NOESY complications with

H is particularly important in NMR experiments because of its high sensitivity and natural abundance. For macromolecules, 1H NMR spectra can become quite complicated. Even a small protein has hundreds of 1H atoms, typically resulting in a one-dimensional NMR spectrum too complex for analysis. Structural analysis of proteins became possible with the advent of two-dimensional NMR techniques (Fig. 3). These methods allow measurement of distance-dependent coupling of nuclear spins in nearby atoms through space (the nuclear Overhauser effect (NOE), in a method dubbed NOESY) or the coupling of nuclear spins in atoms connected by covalent bonds (total correlation spectroscopy, or TOCSY). 

At least three factors complicate the analysis of NOESY spectra. First, COSY signals may be present from scalar couplings and may interfere with interpretations intended to be based entirely on interproton distances. (Vicinal couplings, for example, are largest when the coupled nuclei are farthest apart, a condition that obtains in the antiperiplanar geometry.) Such unwanted spectral features are called artifacts. COSY signals may be reduced through... 

The interpretation of ROESY experiments is not devoid of complications and hence, if the molecular tumbling frequencies allow it, NOESY experiments are to be preferred. One of the complications of ROESY experiments is the possible appearance of cross-peaks originating from scalar coupling instead of cross-relaxation. These crosspeaks have the same origin as the ones that are observed in TOCSY (total-correlation spectroscopy) experiments and can be distinguished from the cross-relaxation peaks because they have opposite sign. However, TOCSY crosspeaks may cancel with NOESY peaks involving the same protons. 

These experiments use pulse sequences similar to that of the spin-echo experiment. There are a number of experiments with different pulse sequences. The different experiments are commonly named with acronyms. For example, COSY (Correlation SpectroscopY) was the original two-dimensional NMR experiment. Some other experiments are NOESY (Nuclear Overhauser Effect SpectroscopY) and HETCOR (HETeronuclear CORrelation). The usual goal of a COSY experiment is to determine which lines belong to which multiplet in a complicated spectrum with overlapping multiplets. We will give a simplified description of the COSY experiment for proton NMR (this is sometimes called HH-COSY). 

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