Effect of mixing, concentration and temperature on the formation of mesostructured solutions and their role in the nucleation of DL-valine crystals

We report investigations on the formation of mesostructured solutions in DL-valine-water-2-propanol mixtures, and the crystallization of DL-valine from these solutions. Mesostructured liquid phases, similar to those previously observed in aqueous solutions of glycine and DL-alanine, were observed using Dynamic Light Scattering and Brownian microscopy, in both undersaturated and supersaturated solutions below a certain transition temperature. Careful experimentation was used to demonstrate that the optically clear mesostructured liquid phase, comprising colloidal mesoscale clusters dispersed within bulk solution, is thermodynamically stable and present in equilibrium with the solid phase at saturation conditions. Solutions prepared by slow cooling contained mesoscale clusters with a narrow size distribution and a mean hydrodynamic diameter of around 200 nm. Solutions of identical composition prepared by rapid isothermal mixing of valine aqueous solutions with 2-propanol contained mesoscale clusters which were significantly larger than those observed in slowly cooled solutions. The presence of larger mesoscale clusters was found to correspond to faster nucleation. Observed induction times were strongly dependent on the rapid initial mixing step, although solutions were left undisturbed afterwards and the induction times observed were up to two orders of magnitude longer than the initial mixing period. We propose that mesoscale clusters above a certain critical size are likely to be the location of productive nucleation events

Formation of valine microcrystals through rapid antisolvent precipitation

In this work we have experimentally studied concentration effects on antisolvent precipitation of valine (an amino acid) from aqueous isopropanol solutions. Our experiments showed that the valine precipitation is very sensitive to both the supersaturation and to the water content in the final solution. Results from spectrophotometric measurements and supersaturation analysis showed that the crystal formation kinetics are strongly dependent on both mixing and concentration profiles in the early stages of the process, even though no visible change in the systems occurs immediately upon mixing with the antisolvent or subsequent dilution. Results from small-angle static light scattering measurements showed that the first visible crystals are of micron size and they grow only little over time, while their number increases gradually. Taken together, these experiments point to intermediate phase separation of (possible amorphous) precursors, being either very small nanoparticles or droplets with their refractive index closely matching that of the continuous phase, which subsequently assemble into micron size valine crystals

Spectator 1960-09-30

vii, 244 p.; 23 cm

Nanoparticle-coated microcrystals

Coprecipitation provides a rapid high-yield method for self-assembly of nanoparticles on the surface of flat water-soluble crystalline surfaces and a simple immobilisation technique prior to storage or thermal and chemical modification

Complementarity and quantum walks

We show that quantum walks interpolate between a coherent `wave walk' and a random walk depending on how strongly the walker's coin state is measured; i.e., the quantum walk exhibits the quintessentially quantum property of complementarity, which is manifested as a trade-off between knowledge of which path the walker takes vs the sharpness of the interference pattern. A physical implementation of a quantum walk (the quantum quincunx) should thus have an identifiable walker and the capacity to demonstrate the interpolation between wave walk and random walk depending on the strength of measurement.Comment: 7 pages, RevTex, 2 figures; v2 adds references; v3 updated to incorporate feedback and updated references; v4 substantially expanded to clarify presentatio

DNA-coated microcrystals

Coprecipitation leads to self-assembly of bioactive DNA on the surface of salt, sugar or amino-acid crystals and provides a rapid inexpensive immobilization method suitable for preparing dry-powder formulations of nucleic acids, useful for storage, imaging and drug delivery

Preferences for the selection of unique tRNA primers revealed from analysis of HIV-1 replication in peripheral blood mononuclear cells

BACKGROUND: All human immunodeficiency virus (HIV-1) uses a host tRNA(Lys,3 )as the primer for reverse transcription. The tRNA(Lys,3 )is bound to a region on the HIV-1 genome, the primer-binding site (PBS), that is complementary to the 18 terminal nucleotides of tRNA(Lys,3). How HIV-1 selects the tRNA from the intracellular milieu is unresolved. RESULTS: HIV-1 tRNA primer selection has been investigated using viruses in which the primer-binding site (PBS) and a sequence within U5 were altered so as to be complementary to tRNA(Met), tRNA(Pro )or tRNA(Ile). Analysis of the replication of these viruses in human peripheral blood mononuclear cells (PBMC) revealed preferences for the selection of certain tRNAs. HIV-1 with the PBS altered to be complementary to tRNA(Met), with and without the additional mutation in U5 to be complementary to the anticodon of tRNA(Met), stably maintains the PBS complementary to tRNA(Met )following extended in vitro culture in PBMC. In contrast, viruses with either the PBS or PBS and U5 mutated to be complementary to tRNA(Ile )were unstable during in vitro replication in PBMC and reverted to utilize tRNA(Lys,3). Viruses with the PBS altered to be complementary to tRNA(Pro )replicated in PBMC but reverted to use tRNA(Lys,3); viruses with mutations in both the U5 and PBS complementary to tRNA(Pro )maintained this PBS, yet replicated poorly in PBMC. CONCLUSION: The results of these studies demonstrate that HIV-1 has preferences for selection of certain tRNAs for high-level replication in PBMC

Atmospheric trace metal concentrations, solubility and deposition fluxes in remote marine air over the south-east Atlantic

Total and soluble trace metal concentrations were determined in atmospheric aerosol and rainwater samples collected during seven cruises in the south-east Atlantic. Back trajectories indicated the samples all represented remote marine air masses, consistent with climatological expectations. Aerosol trace metal loadings were similar to previous measurements in clean, marine air masses. Median total Fe, Al, Mn, V, Co and Zn concentrations were 206, 346, 5, 3, 0.7 and 11 pmol m-3 respectively. Solubility was operationally defined as the fraction extractable using a pH4.7 ammonium acetate leach. Median soluble Fe, Al, Mn, V, Co, Zn, Cu, Ni, Cd and Pb concentrations were 6, 55, 1, 0.7, 0.06, 24, 2, 1, 0.05 and 0.3 pmol m-3 respectively. Large ranges in fractional solubility were observed for all elements except Co; median solubility values for Fe, Al and Mn were below 20% while the median for Zn was 74%. Volume weighted mean rainwater concentrations were 704, 792, 32, 10, 3, 686, 25, 0.02, 0.3 and 10 nmol L-1 for Fe, Al, Mn, V, Co, Zn, Cu, Ni, Cd and Pb respectively (n = 6). Wet deposition fluxes calculated from these values suggest rain makes a significant contribution to total deposition in the study area for all elements except perhaps Ni