We have investigated the structure of mu-conotoxin GIIIA by 2D-NMR methods. The assignment of 1H NMR spectra and a quantitative analysis of NOE and J-coupling data are presented. These results were used for the calculation of secondary structure elements of mu-conotoxin GIIIA. Distance geometry calculations were carried out to define the global folding of the peptide.

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Field desorption (FD) mass spectrometry of positive ions is now an established analytical method, unlike field desorption of negative ions which has not yet been developed. This discrepancy is due to difficulties in producing negative ions under conditions of field electron emission. Because of the low electron affinity of molecules in comparison with the electron work function of normal cathode materials strong electron emission is obtained under conditions where the formation of molecular anions is to be expected by field ionization, i.e. tunnelling of electrons from the cathode into acceptor states of the molecules.The observed and disturbing effect of intense electron emission on the formation of negative field ions can be attributed to several processes: local heating of the surface, sputtering of the surface by impinging positive ions formed near the surface by electron impact, destruction of the surface by a discharge between the cathode and counter anode. Experimentally it is extremely difficult or impossible to detect any type of negative ion above the threshold of the steeply increasing electron current. It is obvious that ionization conditions that enable negative ions to be formed below the threshold of field electron emission are of particular interest.More recently the formation of negative ions by field ionization (FI) without a measurable electron emission current has been demonstrated'32 for a few compounds of higher electron affinity (EA 3 1.8 eV). The experiments performed so far show that this mode of FI cannot be extended to molecules of low electron affinities.FD mass spectrometry under reversed polarity conditions to detect negative ions was first tried by Anbar and St. John' who used the broken end of a thin rod (diameter 1.6 mm) as a cathode. The mass spectra were obtained from a mixture of inorganic samples with polyvinyl alcohol deposited o n the cathode 'tip'.t Authors to whom correspondence should be addressed.Although they have claimed to be able to avoid field electron emission, the spectra exhibit the formation of abundant fragment ions such as [HI-, [OH]-and [C2H]-, which provides evidence for ion formation in the course of a discharge between the cathode and the 50 p m distant anode. A more recent paper by Higuchi et aL4 reports negative ion FD spectra of peptides and sulfonic acids which were obtained under conditions of a high electron emission current. They used a magnetic field parallel to the wire cathode covered with silicon microneedles to deflect the electron beam from the ion optical system. The function of the magnetic field in these experiments is not clear.The present paper reports a method to obtain FD spectra of negative ions below the onset of field electron emission. In this method negative ions are extracted from viscous electrolytic solutions prepared by admixing of a high molecular weight polymer to the aqueous solution of samples. The sample mixtures are similar to those used in the experiments of Anbar and St. John.' Furthermore, it is demonstrated that polar...

The mass spectrometric analysis of salts can be carried out by dissolving the material of interest and polyethylene oxide 4000 (approx. 1:1) in water, applying the mixture to the emitter and introducing the latter into the field‐desorption source of a conventional mass spectrometer. After the application of 4 kV across the emitter, a negative‐ion mass spectrum can be recorded. The method has been applied to a considerable number of inorganic and organic salts.