Triple-resonance nuclear magnetic resonance spectroscopy

Triple resonance experiments are a set of multi-dimensional nuclear magnetic resonance spectroscopy (NMR) experiments that link three types of atomic nuclei, most typically consisting of ¹H, ¹⁵N and ¹³C. These experiments are often used to assign specific resonance signals to specific atoms in an isotopically-enriched protein. The technique was first described in papers by Ad Bax, Mitsuhiko Ikura and Lewis Kay in 1990, and further experiments were then added to the suite of experiments. Many of these experiments have since become the standard set of experiments used for sequential assignment of NMR resonances in the determination of protein structure by NMR. They are now an integral part of solution NMR study of proteins, and they may also be used in solid-state NMR.

There are two main methods of determining protein structure on the atomic level. The first of these is by X-ray crystallography, starting in 1958 when the crystal structure of myoglobin was determined. The second method is by NMR, which began in the 1980s when Kurt Wüthrich outlined the framework for NMR structure determination of proteins and solved the structure of small globular proteins. The early method of structural determination of protein by NMR relied on proton-based homonuclear NMR spectroscopy in which the size of the protein that may be determined is limited to ~10 KDa. This limitation is due to the need to assign NMR signals from the large number of nuclei in the protein – in larger protein, the greater number of nuclei results in overcrowding of resonances, and the increasing size of the protein also broadens the signals, making resonance assignment difficult. These problems may be alleviated by using heteronuclear NMR spectroscopy which allows the proton spectrum to be edited with respect to the ¹⁵N and ¹³C chemical shifts, and also reduces the overlap of resonances by increasing the number of dimensions of the spectrum. In 1990, Ad Bax and coworkers developed the triple resonance technology and experiments on proteins isotopically labelled with ¹⁵N and ¹³C, with the result that the spectra are dramatically simplified, greatly facilitating the process of resonance assignment, and increasing the size of the protein that may be determined by NMR.

These triple resonance experiments utilize the relatively large magnetic couplings between certain pairs of nuclei to establish their connectivity. Specifically, the ¹J_NH, ¹J_CH, ¹J_CC, and ¹J_CN couplings are used to establish the scalar connectivity pathway between nuclei. The magnetization transfer process takes place through multiple, efficient one-bond magnetization transfer steps, rather than a single step through the smaller and variable ³J_HH couplings. The relatively large size and good uniformity of the one-bond couplings allowed the design of efficient magnetization transfer schemes that are effectively uniform across a given protein, nearly independent of conformation. Triple resonance experiments involving ³¹P may also be use for nucleic acid studies.

These experiments are typically named by the nuclei (H, N, and C) involved in the experiment. CO refers to the carbonyl carbon, while CA and CB refer to Cα and Cβ respectively, similarly HA and HB for Hα and Hβ (see diagram for examples of experiments). The nuclei in the name are ordered in the same sequence as in the path of magnetization transfer, those nuclei placed within parentheses are involved in the magnetization transfer pathway but are not recorded. For reason of sensitivity, these experiments generally start on a proton and end on a proton, typically via INEPT and reverse INEPT steps. Therefore, many of these experiments are what may be called "out-and-back" experiments where, although not indicated in the name, the magnetization is transferred back to the starting proton for signal acquisition.

Some of the experiments are used in tandem for the resonance assignment of protein, for example HNCACB may be used together with CBCA(CO)NH as a pair of experiments. Not all of these experiments need to be recorded for sequential assignment (it can be done with as few as two), however extra pairs of experiments are useful for independent assessment of the correctness of the assignment, and the redundancy of information may be necessary when there is ambiguity in the assignments. Other experiments are also necessary to fully assign the side chain resonances.

TROSY versions of many of these experiments exist for improvement in sensitivity. Triple resonance experiments can also be used in sequence-specific backbone resonance assignment of magic angle spinning NMR spectra in solid-state NMR.

A large number triple-resonance NMR experiments have been created, and the experiments listed below is not meant to be exhaustive.

The experiment provides the connectivities between the amide of a residue with the carbonyl carbon of the preceding residues. It is the most sensitive of the triple resonance experiments. The sidechains carboxamides of asparagine and glutamine are also visible in this experiment. Additionally, the guanidino group of arginine, which has similar coupling constant to the carboxamide group, may also appear in this spectrum. This experiment is sometimes used together with HN(CA)CO.