Na)-at m / z 2306 ( m / z 2305 in nominal mass) was selected as the precursor. Ion peaks at m / z 851/835 (821), 573/557 (543), and 469/453 (423) due to cleaved bonds at either side of the ether oxygens allowed us to sequence four blocks (C37<53,C54<57, C58<69, and C 7 M 1 3 5 , Figure 1). Characteristic fragmentations at specific sites of the rings9 provided invaluable information about ring size ( Figure 1). Particularly, they were of great help in assigning rings S and Y because 'H NMR signals of C87/C88 and C108/C109 heavily overlapped and could not be interpreted with confidence.Twdimensional NMR data of 1 enabled us to connect 2 with the rest of the molecule, fragments A and C. DQF-COSY and TOCSY8 revealed spin connections due to H35/H36/H37/H38 and H134/H135/H136/H137. Three vicinal diols in rings K, L, and N cleaved by periodate were reconstructed on the basis of 'JH, H and NOESY data* of 1 (see supplementary material).With these data, the entire structure of maitotoxin (1) is disclosed for the first time: It is a C142 carbon chain of composition Cl64H2&,&Na2 (molecular weight of 3422 in nominal mass as disodium salt),1° encompassing 32 ether rings, 28 hydroxyl groups,II and two sulfate esters. Most of the ether rings are probably trans-fused as is the case with brevetoxini2 except for rings L/M and N/O, for which NOE data suggested cis-fusion (see supplementaiy material). Severe overlapping of both I3C and 'H NMR signals was overcome by repeated spectral measurements in different solvents6*8 and by application of new NMR methods (e.g., 2D HMQC-TOCSY or HSQC). Even so, assignment of NMR signals was sometimes imperfect with only 10 mM of 1 or with 3 mM of 2. Three-dimensional lH-l3C-IH NMR experiments will be attempted with a 13C-enriched sample, which should help to confirm the structure of MTX. Synthesis of labeled MTX in G. toxicus culture will also shed light on mechanisms of action, and on the intriguing relationship of MTX to its companion ciguatoxin. Supplementary Material Available:Summary of NMR experiments showing connectivities assigned by NMR, stereochemical assignments of rings K-0 of 1, ' H NMR assignments of 2 and its acetyl derivative, partial ' H NMR assignments of 1, NOESY and TOCSY of 2, TOCSY of the acetyl derivative of 2, negative FAB MS/MS of 2, IH NMR 1D spectrum of 1, NOESY of 1, DQF-COSY of 1 in two different solvents, HMBC of 1 (21 pages). 'H-'H COSY, TOCSY, and NOESY of 1 in CD30D and 2D HMQC-TOCSY (13C-IH HOHAHA) of 1 in CD3CN-D20, 1 : 1, are available as supplementary material to a previous paper.5 Ordering information is given on any current masthead page. (9) Naoki, H.; Murata, M.; Yasumoto, T. Rapid Commun. Mass Spectrom., in press.(1 0) The presence of nitrogen suggested in a previous paper? was ruled out by elementary analyses with 1.( 1 1) The number of hydroxyl groups was determined on the basis of deuterium shifts in I 'C NMR signals between spectra obtained in CD,CN-D20 and in CD,CN-H?O, and of comparison of 2 with its acetyl derivative in ' H NMR chemical shifts. (12)...

The problem of the convergence of a sequence of Hartree—Fock—Roothaan wavefunctions and energy values to the true Hartree—Fock results is examined for N2(X1Σg+). This critical study is based on a hierarchy of Hartree—Fock—Roothaan wavefunctions which differ in the size and composition of the expansion basis set in terms of STO symmetry orbitals. The concluding basis set gives a total Hartree—Fock energy of −108.9956 hartree and Re(HF)=2.0132 bohr for N2(X1Σg+). Results are also presented from direct calculations for three states of the N2+ molecular ion (X2Σg+, A2Πu, B2Σu+) which are also thought to be very close approximations to the true Hartree—Fock values. The results give EHF=−108.4079, −108.4320, and −108.2702 hartree and Re(HF)=2.0385, 2.134, and 1.934 bohr for the X2Σg+, A2Πu, and B2Σu+ states of N2+, respectively. Extensive calculations for various R values establish that the X2Σg+ and A2Πu states are reversed in order relative to experiment, a short-coming ascribed to the Hartree—Fock approximation.

The effects of various preparation parameters on vanadium reduction in Bi 4V 2 O 11−d have been investigated using EPR, 51V MAS solid state NMR and UV diffuse-reflectance spectroscopies and also SQUID magnetometry and powder neutron diffraction. The results confirm that a greater amount of vanadium reduction is observed in rapidly quenched samples and that significant oxidation occurs when samples are slow cooled. Evidence for spin-spin dipolar coupling is seen in the EPR patterns while uncoupled V4+ spins contribute to weak paramagnetic behaviour. Band gaps of around 2 eV from the UV data suggest there may be a significant electronic component to low temperature conductivities. The 51V NMR data are not inconsistent with the presence of mainly distorted octahedral and tetrahedral coordinations for vanadium.The relationship of the three major polymorphs in Bi 4 V 2 O 11

Restrained molecular dynamics simulations were used to study the interactions between the glycopeptide antibiotic vancomycin and the dipeptide Ac-D-Ala-D-Ala. Restraints were obtained from a combination of homonuclear and heteronuclear two-dimensional NMR experiments (NOESY, ROESY, 1H-15N inverse correlation). The comparison between the structures obtained for vancomycin alone and for the complex suggests a new hypothesis on the binding mode of this system. The numerical simulations were not straightforward because vancomycin is made of building blocks for which standard force-fields are not available. The representation of unusual chemical environments is also mandatory. We believe that our extension of the force-field parameters to our system could be of more general interest. Furthermore, we consider vancomycin and its complex a good example for exploring the more general problem of molecular recognition, a challenge that has been widely approached in the past few years but for which no unique and general methodology has, so far, been recognized.

The electronic spectra, magnetic susceptibilities, and preliminary e.s.r. data are reported for the trigonal compounds M[N(SiMe,),],where M = Sc,Ti, V, Cr,and Fe. The data are interpreted on the basis of crystal-field calculations for d" systems in D3, symmetry.