Assignment Techniques

The assignment of signals to specific carbon atoms is a necessary prerequisite to the application of, 3C-n.m.r. spectroscopy in structural investigations. As assignment techniques have been described in numerous reviews and books,3,12 ,5 17 this area will be treated relatively 

A number of more complicated rules on the influence of axial or equatorial substituents on the chemical shifts of a-, /8-, or y-carbon atoms may be safely applied to simple, alicyclic molecules.3,37-39 In the authors opinion, however, such rules are generally of limited value for pyranoses or furanoses, because these contain several, mutually interacting substituents, and use of these rules has, in several instances, led to erroneous assignments. 

Whereas introduction of, 3C or deuterium onto carbon atoms requires more-or-less laborious syntheses, O-deuteration of hydroxyl groups or N-deuteration of amino groups is readily achieved by exchange of protons by deuterons with D20. In the deuterated carbohydrates thus obtained, only small isotopic-shifts are observed in the l3C-n.m.r. spectra however, when measured under appropriate conditions, these shifts are very useful for the assignment of 13C-sig-nals.50,57-61 

Correlation between proton and carbon chemical-shifts and coupling-constants may also be obtained through heteronuclear, two-dimensional, n.m.r. experiments.73,74 

Carbon-13 relaxation-rates of monosaccharides are dominated by dipolar-relaxation mechanisms,18,22 and primarily give information about molecular motion,75,76 in addition to the somewhat trivial distinction between C, CH, CH2, and CH3 groups. However, by measuring spectra with a suitable pulse-sequence, the differences in spin-lattice relaxation-rates can be used for the assignment of signals from overlapping CH and CH2 groups.77 

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The aforementioned assignment techniques based on dipole-dipole interactions are sufficient for the fibril samples with limited structural dynamics. In the presence of substantial dynamics, which may significantly attenuate the dipole-dipole... 

Nowadays, the NMR specialist finds an arsenal of assignment techniques at his disposal for structural determination. Long before they became available this responsibility lay mostly in the hands of chemists who could treat the information gleaned from coupling constants. Older NMR texts dedicated a large part of their content to the descriptions of spin-spin coupling patterns (order of spectra, two and higher order systems) as exemplified by Reference 5, which contains relevant J values. This content has been updated and placed on a very sound theoretical basis that is described in recent reviews. ... 

Be familiar with the use of various C assignment techniques (DEPT, off resonance, HMBC and HMQC)... 

There are many assignment techniques which can be run to aid interpretation and assignment of difficult NMR spectra, with the most commonly used being ... 

The sampling and assignment techniques used for the measurement of 13C-n.m.r. spectra of oligosaccharides are identical to those discussed previously for monosaccharides1. Particularly for oligosaccharides, it is important, when data are to be compared accurately, that is, with a precision better than 0.5 p.p.m., to measure the spectra at the same temperature. 

Signal assignment in routine NMR spectroscopy is usually facilitated by the application of two-dimensional assignment techniques such as homonuclear proton-proton shift correlated spectroscopy ( H/ H-COSY). 

Selective Hj NOE of quaternary carbons have been observed in selective decoupling experiments. (296,297) However, a study (517) of the frequency dependence of the NOE in [28] shows that, although differential NOE are observed for C-1 and C-3 when H-2 is irradiated with a field of strength yB2/2n 20 Hz, the frequency selectivity is not very good. The utility of the experiment as an assignment technique may therefore be somewhat restricted if the protons responsible for the selective NOE cannot be identified. 

NMR spectroscopy is the most important technique for the examination of stmcture and conformation in solution [370,371,372,373]. Many aspects of NMR spectroscopy can be employed for the study of carbohydrates [130,351,374,375,376,377]. Assignment techniques are discussed in many books and will not be considered here. Chapters by Widmalm [354] and Serianni [376] include sections on assignment of carbohydrate spectra. 

Several new assignment techniques for C solid-state NMR spectra have recently been proposed Spectral Editing by Multiple-Quantum Traps (SEMUT) [15], separated local field MAS [17-19], Windowless /sotropic Mixing for Spectral Editing (WIMSE) [20], Off-/ esonance Proton /rradiation (ORPI) [21,22], and cross-polarization with fast MAS ]23]. Their detailed performance has yet to be assessed, but it is already clear that the latter two methods are simple and eminently worthwhile. 

To determine the conformation of our complex CaM/C20W, the C20W peptide needs to be assigned as well. The techniques that were used for the protein caimot be applied, since the peptide is not labelled with and C. Thus, the traditional assignment techniques based on COSY, NOESY, and TOCSY are applied for the assignment [71]. The COSY and TOCSY spectra reveal the peak patterns of the amino acids, whereas the NOESY provides the sequential assignment of the peptide. The pulse sequences need to filter all... 

Besides the aforementioned time-domain approaches, many frequency-domain methods have also been developed and widely used. Examples are the complex curve fitting method [153], the maximum entropy method [4,263], the pole/zero assignment technique [271], the simultaneous frequency-domain approach [62], the rational fraction polynomial approach [219], the orthogonal polynomial approach [264], the polyreference frequency-domain approach [73], the multi-reference simultaneous frequency-domain approach [64] and the best-fit reciprocal vectors method [173]. 

The assignment techniques can genmlly be divided into gnq)h based methods and non grai based methods. First the non graph based methods will be presented. An expert system approach is used in the DAA system [KoTh85]. 

FAR 89] Farineau J., Lateral electric flight control laws of a civil aircraft based upon eigen structure assignment technique . Proceedings of AIAA Guidance, Navigation and Control Conference, Boston, MA, USA, 1989. 

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