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Langmuir ; 29(31): 9874-80, 2013 Aug 06.
Artigo em Inglês | MEDLINE | ID: mdl-23837868


Lipid cubic phases are complex nanostructures that form naturally in a variety of biological systems, with applications including drug delivery and nanotemplating. Most X-ray scattering studies on lipid cubic phases have used unoriented polydomain samples as either bulk gels or suspensions of micrometer-sized cubosomes. We present a method of investigating cubic phases in a new form, as supported thin films that can be analyzed using grazing incidence small-angle X-ray scattering (GISAXS). We present GISAXS data on three lipid systems: phytantriol and two grades of monoolein (research and industrial). The use of thin films brings a number of advantages. First, the samples exhibit a high degree of uniaxial orientation about the substrate normal. Second, the new morphology allows precise control of the substrate mesophase geometry and lattice parameter using a controlled temperature and humidity environment, and we demonstrate the controllable formation of oriented diamond and gyroid inverse bicontinuous cubic along with lamellar phases. Finally, the thin film morphology allows the induction of reversible phase transitions between these mesophase structures by changes in humidity on subminute time scales, and we present time-resolved GISAXS data monitoring these transformations.

Lipídeos/química , Tamanho da Partícula , Espalhamento a Baixo Ângulo , Propriedades de Superfície , Difração de Raios X
Phys Chem Chem Phys ; 14(1): 353-66, 2012 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-22089140


Linear dichroism (LD), a spectroscopic method for aligned samples, has been used with a synchrotron radiation source to reveal insights into the structure and stability of DNA with increasing salt concentrations (thus stabilizing the base pairing) and increasing temperature while remaining below the melting point (thus destabilizing the base pairing). Measurements have been made from 350 nm to 182 nm, and the spectral changes observed quantified using a Bayesian Markov chain Monte Carlo (MCMC) algorithm, which uses statistical methods to fit to experimental data. Based on literature H-D exchange experiments, we surmise that the cause of the spectral variations is the induction of transient single stranding of tracts in the DNA polymer, particularly those with significant content of the weaker AT base pairs. More detailed analysis of the LD data will require better nucleotide transition polarization assignments.

DNA/química , Conformação de Ácido Nucleico , Análise Espectral , Algoritmos , Pareamento de Bases , Teorema de Bayes , Método de Monte Carlo , Sais , Soluções , Síncrotrons , Temperatura , Termodinâmica
Sci Prog ; 92(Pt 2): 163-204, 2009.
Artigo em Inglês | MEDLINE | ID: mdl-19697713


Persistence length is the foremost measure of DNA flexibility. Its origins lie in polymer theory which was adapted for DNA following the determination of BDNA structure in 1953. There is no single definition of persistence length used, and the links between published definitions are based on assumptions which may, or may not be, clearly stated. DNA flexibility is affected by local ionic strength, solvent environment, bound ligands and intrinsic sequence-dependent flexibility. This article is a review of persistence length providing a mathematical treatment of the relationships between four definitions of persistence length, including: correlation, Kuhn length, bending, and curvature. Persistence length has been measured using various microscopy, force extension and solution methods such as linear dichroism and transient electric birefringence. For each experimental method a model of DNA is required to interpret the data. The importance of understanding the underlying models, along with the assumptions required by each definition to determine a value of persistence length, is highlighted for linear dichroism data, where it transpires that no model is currently available for long DNA or medium to high shear rate experiments.

DNA/química , Animais , Biopolímeros/química , Bovinos , Técnicas In Vitro , Modelos Moleculares , Estrutura Molecular , Conformação de Ácido Nucleico , Análise Espectral , Viscosidade