Mark W Tate | PubMed Health

PubMed2002

Rapid compaction during RNA folding.

We have used small angle x-ray scattering and computer simulations with a coarse-grained model to provide a time-resolved picture of the global folding process of the Tetrahymena group I RNA over a time window of more than five orders of magnitude. A substantial phase of compaction is observed on the low millisecond timescale, and the overall compaction and global shape changes are largely complete within one second, earlier than any known tertiary contacts are formed. This finding indicates that the RNA forms a nonspecifically collapsed intermediate and then searches for its tertiary contacts within a highly restricted subset of conformational space. The collapsed intermediate early in folding of this RNA is grossly akin to molten globule intermediates in protein folding.

Animals↗

PubMed2002

X-ray imaging of shock waves generated by high-pressure fuel sprays.

Synchrotron x-radiography and a fast x-ray detector were used to record the time evolution of the transient fuel sprays from a high-pressure injector. A succession of 5.1-microsecond radiographs captured the propagation of the spray-induced shock waves in a gaseous medium and revealed the complex nature of the spray hydrodynamics. The monochromatic x-radiographs also allow quantitative analysis of the shock waves that has been difficult if not impossible with optical imaging. Under injection conditions similar to those found in operating engines, the fuel jets can exceed supersonic speeds and result in gaseous shock waves.

Cerium↗

PubMed2002

Determination of L(alpha)-H(II) phase transition temperature for 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine.

The thermodynamic properties of fully-hydrated lipids provide important information about the stability of membranes and the energetic interactions of lipid bilayers with membrane proteins (Nagle and Scott, Physics Today, 2:39, 1978). The lamellar/inverse hexagonal (L(alpha)-H(II)) phase transition of 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine (DOPE) water mixtures is a first-order transition and, therefore, at constant pressure, must have a thermodynamically well-defined equilibrium transition temperature. The observed transition temperature is known to be dependent upon the rate at which the temperature is changed, which accounts for the many different values in the literature. X-ray diffraction was used to study the phase transition of fully-hydrated DOPE to determine the rate-independent transition temperature, T(LH). Samples were heated or cooled for a range of rates, 0.212 < r < 225 degrees C/hr, and the rate-dependent apparent phase transition temperatures, T(A)(r) were determined from the x-ray data. By use of a model-free extrapolation method, the transition temperature was found to be T(LH) = 3.33 +/- 0.16 degrees C. The hysteresis, /T(A)(r) - T(LH)/, was identical for heating and cooling rates, +/-r, and varied as /r/beta for beta approximately 1/4. This unexpected power-law relationship is consistent with a previous study (Tate et al., Biochemistry, 31:1081-1092, 1992) but differs markedly from the exponential behavior of activation barrier kinetics. The methods used in this study are general and provide a simple way to determine the true mesomorphic phase transition temperatures of other lipid and lyotropic systems.

Hot Temperature↗