Reactions of sodium sulphide—I : With compounds containing hydroxyl groups

Aqueous solutions of sodium sulphide (ca 10%) reagent were reacted at 250° with compounds containing OH groups. This work was part of a programme designed to test the usefulness of the Na₂S solution degradation technique for determining the primary structure of humic acids. The compounds reacted here and the principal products detected (shown in brackets) were: cinnamyl alcohol (benzyl alcohol, benzoic acid, 3-phenylpropan-1-ol, 3-phenylpropionic acid), benzyl alcohol (little reaction), 3-phenylpropan-1-ol (little reaction), 1,2-dihydroxy-1-phenylethane (phenylacetic acid, acetophenone, 1-phenylethanol), catechol and hydroquinone (recovered starting compounds+ polymer products). Sodium hydroxide gave the same products with cinnamyl alcohol, but in different proportions. Possible mechanisms for these reactions are discussed.     

A finite element method for modelling dissipative mufflers with a locally reactive lining

The transmission loss of a lined expansion chamber muffler with zero mean flow is studied by using a finite element model of the system. In the cases considered the lining is arranged in small cells so that it is reasonable to assume a locally reacting effect. An existing empirical relationship is used to calculate the lining impedance. The results show that the lining always increases the transmission loss over the unlined chamber valuys; with a thin lining its effect is similar to that of a side branch filter. However, as the thickness increases and the lining impedance approaches the characteristic impedance for the absorption material, the transmission loss develops a single maximum similar to the attenuation produced by a thick lining on the primary mode in a lined duct.     

Evidence of an intermediate and parallel pathways in protein unfolding from single-molecule fluorescence

Determining how proteins fold into their native structures is a subject of great importance, since ultimately it will allow protein structure and function to be predicted from primary sequence data. In addition, there is now a clear link between protein unfolding and misfolding events and many disease states. However, since proteins fold over rugged, multidimensional energy landscapes, this is a challenging experimental and theoretical problem. Single-molecule fluorescence methods developed over the past decade have the potential to follow the unfolding/folding of individual molecules. Mapping out the landscape without ensemble averaging will enable the identification of intermediate states which may not be significantly populated, in addition to the presence of multiple pathways. To date, there have been only a limited number of single-molecule folding/unfolding studies under nonequilibrium conditions and no intermediates have been observed. Here, for the first time, we present a single-molecule study of the unfolding of a large autofluorescent protein, Citrine, a variant of green fluorescent protein. Single-molecule fluorescence techniques are used to directly detect an intermediate on the unfolding/folding pathway and the existence of parallel unfolding pathways. This work, and the novel methods used, shows that single-molecule fluorescence can now provide new, hitherto experimentally inaccessible, insights into the folding/unfolding of proteins.     

19F NMR studies of the native and denatured states of green fluorescent protein

Biosynthetic preparation and (19)F NMR experiments on uniformly 3-fluorotyrosine-labeled green fluorescent protein (GFP) are described. The (19)F NMR signals of all 10 fluorotyrosines are resolved in the protein spectrum with signals spread over 10 ppm. Each tyrosine in GFP was mutated in turn to phenylalanine. The spectra of the Tyr --> Phe mutants, in conjunction with relaxation data and results from (19)F photo-CIDNP (chemically induced dynamic nuclear polarization) experiments, yielded a full (19)F NMR assignment. Two (19)F-Tyr residues (Y92 and Y143) were found to yield pairs of signals originating from ring-flip conformers; these two residues must therefore be immobilized in the native structure and have (19)F nuclei in two magnetically distinct positions depending on the orientation of the aromatic ring. Photo-CIDNP experiments were undertaken to probe further the structure of the native and denatured states. The observed NMR signal enhancements were found to be consistent with calculations of the HOMO (highest occupied molecular orbital) accessibilities of the tyrosine residues. The photo-CIDNP spectrum of native GFP shows four peaks corresponding to the four tyrosine residues that have solvent-exposed HOMOs. In contrast, the photo-CIDNP spectra of various denatured states of GFP show only two peaks corresponding to the (19)F-labeled tyrosine side chains and the (19)F-labeled Y66 of the chromophore. These data suggest that the pH-denatured and GdnDCl-denatured states are similar in terms of the chemical environments of the tyrosine residues. Further analysis of the sign and amplitude of the photo-CIDNP effect, however, provided strong evidence that the denatured state at pH 2.9 has significantly different properties and appears to be heterogeneous, containing subensembles with significantly different rotational correlation times.     

An Experiment in Teaching Applied Mathematics in an Australian University

This article describes an experiment in the teaching of an ‘Applied Mathematics’ course at the University of Melbourne. The syllabus covered was not that of the conventional classical mechanics course but included topics felt to be attractive to first‐year science undergraduates.     

PVC ion-selective electrodes based on calcium bis-[dialkyl- and di-(4-alkylphenyl) phosphates] and mixed solvent mediators

PVC calcium ion-selective electrodes based on either calcium bis-di(n-decyl)phosphate or calcium bis-di[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphate with various solvent mediators (alone or in pairs) have been evaluated with particular respect to interference from Na(+), K(+), Mg(2+), Sr(2+), Ba(2+), Mn(2+), Cu(2+), Ni(2+) and Zn(2+) ions. PVC calcium ion-selective electrodes based on calcium bis-di[4-(n-octyl)phenyl]phosphate with varied amounts of decan-l-ol plus di-n-octyl phenylphosphonate showed a continuous gradation in selectivity coefficients on going from a high fraction of decan-l-ol to a high fraction of the second mediator. Thus, k(pot)(Ca,Mg) changed from 1.6 for an electrode based exclusively on decan-l-ol to 4.9 x 10(-4) for one based completely on di-n-octyl phenylphosphonate. The corresponding k(pot)(Ca,Na) values were 7.0 x 10(-2) and 1.1 x 10(-3).     

On the algebra of handle operations in 4-manifolds

The possibility is considered that the Andrews-Curtis conjecture, if false, may provide obstructions to the cancellation of 1- and 3-handles in a simply connected 4-manifold. An algebra is developed for the simultaneous algebraic reading of attaching maps for 2-handles and their duals, and it is shown that obstructions to 1- and 3-handle cancellation cannot bedetected in this algebra.