Pm Cullis

Calorimetric studies of macromolecular aqueous solutions

DoUi tilration and differential scanning microcalorimelric techniques are shown to yield important information concerning the properties of macromolecules in aqueous solution. Application of titration calorimetry is examined in the context of &aggregation of cationic miceiles (e.g. hexadecyilrimethyiaonium bromide), of guest-host interactions and of enzyme-substrale interactions (e.g. CAT,,, with cllloramphenicol). Experimenlal data oblained using DSC are reviewed with reference lo the thermal stability of enzymes (e.g. DNA-Gyrase) and lo the gel to liquid crystal transition characlerising vesicles foonned by dialkylphosphates. In Uw: latter, the dependcnce of the characteristic melting temperature T,,, on chain length, counter cation and added solute is noted.

Effects of added urea and alkylureas on gel to liquid-crystal transitions in DOAB vesicles

The gel to liquid-crystal transition for vesicles in aqueous solution formed by dimethyldi-n-octadecylammonium bromide (DOAB) occurs at 44.7°C. Moreover, the shapes of the scans recorded by a sensitive DSC microcalorimeter are very similar when the vesicular solutions are prepared starting with solid DOAB and comparable amounts of either solid urea or solid alkylureas. Therefore, the DOAB vesicles in aqueous solution accommodate this class of solutes without marked changes in the melting temperature and the enthalpy of the transition. The contrast with effects of added surfactants and simple organic solutes such as THF and ethanol is particularly significant.

Effects of added dotab on thecmc and enthalpy of micelle formation at 298.2 K for CTAB(aq)

In a titration calorimetric study an aqueous solution held in a syringe and containing hexadecyltrimethylammonium bromide (CTAB; 15.4×10−3 mol dm−3) is injected in aliquots (5×l0−3 dm3) into a sample cell containing initially water. Analysis of the data shows that thecmc equals 0.97×l0−3 dm−3 and the enthalpy of micelle formation equals −10.3 kJ mol−1. When the solution in the syringe is replaced by a mixed surfactant solution, CTAB+dodecyltrimethylammonium bromide, at the same total concentration of surfactant, thecmc of CTAB decreases gradually with increasing mole fraction of DOTAB but the enthalpy of CTAB micelle formation is hardly affected. We conclude, therefore, that incorporation of DOTAB monomers into the CTAB micelles stabilizes entropically the CTAB micelles.

Hexadecyltrimethylammonium chloride and bromide in aqueous solutions containing urea and several alkylureas: A differential scanning calorimetric study

With increase to the concentration of urea over the range 0 ⩽[urea]/mol dm–3⩽ 3.0, the extremum in the differential heat capacity, δCp, of hexadecyltrimethylammonium bromide (CTAB; aq; 0.02 mol dm–3; reference, water) moves from 45 °C to lower temperatures and the associated enthalpy change characterising cluster reorganisation decreases. Addition of ethylurea to this CTAB solution produces a more dramatic shift in the extremum; the effect is even more striking when 1, 3-diethylurea is added. Addition of other alkylureas including, 1,1-diethylurea both shifts the extremum in δCp and reduces its magnitude. A similar pattern emerges for aqueous solutions containing hexadecyltrimethylammonium chloride (CTAC). The effect of these ureas on CTAB(aq) is attributed to adsorption and penetration of the alkylureas into the CTAB micelles and clusters.

Differential scanning calorimetric study of n-hexadecyltrimethylammonium chloride in aqueous solutions

The isobaric heat capacity of n-hexadecyltrimethylammonium chloride (CTAC) in aqueous solutions containing 0.02 (mol monomer) dm–3 passes through a maximum at 294 K. The temperature, Tm, of the maximum shifts to higher temperatures with increase in concentration of CTAC. These extrema at Tm in the dependence on temperature of the isobaric heat capacity, Cp, measured using a differential scanning calorimeter, point to structural transitions in aqueous solutions containing CTAC. The processes responsible for the extrema at Tm are sensitive to added alcohol and added KCI. Replacement of water by D2O produces a shift in Tm to higher temperatures, whereas addition of urea produces a shift in Tm to lower temperatures. Tm shifts gradually to higher temperatures as the proportion of n-hexadecyltrimethylammonium bromide (CTAB) in a mixture of CTAB (aq) and CTAC(aq) increases. The data are quantitatively accounted for in terms of micelle clustering. The stoichiometries required to account for the dependence of Cp on temperature in the region of Tm point to processes involving several hundred monomers, a consequence of micelle clustering in these reasonably concentrated solutions. In the case of CTAC a simple two-state model suffices, whereas a more complex description is required for related CTAB solutions for which the dependence of Cp on temperature is characterised by extrema at 40.6 and 49.8 °C with an aggregation number of 650 where [CTAB]= 0.03 (mol monomer) dm–3.

Marked Effect of Buffers on Yield of Single- and Double-strand Breaks in DNA Irradiated at Room Temperature and at 77K

We have shown that the protocol for handling plasmid (pBR 322) DNA for radiation studies, which normally involves using quite high concentrations of Tris or phosphate buffers, is equally satisfactory in the absence of added buffers provided samples are stored at 0 degree C, or used directly. On exposure of aqueous solutions at 77 K, Tris buffer acts as a weak protecting agent, but phosphate buffers at the same pH act as remarkably effective sensitizing agents, giving ca. 100-fold increases in strand breaks. At room temperature there is again a marked difference between the aqueous and buffered systems, but in this case both the Tris and the phosphate systems are protective. Possible reasons for these contrasting results are discussed, and the advantages of using simple aqueous solutions are stressed.

Cluster reorganization in aqueous solutions containing cationic surfactants as recorded using differential scanning calorimetry

Abstract Dependences on temperature are reported for the isobaric heat capacities of moderately concentrated aqueous solutions containing the cationic surfactants hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, tetradecyltrimethylammonium bromide, and 1-methyl-4- n -dodecylpyridinium iodide. The dependences pass through maxima at temperatures characteristic of the solute. These extrema are formed by bell-shaped dependences of differential heat capacities on temperature. The bell shape can be accounted for by two-state equilibria of the general form X (aq) ⇌ Y (aq). Quantitative analysis shows that the equilibria involve several hundred surfactant monomers.

ELECTRON-PARAMAGNETIC RESONANCE STUDIES OF THE EFFECTS OF 1/1 ELECTROLYTES ON THE ACTION OF IONIZING-RADIATION ON AQUEOUS DNA

Exposure of dilute aqueous solutions of DNA to 60Co γ-rays at 77 K gives EPR spectra characteristic of ˙OH radicals in ice, and DNA radicals thought to be mainly G˙+ radical cations and a mixture of C˙– and T˙– radical anions. These DNA radicals are probably stabilized by rapid gain or loss of protons. The ˙OH radicals form H2O2 in the ice-phase at ca. 130 K. The T˙– centres are irreversibly converted to ˙TH radicals by protonation at C6 on annealing to ca. 200 K, but no other intermediate radical centres are clearly defined. All radical centres are lost prior to complete melting.On the addition of LiCl or NaCl in the 0–1.0 mol dm–3 region, there is a two-fold increase in the yields of DNA radicals, this being mainly in the anionic centres. Trapped Cl2˙– ions grow in with increasing concentration. On annealing, ˙TH radicals are formed, but, especially for LiCl systems, these are lost increasingly rapidly as the LiCl concentration increases. Despite the increases in target volume, very little change in the yields of strand-breaks is induced by these salts. Sodium bromide gives a smaller initial increase and more rapid loss of DNA radicals on annealing. The yields of ˙TH are considerably reduced indicating protection despite the increased target volume. This is reflected in a reduction in the yields of strand-breaks.The tetroxy salt sodium perchlorate also gives rise to an increase of initial DNA radical concentration. In contrast to the halide salts, however, this is assigned to an increase in [G˙+] with little or no effect upon the concentration of anionic centres. Strand-break studies show a marked protection of DNA by the perchlorate ion.These results are discussed in terms of the increase in effective target volume and the reactivities of the salt radicals Cl2˙–, Br2˙– and O˙–, all of which are detected by EPR spectroscopy.

Titration microcalorimetry of micelle formation in aqueous solutions containing alkylpolyoxyethylene glycol ethers, CH(OCHCH)(m)OH where m = 3, 4 and 5, and octadecyltrimethylammonium bromide

Titration microcalorimetric results are reported for alkylpolyoxyethylene glycol ethers, C12H25(OCH2CH2)mOH where m=3, 4 or 5 at ambient pressure and 298.2 K. The enthalpograms yield estimates of critical micelle concentrations (c.m.c.) which for these surfactants cover the range from 3.25(±0.22)×10−5 to 5.26(±0.49)×10−5 mol dm−3 with increasing integer m. The enthalpograms are complicated. Although the recorded enthalpies of injection are exothermic, the dominant process accompanying injection of an aliquot into the sample cell is not simple micelle deaggregation. Rather these surfactant solutions above the c.m.c. contain both micelles and higher aggregates. In a similar fashion, enthalpograms for aqueous solutions containing octadecyltrimethylammonium bromide, C18H37N+ Me3Br− (OCTAB) do not follow the previously reported pattern for n-hexadecyltrimethylammonium bromide (CTAB), pointing to the dramatic impact of an additional methylene group in the alkyl chain of the cationic surfactant on the aggregation of this surfactant in aqueous solution.

Vesicle - α-amino acid and vesicle-amide interactions: Effects of added α-amino acids and amides on gel to liquid-crystal transitions for four aqueous vesicular systems

Interactions of α-amino acids, glycine (gly), valine (val), leucine (leu) and phenylalanine (phe) and the closely related phenylalaninamide (phea) with cationic vesicles formed by dimethyldioctadecylammonium bromide (DOAB), dihexadecyldimethylammonium bromide (DHAB), with anionic vesicles formed by sodium ditetradecyl phosphate (DTP) and with zwitterionic vesicles formed by dimyristoyl (C14:0) phosphatidylcholine (DMPC) have been probed using differential scanning microcalorimetry (DSC). Gel-to-liquid transitions for DTP, DOAB, DHAB and DMPC vesicular systems are relatively unaffected by added α-amino acids. In contrast added phea markedly changes the recorded scans for DSC for DTP and DMPC vesicular systems indicating that small changes in the structure of added solutes can have dramatic effects on the structures and thermal characteristics of the vesicles in aqueous suspensions. The results indicate, perhaps surprisingly, that α-amino acids are not incorporated into the bilayers whereas phea is incorporated into DTP and DMPC bilayers.