COSY-type spectra

Multidimensional NMR spectra are not restricted to cases where the separate frequency axes encode signals from different nuclear types. Indeed, much of the early work on the development of 2D NMR was performed on cases where both axes involved chemipal shifts. The main value in such spectra comes from the information content in cross peaks between pairs of protons. In COSY-type spectra (COSY = Correlation SpectroscopY) cross peaks occur only between protons that are scalar coupled (i.e., within 2 or 3 bonds) to each other, whereas in NOESY (NOE Spectroscopy) spectra cross peaks occur for protons that are physically close in space (<5 A apart). A combination of these two types of 2D spectra may be used to assign the NMR signals of small proteins and provides sufficient information on internuclear distances to calculate three-dimensional structures. Figure 12.3 includes a panel showing the COSY spectrum of cyclosporin and highlights the relationships between ID H-NMR spectra and corresponding 2D homonuclear (COSY) and heteronuclear (HSQC) spectra. 

Generation of spin coupling patterns (ISNets) from COSY-type spectra... 

Schmieder P, Stem AS, Wagner G, Hoch JC (1993) Application of nonlinear sampling schemes to COSY-type spectra. J Biomol NMR 3 569-576... 

FIGURE 9 The interpretation of a COSY-type spectrum and a TOCSY-type spectrum. Only half side peaks are listed. Two spectra are overlaid. T is the merger tolerance for comparing two peaks in the same spectrum (here a COSY spectrum) is the comparison tolerance for comparing two peaks from different spectra (here COSY and TOCSY spectra). 

Multiple-quantum correlation spectra provide information about through-bond connectivities as all COSY type experiments do. In addition, direct topology information is also available from the same spectrum through remote and combination peaks [5]. Correlation peaks between spins with small chemical shift difference can be examined, too, since there are no diagonal peaks. In this sense, a correlation of MQ coherences with those... 

Short mixing times (e.g., 30 ms) lead to INEPT-type spectra (or COSY-type 2D spectra), where transfer is mostly limited to a single jump over one J coupling. Unlike INEPT and COSY, however, the transfer results in an in-phase rather than antiphase signal. This is a significant advantage as the peaks have the same shape and pattern as they do in a ID spectrum. 

In Check its 2.33.7 and 2.3.3.8 a phase-cycled COSY magnitude spectrum is converted into a gradient selected COSY spectrum. The Check its also show how the choice of either the n- or p-type CT pathways influences the frequency display in the fl dimension. 

Multiple-quantum homonuclear experiment Incorporation of a multiple-quantum filter into a COSY-type sequence reduces the complexity of the spectrum, and aids in the assignment process. 

A second 2D NMR method called HETCOR (heteronuclear chemical shift correlation) is a type of COSY in which the two frequency axes are the chemical shifts for different nuclei usually H and With HETCOR it is possible to relate a peak m a C spectrum to the H signal of the protons attached to that carbon As we did with COSY we 11 use 2 hexanone to illustrate the technique... 

It was straightforward to identify the spin systems of four valines, five threonines, four alanines, three glycines, two glutamates, and AMX type residues (Asp, Cys, Phe, Tyr, Trp, and Ser) of this protein from the COSY, RELAY, and NOESY spectra in D O solution. The RELAY spectrum was particularly useful in identification of Val, Ala, Thr, and Glu spin systems. 

Figure 7.16 When soft pulses are used for excitation and mixing in a 2D experiment, it becomes a 2D soft experiment. The spectrum of the 2D soft experiment has reduced frequency ranges in F, and Fj. The excitation ranges of the selective pulse depend on the type of experiment. For example, in a soft COSY-COSY experiment, one multiplet is excited, while in the soft NOESYexperiment the whole resonance region of a group of signals is excited. (Reprinted from Mag. Reson. Chem. 29, H. Kessler et al, 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

Figure 18. A 0.2-s delayed COSY spectrum of the aliphatic region of 10 (2mg, CDCLj). Long-range "W-type" coupling of 19 and 21 axial protons to 30-CHj and coupling across the gem dimethyls from I9eq to 21 eq establish the position of oxidation at C-22. The spectrum was obtained under conditions similar to those in Figure 1, except that 32 transients were acquired for each of 128 x 512 data point spectra (17).

A theoretical COSY spectrum for two coupled methine protons (AX system) is shown in Figure 9.44, the chemical shifts of the two protons being 8A and 8X ppm. The theoretical 1 -D proton spectrum is shown alongside each frequency axis, Ft and F2, the two scales being equal. Two types of... 

Schematic COSY spectrum of a two coupled spins, denoted A and X. For convenience, the normal one-dimensional spectrum is plotted alongside the F and F2 axes and the diagonal (F t = F2) is indicated by a dashed line. This spectrum shows two types of multiplets those centred at the same F t and F2 frequencies, called diagonal-peak multiplets, and those centred at different frequencies in the two dimensions, called cross-peak multiplets. Each multiplet has four component peaks. The appearance of a cross-peaked multiplet centred at I = A, F2 = 8x indicates that the proton with shift A is coupled to the proton with shift A. This observation is all that is required to interpret a COSY spectrum.

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