Description file belonging to 15N-1H and 13C-1H CPMG-HSQC sequences.
For Varian Unity Inova 500 MHz (should also work on Unity Plus)
(Frans Mulder 22 Sept 1999)

The following experiments allow one to perform HSQC experiments
with improved performance in the presence of exchange-broadened
signals (ref.1). In addition, intensities in the CPMG-HSQC spectrum
can be compared with a reference HSQC spectrum to identify residues
which are affected by millisecond to microsecond time scale motion
(ref.2). In the case of 15N-1H two separate sequences are provided,
for 13C-1H a flag has been added to select for regular or CPMG-
enhanced INEPT.

General considerations:
The sequences for 15N-1H HSQC experiments are best performed with
the carrier in the center of the amide region to reduce offset
effects (as in ref.2). Water is being dephased at the beginning of the sequence
by an off-resonance selective pulse (which does not come with the sequence, and has to be generated by the experimenter).
Alternatively, a water flip-back strategy may be followed in cases where
fast exchange of protons with solvent water is an issue. In that case
one must place the carrier at the water frequency throughout the sequence 
and preferably implement weak gradients during t1, to prevent radiation
damping (as in ref.1).

References:
1. Frans A.A. Mulder, Chris A.E.M. Spronk, Monique Slijper, Robert Kaptein
and Rolf Boelens "Improved HSQC experiments for the observation of
exchange broadened signals" J. Biomol. NMR, 8 (1996), 223-228

2. Paul J.A. van Tilborg, Frans A.A. Mulder, Maaike M.E. de Backer, Margie
Nair, Erika C. van Heerde, Gert Folkers, Paul T. van der Saag, Yasmin
Karimi-Nejad, Rolf Boelens and Rob Kaptein "Millisecond to microsecond
time scale dynamics of the retinoid X and retinoic acid receptor DNA-binding
domains and dimeric complex formation" Biochemistry, 38 (1999),
1951-1956

Acknowledgement: I gratefully acknowledge the help of Lewis E Kay
and Ranjith Muhandiram in implementing these sequences.