0)-
370(180)-340(0)-350(180)-260(0)-270(180)-30(0)-225(180)-365(0)-255(180)-395(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.

The relative length of several mixing sequences are:

WALTZ-16 1.00
DIPSI-2  1.20
DIPSI-3  2.25
FLOPSY-8 0.98

The DIPSI-2 offers an 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

A modified DIPSI sequence has been proposed to suppress cross-relaxation effects in TOCSY spectra ( 92JMR670-96 ) in which a t delay is adjusted to minimize the net cross-relaxation rate.

DIPSI-2rc: R(0)R(180)R(180)R(0) where
R=180(0)-t-140(0)-320(180)-t-90(180)-270(0)-t-20(0)-200(180)-t-85(180)-30(0)- 125(180)-t-120(180)-300(0)-t-75((0)-225(180)-t-10(180)-190(0)-t-180(0)-t

The DIPSI schemes are also widely used for effective heteronuclear cross-polarization purposes ( 91JMR413-91 )  

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.