Homonuclear spin exchange is an essential tool in NMR tive to the direction of the applied magnetic field. Since the relevant dipolar coupling is weak, the homonuclear 15 N spin-spectroscopy (1). It is widely used to measure short-range internuclear distances, which provide constraints for struc-exchange process can be quite slow. Typically, a mix time in the laboratory frame of several seconds is required to ture determination and can lead to resonance assignments. It can also result in substantial improvements in spectral effect significant spin exchange between 15 N sites separated by more than 3 A ห. Thus, the mix time adds significantly to resolution by providing a mechanisms for generation of additional frequency dimensions. In this Communication, we de-the total experimental time, and in some cases, the 15 N T 1 's are so short that they provide competitive pathways for loss scribe a three-dimensional solid-state NMR experiment that incorporates homonuclear spin exchange between abundant of magnetization. Nonetheless, there are a variety of situations where dilute spin-exchange experiments are extremely spins and heteronuclear correlation between directly bonded spins. The experiment is shown to correlate the 1 H and 15 N valuable, for example, the recent implementation of a fourdimensional solid-state NMR correlation experiment (8). resonance frequencies of adjacent amide NH sites in a singlecrystal sample of a peptide. Since the ranges of internuclear Abundant ( 1 H) spin exchange is routinely used in homonuclear NOE measurements in solution NMR experiments. 1 H-1 H distances in various protein secondary structures are well established and used in resonance assignments and However, homonuclear 1 H-1 H dipolar couplings are generally so strong in solids that they present major difficulties structure determinations in solution (2), there is considerable background for applications to polypeptides.
for the resolution of resonances and the selective measurement of spectral parameters. Homonuclear 1 H spin exchange Homonuclear spin exchange can be effected by spin-diffusion or cross-relaxation processes originating in dipole-di-has been observed in a number of examples (9-12). However, the applications have been restricted to fairly simple pole interactions (3). Even though it can be difficult to identify the dominant mechanism in a solid sample, since molecules and synthetic polymers because of the limited resolution achievable in one-dimensional solid-state 1 H the effects of the dipolar interactions on exchange of magnetization always fall off rapidly as a function of distance, spin-NMR spectra. As a consequence, solid-state NMR studies of complex molecules (4-6, 13, 14), especially proteins and exchange experiments strongly discriminate toward pairs of spins that are in close proximity and can be used with consid-nucleic acids, have relied primarily on dilute ( 13 C or 15 N) spin-exchange experiments. erable reliability for qualitative distance measurements and the assignment of resonances. While the angular dependence Many of the limitations of homonuclear spin-exchange spectroscopy can be overcome by combining various aspects of the dipolar interaction is a potential complication, it may provide an additional source of structural information.
of heteronuclear and homonuclear correlation spectroscopy with 1 H spin exchange in the laboratory frame. This can be Two-dimensional homonuclear 15 N spin-exchange experiments have been applied to selectively and uniformly 15 N-accomplished with the pulse sequence diagrammed in Fig.
- This three-dimensional experiment enables the selective labeled proteins (4) and nucleic acids (5, 6). In these systems, all of the homonuclear dipolar couplings between observation of 1 H spin exchange between amide hydrogens through their 15 N signals, which are detected following polar-neighboring 15 N nuclei are weak due to their relatively long internuclear distances and low gyromagnetic ratio. For ex-ization transfer via the heteronuclear 1 H-15 N dipolar coupling between directly bonded nuclei. We refer to this exper-ample, in the double-labeled dipeptide shown in Fig. 1, Nacetyl-15 N-Val-15 N-Leu, the intramolecular 15 N-15 N in-iment by the acronym PSEUDO, which stands for proton spin exchange unraveled by dilute-spin observation; it is a ternuclear distance is 3.5 A ห(7), and the corresponding 15 N-15 N dipolar coupling varies between about 0 and 30 Hz as member of a family of heteronuclear multidimensional solidstate NMR experiments that provide high resolution among a function of the orientation of the internuclear vector rela-81