Homonuclear correlations

The LR COSY-45° spectrum and H-NMR chemical shifts of an isoprenyl coumarin are given below. Determine the long-range H/ H homonuclear correlations based on the long-range COSV45° spectrum. Demonstrate with reference to problem -5.15 how they can be helpful in interconnecting different spin systems ... 

The HOHAHA spectrum (100 ms) of podophyllotoxin is presented. The HOHAHA, or TOCSY (total correlation spectroscopy), spectrum (100 ms) shows coupling interactions of all protons within a spin network, irrespective of whether they are directly coupled to one another or not. As in COSY spectra, peaks on the diagonal are ignored as they arise due to magnetization that is not modulated by coupling interactions. Podophyllotoxin has only one large spin system, extending from the C-1 proton to the C4 and 015 protons. Identify all homonuclear correlations of protons within this spin system based on the crosspeaks in the spectrum. 

The HMQC spectrum, H-NMR chemical shift assignments, and C-NMR data of vasicinone are shown. Consider the homonuclear correlations obtained from the COSY spectrum in Problem 5.14, and then determine the carbon framework of the spin systems. 

Fig. 8 Pulse sequence for the measurement of homonuclear correlation spectra with 7 filtering. Narrows bars indicate 7t/2 pulses and broad bars indicate 71 pulses. Experimental conditions employed for a-synuclein fibrils samples static field strength 9.4 T temperature —5 °C spinning frequency 8 kHz T2 filter 100 ms CP contact time 2.5 ms TOBSY mixing time 6 ms. The rf phases can be found in the original figure and caption. (Figure and caption adapted from the Supporting Information of [134]. Copyright (2005) National Academy of Sciences, USA)...

More recently, homonuclear correlation techniques relying on J-couplings have also been developed [86, 87, 89] and applied to the assignment of spin systems of amino-acid residues in uniformly labeled proteins and peptides [90]. These have, in some cases, a higher information content than the comparable dipolar-mediated experiments, as relayed correlations throughout the continuous 13C-13C network are more easily realized at high B0 fields [90]. 

Figures 13.7 and 13.8 are two examples of two-dimensional NMR spectroscopy applied to polymers. Figure 13.7 is the proton homonuclear correlated spectroscopy (COSY) contour plot of Allied 8207A poly(amide) 6 [29]. In this experiment, the normal NMR spectrum is along the diagonal. Whenever a cross peak occurs, it is indicative of protons that are three bonds apart. Consequently, the backbone methylenes of this particular polymer can be traced through their J-coupling. Figure 13.8 is the proton-carbon correlated (HETCOR) contour plot of Nylon 6 [29]. This experiment permits the mapping of the proton resonances into the carbon-13 resonances.

Fig. 13.7 Proton homonuclear correlated spectroscopy (COSY) contour plot of Allied 8207A, a poly(amide) 6.

Wemmer, D. E., Homonuclear Correlated Spectroscopy (COSY). Concepts in Magnetic Resonance An Educational Journal, 1989,1, (No 2)... 

Homonuclear correlation spectroscopy (COSY) experiments (see Chapter 9) substantiate the theoretical predictions, based on molecular orbital calculation, of the pattern of spin delocalization in the 3e orbitals of low-spin Fe(III) complexes of unsymmetrically substituted tetraphenylporphyrins [46]. Furthermore, the correlations observed show that this n electron spin density distribution is differently modified by the electronic properties of a mono-orf/io-substituted derivative, depending on the distribution of the electronic effect over both sets of pyrrole rings or only over the immediately adjacent pyrrole rings [46]. No NOESY cross peaks are detectable, consistently with expectations of small NOEs for relatively small molecules and effective paramagnetic relaxation [47]. 

Additional information on molecular conformation can be obtained by NOE, transfer NOE or 2-D homonuclear correlated NMR spectroscopy (COSY), which is used to measure the distance between nuclei. For detailed information on the various techniques see refs. 86 and 87. 

By way of example, useful 2-D techniques are homonuclear correlation spectroscopy (COSY), total correlation spectroscopy (TOCSY), heteronuclear multiple quantum coherence (HMQC) spectroscopy, and heteronuclear multiple quantum coherence/total... 

Homonuclear Correlations via direct (dipolar) couplings, D-HOMCOR... 

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