A theory of the chain melting phase transition of aqueous phospholipid dispersions

A model for the chain melting phase transition in dilute aqueous phospholipid bilayer dispersions is presented. This model includes interactions between head groups, between hydrocarbon chains, and within the chains. The head groups are modeled as hard disks which are constrained to lie on a two-dimensional surface separating the aqueous and hydrocarbon regions. The chain statistics problem is treated in an approximate manner using an approach motivated by scaled particle theory to describe the inter-chain steric repulsions in a mathematically tractable way. In this approach the whole system interacts with any given chain through an average lateral pressure which is proportional to the hard disk pressure. Following Nagle, we assume that the steric repulsions between chains and between head groups and the trans-gauche rotation energies are the dominant interactions in determining the transition and we describe the effect of the other interactions with a mean field approximation. Using the known transition temperature of a series of 1,2-diacyl phosphatidyl cholines to adjust two parameters in the theory, the model gives enthalpy and area changes that are in quite reasonable agreement with experiment. Moreover, the curvature observed in the plot of the transition temperature against acyl chain length is reproduced

Making sense: dopamine activates conscious self-monitoring through medial prefrontal cortex

When experiences become meaningful to the self, they are linked to synchronous activity in a paralimbic network of self-awareness and dopaminergic activity. This network includes medial prefrontal and medial parietal/posterior cingulate cortices, where transcranial magnetic stimulation may transiently impair self-awareness. Conversely, we hypothesize that dopaminergic stimulation may improve self-awareness and metacognition (i.e., the ability of the brain to consciously monitor its own cognitive processes). Here, we demonstrate improved noetic (conscious) metacognition by oral administration of 100 mg dopamine in minimal self-awareness. In a separate experiment with extended self-awareness dopamine improved the retrieval accuracy of memories of self-judgment (autonoetic, i.e., explicitly self-conscious) metacognition. Concomitantly, magnetoencephalography (MEG) showed increased amplitudes of oscillations (power) preferentially in the medial prefrontal cortex. Given that electromagnetic activity in this region is instrumental in self-awareness, this explains the specific effect of dopamine on explicit self-awareness and autonoetic metacognition

Ca(2+ )binding to complement-type repeat domains 5 and 6 from the low-density lipoprotein receptor-related protein

BACKGROUND: The binding of ligands to clusters of complement-type repeat (CR)-domains in proteins of the low-density lipoprotein receptor (LDLR) family is dependent on Ca(2+ )ions. One reason for this cation requirement was identified from the crystal structure data for a CR-domain from the prototypic LDLR, which showed the burial of a Ca(2+ )ion as a necessity for correct folding and stabilization of this protein module. Additional Ca(2+ )binding data to other CR-domains from both LDLR and the LDLR-related protein (LRP) have suggested the presence of a conserved Ca(2+ )cage within CR-domains from this family of receptors that function in endocytosis and signalling. RESULTS: We have previously described the binding of several ligands to a fragment comprising the fifth and the sixth CR-domain (CR56) from LRP, as well as qualitatively described the binding of Ca(2+ )ions to this CR-domain pair. In the present study we have applied the rate dialysis method to measure the affinity for Ca(2+), and show that CR56 binds 2 Ca(2+ )ions with an average affinity of K(D )= 10.6 microM, and there is no indication of additional Ca(2+ )binding sites within this receptor fragment. CONCLUSIONS: Both CR-domains of CR56 bind a single Ca(2+ )ion with an affinity of 10.6 microM within the range of affinities demonstrated for several other CR-domains

Analysis of time-to-event for observational studies: Guidance to the use of intensity models

This paper provides guidance for researchers with some mathematical background on the conduct of time-to-event analysis in observational studies based on intensity (hazard) models. Discussions of basic concepts like time axis, event definition and censoring are given. Hazard models are introduced, with special emphasis on the Cox proportional hazards regression model. We provide check lists that may be useful both when fitting the model and assessing its goodness of fit and when interpreting the results. Special attention is paid to how to avoid problems with immortal time bias by introducing time-dependent covariates. We discuss prediction based on hazard models and difficulties when attempting to draw proper causal conclusions from such models. Finally, we present a series of examples where the methods and check lists are exemplified. Computational details and implementation using the freely available R software are documented in Supplementary Material. The paper was prepared as part of the STRATOS initiative.Comment: 28 pages, 12 figures. For associated Supplementary material, see http://publicifsv.sund.ku.dk/~pka/STRATOSTG8

A diagrammatic formulation of the kinetic theory of fluctuations in equilibrium classical fluids. VI. Binary collision approximations for the memory function for self correlation functions

We use computer simulation results for a dense Lennard-Jones fluid for a range of temperatures to test the accuracy of various binary collision approximations for the memory function for density fluctuations in liquids. The approximations tested include the moderate density approximation of the generalized Boltzmann-Enskog memory function (MGBE) of Mazenko and Yip, the binary collision approximation (BCA) and the short time approximation (STA) of Ranganathan and Andersen, and various other approximations derived by us using diagrammatic methods. The tests are of twotypes. The first is a comparison of the correlation functions predicted by each approximate memory function with the simulation results, especially for the self longitudinal current correlation function (SLCC). The second is a direct comparison of each approximate memory function with a memory function numerically extracted from the correlation function data. The MGBE memory function is accurate at short times but decays to zero too slowly and gives a poor description of the correlation function at intermediate times. The BCA is exact at zero time, but it predicts a correlation function that diverges at long times. The STA gives a reasonable description of the SLCC but does not predict the correct temperature dependence of the negative dip in the function that is associated with caging at low temperatures. None of the other binary collision approximations is a systematic improvement upon the STA. The extracted memory functions have a rapidly decaying short time part, much like the STA, and a much smaller, more slowly decaying part of the type predicted by mode coupling theory. Theories that use mode coupling commonly include a binary collision term in the memory function but do not discuss in detail the nature of that term. ...Comment: 18 pages, 10 figure

Scaling behavior in the β\beta-relaxation regime of a supercooled Lennard-Jones mixture

We report the results of a molecular dynamics simulation of a supercooled binary Lennard-Jones mixture. By plotting the self intermediate scattering functions vs. rescaled time, we find a master curve in the $\beta$-relaxation regime. This master curve can be fitted well by a power-law for almost three decades in rescaled time and the scaling time, or relaxation time, has a power-law dependence on temperature. Thus the predictions of mode-coupling-theory on the existence of a von Schweidler law are found to hold for this system; moreover, the exponents in these two power-laws are very close to satisfying the exponent relationship predicted by the mode-coupling-theory. At low temperatures, the diffusion constants also show a power-law behavior with the same critical temperature. However, the exponent for diffusion differs from that of the relaxation time, a result that is in disagreement with the theory.Comment: 8 pages, RevTex, four postscript figures available on request, MZ-Physics-10