Structural elucidation studies of erythromycins by electrospray tandem mass spectrometry

Erythromycin A (EryA) was studied by electrospray ionisation tandem mass spectrometry (ESI-MS/MS) with the aim of developing a methodology for the structural elucidation of novel erythromycins developed by biological synthetic methods. Skimmer dissociation along with sequential mass spectrometry studies (up to MS5) have been employed in this study. In the low-resolution MS/MS analysis of the polyketides, there are several fragment ions that are easily assigned to various neutral losses. These have all been confirmed by accurate-mass measurements. There is also a series of peaks due to ring opening and fragmentation that can only be assigned by high-resolution MSn analysis. Further experiments were performed in deuterated media (D2O/CD3OD 50%) which, along with the high-resolution MSn of erythromycin analogues, has enabled us to identify some of the steps in the ring fragmentation, particularly the loss of the polyketide starter acid. This is an essential step for determining structural alterations in the novel polyketides, but further labelling experiments and studies on more erythromycin analogues are required before the complete fragmentation pathway can be confirmed.     

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of dextran and dextrin derivatives

Application of matrix-assisted laser desorption/ionization (MALDI) to the analysis of dextran and dextrin derivatives, specifically glucose saccharides, by time-of-flight (TOF) mass spectrometry has been reported. MALDI-TOF analysis was carried out on alpha-, beta- and gamma-cyclodextrin, two O-methylated-beta-cyclodextrins of differing degrees of substitution (DS) and dextrans (a linear glucose saccharide), as pure and doped solutions and as mixtures of two or more of these. Doping was carried out with trace amounts of inorganic salts. The purpose of the analysis of the cyclodextrins was to determine whether they would form inclusion complexes with the various cations added, or whether less specific cation addition/exchange was occurring either prior to desorption or in the gas phase.     

Fabrication and characterization of photonic crystals with well-controlled thickness and stop-band attenuation

Photonic crystals with stop bands located in the visible region have been fabricated by crystallizing monodispersed spherical colloids (made of polystyrene or silica) into cubic-close-packed lattices within specially designed packing cells. These crystals were oriented with their (111) planes parallel to their solid supports, and the number of these planes could be conveniently controlled from 13 to 127 layers by varying the thickness of packing cells. In accordance, the stop-band attenuation of these crystals monotonically increased from 1 to 21 dB. Our transmission spectral measurements indicated that there exists a non-linear dependence between the stop-band attenuation and the total number of (111) planes, and this dependence could be quantitatively simulated using the dynamic light scattering model or the photonic analogy to KKR method. The colloidal crystals presented here should find use as components in fabricating optical devices that include sensors, mirrors, filters, switches and waveguides. Received: 17 May 2002 / Accepted: 25 July 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +1-206/685-8665, E-mail: xia@chem.washington.edu     

A comment on superspace Bianchi identities and six-dimensional spacetime

We investigate the Bianchi identities for a superspace with a six-dimensional spacetime, under the assumption of certain superspace kinematic constraints. It is shown that these constraints lead to the supergravity theory recently discussed by Breitenlohner and Kabelschacht. However, we conclude that the resulting theory is consistent only if the curvature scalar of the six-dimensional spacetime vanishes.     

Rigorous results in the mean-field theory of freezing

The mean-field theory of freezing, of which the Kirkwood-Monroe theory is a special case, is formulated in terms of a variational principle which can be derived from a statistical model. New upper and lower bounds on the free energy are found, and used to prove the existence of and to locate a freezing transition. The Kirkwood-Monroe theory is shown to obey a scaling law which implies that the freezing transition has no critical point and that the pressure-temperature phase diagram is a parabola. For a suitable choice of interaction potential, the free energy and density distribution of a crystalline state are calculated with very small error. The variational principle yields an integral equation found previously by van Kampen. This equation is derived rigorously, and shown to admit solutions of any chosen periodicity. The equation is transformed into a nonlinear integral equation of the Hammerstein type, and some standard uniqueness theorems applied. The equation is also transformed into a set of linear equations closely resembling the Kirkwood-Salsburg equations. Many unsolved problems are raised.     

An alternative formulation of rigorous mean-field theory

It is shown how to derive rigorous mean-field theory from a type of many-body interaction.     

The van der Waals limit for classical systems. I. A variational principle

We consider the thermodynamic pressurep(, ) of a classical system of particles with the two-body interaction potentialq(r)+ ^v K(r), where is the number of space dimensions, is a positive parameter, and is the chemical potential. The temperature is not shown in the notation. We prove rigorously, for hard-core potentialsq(r) and for a very general class of functionsK(s), that the limit 0 of the pressurep(, ) exists and is given by where the limit and the supremum can be interchanged. Here is a certain class of nonnegative, Riemann integrable functions,D is a cube of volume |D|, anda ⁰() is the free energy density of a system withK=0 and density . A similar result is proved for the free energy.     

Analysis of the products of the anionic oligomerisation of a phosphaalkene using MALDI-TOF mass spectrometry

The first use of MALDI-TOF MS in the study of the products of RLi (R = Me, Bu) initiated oligomerisation of P[double bond, length as m-dash]C bonds is reported. These studies may be considered as models for analogous polymerisation reactions. The detected linear products with R-P and C-H end-groups are consistent with a chain growth mechanism similar to that known for olefins. Interestingly, the results suggest that backbiting may compete with chain growth.