Double resonance biomolecules

As virtually all biological molecules possess low vapour pressures gas-phase molecular spectroscopy methods to investigate isolated neutral biomolecules require volatilization methods other than thermal evaporation. The combination of laser desorption with a supersonic molecular beam expansion together with the selectivity of IR and UV double resonance methods opened up the possibility of characterizing isolated, neutral biomolecules and their clusters with the biological environment. 

The preceding section on gramicidin S demonstrates how the combination of buffer-gas cooling and IR-UV double-resonance spectroscopy can simplify the spectrum of a decapeptide and allow its precise structural determination in the gas phase as well as elucidate the key structural changes upon stepwise solvation. As one pushes these techniques to larger biomolecules, the increasing number of stable conformations they can adopt increase the degree of spectral complexity,... 

A chapter of this special issue is fully dedicated to microwave spectroscopy of biomolecules and contains a full section on CBHs. The observation of the pure rotational spectrum offers the advantage of not requiring additional aromatic chromophores for spectroscopic detection, so it may complement the vibrational data obtained from double resonance spectroscopy experiments, especially for the smallest CBHs. 

Probably, one of the most valuable advances in this field has dealt with the first chemoenzymatic synthesis of the stable isotope-enriched heparin from a uniformly double labelled 13C, 15N /V-acetylheparosan from E. coli K5. Heteronuclear, multidimensional nuclear magnetic resonance spectroscopy was employed to analyze the chemical composition and solution conformation of N-acety 1 hcparosan, the precursors, and heparin. Isotopic enrichment was found to provide well-resolved 13C spectra with the high sensitivity required for conformational studies of these biomolecules. Stable isotope-labelled heparin was indistinguishable from heparin derived from animal tissues and might be employed as a novel tool for studying the interaction of heparin with different receptors.30... 

For relaxation studies of biomolecules in solution (which is no specialty of the authors of this chapter), it is often essential to use inverse-detection schemes to obtain reasonable sensitivity. Furthermore, besides problems with poor sensitivity, the carbon-13 and nitrogen-15 spectra are often too crowded to allow measurement of individual relaxation rates for different I nuclei, either by direct detection or by indirectly detecting the protons. If this is the situation, one can spread out the I nuclei signals for better resolution of individual resonances by detecting a two-dimensional H-I correlation spectrum. Relaxation experiments of this type can be considered a modification of the double polarization-transfer IS correlation experiment [7, 17] ... 

Nitrogen is closely concerned with many or most vital processes. With 95%-99% bioenrichment, which can be achieved with relatively cheap materials such as NH salts, N is several times more NMR sensitive than naturally-abundant C, and can give useful labeling information. Sensitivity enhancement methods such as DEPT or INEPT are of great value, with or without N-enrichment. ° ° Also useful, particularly for in vivo work, is indirect detection of N by double quantum proton NMR. Applications of solid state techniques to biomolecules in N or N resonance were described in Section 1.3. 

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