Short transverse I3C relaxation times, T2, constitute the principal barrier for the application of heteronuclear Jcorrelation N M R techniques to larger proteins uniformly enriched with I3C and 15N.1-6 The 13C T2 is dominated by the strong dipolar interaction with its attached protons.' As themagnetogyricratio of 2H is -6.5 times lower than that of IH, the heteronuclear dipolar interaction is greatly reduced by deuteration. Because of the large 2H quadrupolar interaction (170 kHz), the 2H spin lattice relaxation time, TI, in proteins is in the millisecond range at a magnetic field strength of 14 T. Therefore, the 2H-13C J coupling (-22 Hz) does not result in the triplet shape, expected for a 13C nucleus coupled to a spin-1 nucleus, but gives rise to a collapsed singlet resonance that is broadened by scalar relaxation of the second k i r ~ d . ~ , ~ High-power (-2.5 W) 2H decoupling with an R F field strength much stronger than the inverse 2H TI effectively removes this broadening and results in a 13C line width that is much narrower than for the protonated I3C. One of the triple resonance Jcorrelation experiments affected most by the 13C line width is the H(CA)NH experimenti0%] I which relies on magnetization transfer from Ca to the backbone I5N nucleus via the relatively small I J N c ~ (1 1 Hz) and 2 J ~ ~ a (5-8 Hz) couplings. Although experiments have been proposed to alleviate this difficult J correlation step,l3~l4 the sequential assignment procedure which is based on J correlation between the intraresidue IH/l5N and IHa/l3Ca resonances and between the 1Hm/l3Ca of residue i and IH/lSN of residue i + 1 is complicated by the high degree of overlap among IHa/l3Ca correlations. Here we describe a procedure which allows J correlation between the much better resolved IH/l5N resonances of sequential residues, thereby bypassing the overlapping 'Ha/ I3Ca pairs. Efficient transfer of magnetization from I3Ca to I5N is possible in the present case because of the I3Ca line narrowing afforded by deuteration and 2H decoupling.