Hydrogen NMR

Two features of the NMR structure present relevant new information. Phe-82 and Phe-29 are tyrosine and tryptophan, respectively, in the algal protein, and are apparently now, according to NMR, hydrogen bonded (internally) to each other. The deletion of the two residues 57 and 58 leads to an alteration of the shape of the acidic patch, a step back to the absence of the patch in evolutionarily more primitive plastocyanin. 

Most aldohexopyranoses exist in a chair form in which the hydroxymethyl group at C(5) assumes an equatorial position. All the P-D-hexopyranoses exist predominantly in the 4Ci form since the alternative C4 conformer involves a large unfavourable xyn-diaxial interaction between the hydroxymethyl and anomeric group (Figure 1.8). Most of the a-D-hexopyranosides also adopt the 4Ci conformation preferentially. Only ot-idopyranoside and a-D-altropyranose show a tendency to exist in the C4 conformation, and they coexist with the alternative 4Ci conformations according to H-NMR (hydrogen nuclear magnetic resonance) spectroscopy studies. 

Ditchfield, R. and McKinney, R. E., Molecular orbital studies of the NMR hydrogen bond shift, Chem. Phys. 13, 187-194 (1976). 

Often, experimental studies of lipid systems are based on spectroscopic approaches, which in turn frequently employ probes for enhancement of sensitivity and resolution. For example, in NMR, hydrogen atoms of lipids are replaced with deuterium, and in fluorescence spectroscopy and imaging, native lipid molecules are replaced with lipids in which one of the hydrocarbon chains is linked covalently to a fluorescent marker such as pyrene or diphenylhexatriene. Fluorescent markers allow one to follow numerous cellular processes in real time, such as intracellular trafficking of molecules and formation of domains within a biomembrane, see Fig. 3. The downside is that the probes tend to perturb their environment and affect the thermodynamic state of the system. Experiments have shown, for example, that probes may change the main transition temperature of a lipid membrane, and that the dynamics of probes may deviate considerably from the dynamics of corresponding native molecules (see discussion in Reference 27). Therefore, we wish to pose several questions. What is the range of perturbations induced by the probe How significant are these perturbations actually ... 

Other correlations of NMR hydrogen-bonding shifts have been noted, e.g. with pK of proton donor, with for OD stretchings and for C=C stretching (of propargyl bromide),and with AG of... 

The most reliable method for characterization of the product H-phosphonate IS H and P NMR. Hydrogen-phosphonates have a characteristic coupling constant of the P-H (Jp.n = 600-605 Hz) that can be seen in tire H-NMR spectrum as well as in the H-coupled P NMR spectrum. Furthermore, the P NMR spectrum of a nucleoside 3 -H-phos-phonate is split into another doublet due to the C-3 hydrogen (Jp.n = 8-12 Hz). 

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