摘要:

Ion channels are deeply involved in sea urchin sperm activation, motility, chemotaxis and in the acrosome reaction. Unraveling ion channel function and regulation in sperm behavior has required a combination of complementary approaches since spermatozoa are very tiny cells. Planar bilayer and patch clamp techniques have allowed us to detect, for the first time, the activity of single channels in the plasma membrane of these cells. Unlike intact sperm, swollen sperm can be much more easily patch clamped and single channel activity recorded. These techniques, together with studies of membrane potential, intracellular Ca2+and pH in whole sperm, have established the presence of K+, Ca2+, and Cl−channels in this cell. The strategies developed to study sea urchin sperm channels are applicable to mammalian spermatozoa. We recently detected a Ca2+channel resembling one found inS.purpuratussperm in planar bilayers containing mouse sperm plasma membranes. The presence of this Ca2+channel in such diverse species suggests it is important in sperm function.

ISSN:0094-243X    

DOI:10.1063/1.48825

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Enzymes are complex structures that have been produced by evolution to function in water. The interrelation between water and proteins is not completely understood. However, data of the last decades have shown that water is indeed an essential component of enzymes. It is central in their formation, often it forms an essential part of their three‐dimensional structure, and it is fundamental for supporting enzyme action, at least at rates compatible with life. It is precisely because of the intimate relation that exists between water and enzymes that water may be used to gain deeper insight into how enzymes are formed and function.

ISSN:0094-243X    

DOI:10.1063/1.48826

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Abinitioquantum mechanical calculations are valuable tools for interpretation and elucidation of elemental processes in biochemical systems. With theabinitioapproach one can calculate data that sometimes are difficult to obtain by experimental techniques. The most popular computational theoretical methods include the Hartree‐Fock method as well as some lower‐level variational and perturbational post‐Hartree Fock approaches which allow to predict molecular structures and to calculate spectral properties. We have been involved in a number of joined theoretical and experimental studies in the past and some examples of these studies are given in this presentation. The systems chosen cover a wide variety of simple biomolecules, such as precursors of nucleic acids, double‐proton transferring molecules, and simple systems involved in processes related to first stages of substrate‐enzyme interactions. In particular, examples of someabinitiocalculations used in the assignment of IR spectra of matrix isolated pyrimidine nucleic bases are shown. Some radiation‐induced transformations in model chromophores are also presented. Lastly, we demonstrate how theab‐initioapproach can be used to determine the initial several steps of the molecular mechanism of thymidylate synthase inhibition by dUMP analogues.

ISSN:0094-243X    

DOI:10.1063/1.48840

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The molecular study of liquids is a field that recently has had a strong development, both, because of the appearance of new experimental techniques, and the increasing power of computers. Here we present a study of hydration looking into the molecular basis of the phenomenon. We compare the molecular patterns involved in aqueous solutions ofMg2+, Urea, and Methanol. All studies were performed by Monte Carlo simulations, using highly refined potentials. Care was taken on including polarization and nonadditive effects. We present an analysis of the different contributions to the hydration energy, coming from solute‐water interactions, or modification of the water–water interaction as a result of the presence of the solute. We also present the structural consequences of hydration, from the very dramatic ones for the ion to the very small ones for urea. Of course the main interest is in gaining understanding of hydration. We advance proposals such as: a) that electrostriction is a very local effect, due to the hydration shells of the ion rather than changes in the density of the solute, b) the coexistence of a urea network with the water network and c) the very small disruption by methanol of the water network.

ISSN:0094-243X    

DOI:10.1063/1.48827

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A new statistical mechanics approach to calculating probabilities of chemical bonds being open is developed. The method uses effective phonon theory and becomes a mean field theory which shows the observed second order phase transition on helix melting. The temperature and width of the transition is accurately calculated without fitting parameters to melting. The method was applied to the melting of a DNA with a hydration spine. The effect on the DNA and the melting of the spine are explored.

ISSN:0094-243X    

DOI:10.1063/1.48828

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The advancement of experimental techniques in Biochemistry calls for a more accurate description of various phenomena at the molecular level. Numerical simulations are a convenient tool to provide it, but refined model potentials are needed to account for the changes that local conditions impose to the structure and energetics of the liquids. This work presents a discussion on the methods to design and test such models. An application to refine theMCHO(1) water potential shows the usefulness of theabinitioapproach, in which successive refinements are possible. This not only allows for a better description of the liquids, but also improves our understanding of the relevance of the different contributions to the intermolecular interactions.

ISSN:0094-243X    

DOI:10.1063/1.48829

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Using a recent prescription to obtain numerical ‘time series’ from a DNA chain, we associate a fractal with the series, and use its dimension as a measure of the chain’s complexity. We justify our approach on the assumption that genetic evolution can be described by a dynamical system, and that the observed genes and genomes in a given species correspond to attractors in the space of solutions of such a system. To illustrate our method we calculate the fractal dimension for a viral genome and a few eukariotic genes, the latter differing in size and intronic content. The fractal dimension can distinguish among these sequences, but its biological relevance will depend on increasing the numerical precision of the calculations involved.

ISSN:0094-243X    

DOI:10.1063/1.48830

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Various types of magnetic phases in two dimensional systems with competing interactions are studied by means of a simulation method. We consider only first (J1) and second (J2) antiferromagnetic neighbor interactions. The simulation is based on the Crystal Growth Probability Method using the cluster probabilities obtained within the Kramer–Wannier approximation. We focus our attention on the evolution of the magnetic ordering as a function of temperature and of the ratioV(≡J2/J1). We notice that the pattern of the excitations changes drastically close toV=0.5, and is very different from systems withVvalues distant from that multicritical point.

ISSN:0094-243X    

DOI:10.1063/1.48831

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A general analytic method for calculation and simulation of theoretical powder magnetic resonance spectra, based on precise analytical expressions for powder patterns built from the intersections of isotimic surfaces,H(&Vthgr;,&fgr;)=C, of the resonance field with unit sphere, is reviewed and extended to include the effect of field‐dependent transition probability. Full powder magnetic resonance spectra in axial symmetry is expressed by a convolution integral over the magnetic field variable. Several important powder patterns obtained by this method for the first time are reviewed, including the recently found one for ag‐orthorhombic system. This is also the first analytically exact powder pattern found for a non‐axial system. Theoretical powder EPR spectra based on these analytical powder patterns are shown to be easily handled by means of modern computation languages such asMathematica.

ISSN:0094-243X    

DOI:10.1063/1.48832

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In a single tight binding band with modest effective mass anisotropy, the resulting superconducting gap, in the nearly antiferromagnetic Fermi liquid, has a main component exhibiting dx2−y2symmetry but also has a minor extended s‐wave contribution (including a constant part). This leads to anisotropy in the in plane penetration depth as is measured and can explain the recent observation of a DC Josephson current between Pb and YBa2Cu3O7−&dgr;.

ISSN:0094-243X    

DOI:10.1063/1.48837

出版商:AIP    

年代:1995

数据来源: AIP