Ernesto Freire

Estimation of molecular averages and equilibrium fluctuations in lipid bilayer systems from the excess heat capacity function

It is demonstrated that the bilayer partition function can be numerically obtained from scanning calorimetric data without assuming a particular model for the gel-liquid crystalline transition. From this partition function, the enthalpy, entropy and volume changes accompanying the transition can be calculated. In the limit of very large systems, the method of the grand partition function allows calculation of cluster model distribution functions from which average sizes of gel and liquid-crystal clusters, cluster densities and equilibrium fluctuations are obtained. These results indicate that the main transition in phospholipid bilayers proceeds through the formation of clusters and that these clusters are not static domains but highly fluctuating entities. These fluctuations in cluster size are approximately equal to the average cluster size and give rise to localized density and volume fluctuations. The magnitude of these fluctuations is affected by the radius of curvature of the bilayer and by the addition of small molecular weight compounds to the system.

Monte Carlo studies of the lateral organization of molecules in two-component lipid bilayers.

The lateral organization of two-component phosphatidylcholine bilayers has been investigated using Monte Carlo calculations based upon non-ideality parameters deduced from the phase diagrams of these mixtures. The results are used to develop a quantitative description of the distribution and spatial localization of compositional regions along the bilayer plane in both the gel and liquid crystalline phases. In particular, a detailed analysis of the physical extension (lateral connectivity) and compactness of the compositional clusters is made. It is concluded that the chemical composition of the membrane, the physical state of the bilayer and the interaction energies between molecules greatly influence the lateral connectivity and compactness of compositional regions and that these parameters might play an important role in the formation of diffusional pathways along the membrane plane.

Modification of a vibrating-tube density meter for precise temperature scanning

Abstract We have devised a method of temperature scanning with a vibrating-U-tube density meter in which temperature fluctuations are much reduced compared to those using a constant or programmable thermostat. The standard error of a density measurement is 5 × 10 −7 g/ml. Volume changes associated with conformational changes of macromolecular systems can be precisely measured. Using this instrument the volume expansion-melting curves of lipid dispersions have been obtained. The curves are similar in shape and resolution to the excess heat-capacity curves derived from differential scanning calorimetry performed on the same sample. Temperature scanning allows measurements of expansivity as well as apparent volume throughout a temperature range of interest.

Statistical thermodynamic analysis of the excess heat capacity function of macromolecular systems

The deconvolution theory of thermal transitions has proven to be a powerful method with which to analyze the heat capacity function of macromolecular systems. In this article, the basic results of the theory will be presented and their application to multistate transitions and general cooperative transitions of biopolymers and phospholipid membranes will be discussed. INTRODUCTION The development of highly precise differential scanning calorimeters has made possible the accurate definition of the heat capacity function associated with thermally-induced transitions of proteins, polypeptides, nucleic acids, lipid bilayers and other macromolecular systems. The importance of having experimental access to the exact shape of this function is that it contains all the information necessary to develop a complete thermodynamic description of a thermally-induced transition. In fact, it has been demonstrated that the excess heat capacity function can be appropriately transformed to yield the partition function of such a system and that this partition function can be used to deduce the microscopic mechanism of the transition (Refs. 1-6). In this article the analytical methods directed to obtaining a detailed statistical thermodynamic description of complex macromolecular systems will be presented. These methods constitute the basis of the deconvolution theory of thermal-transitions in macromolecules and, thus f.ar, have been applied to the study of protein unfolding reactions, helix-coil transitions in polynucleotides, thermal-transitions of transfer ribonucleic acids (tRNA) and biomembrane phase transitions. MACROMOLECULAR CONFORMATIONAL TRANSITIONS Most theories directed toward describing the molecular basis of function and modulation of biochemical systems include structural variations of the relevant macromolecules. Thus important questions relate to what are the characteristics of the equilibrium and dynamic fluctuations within an ensemble of such states. In general, the accessible structural states of a macromolecular system can be represented by the following reaction scheme: A+A A A (1) 0+ 1+ 2 n where the indexing of states is such that the enthalpy of state i (H.) is greater than the i-l state. (i.e. H>H.,). Thus as temperature i increased the population distribution mohotonically progresses toward state n. A normalized partition function for this system can be written in terms of the Gibbs energy differences as: