2D-HSQC spectra (Mori et al. 1995) were recorded on a 500-MHz DRX Bruker spectrometer either on 0.5 mM uniformly 15 N-labeled N TAILHN or on 0.125 mM uniformly 15 N-labeled N TAILD3 in 10 mM sodium phosphate buffer at pH 7.0 containing 10% D 2 O (v/v) alone or after addition of a twofold molar excess of XD. The temperature was set to 283 K and the spectra were recorded with 2048 complex points in the directly acquired dimension and 256 points in the indirectly detected dimension, for 6 h each. Solvent suppression was achieved by the WATERGATE 3-9-19 pulse (Piotto et al. 1992) . The data were processed using the UXNMR software; they were multiplied by a sine-squared bell and zero-filled to 1 K in first dimension prior to Fourier transformation. For the spectra recorded after addition of XD, the number of scans was multiplied by 4.