L. Cruzeiro-Hansson

Passive ion permeability of lipid membranes modelled via lipid-domain interfacial area

A microscopic interaction model of the gel-to-fluid chain-melting phase transition of fully hydrated lipid bilayer membranes is used as a basis for modelling the temperature dependence of passive transmembrane permeability of small ions, e.g. Na+. Computer simulation of the model shows that the phase transition is accompanied by strong lateral density fluctuations which manifest themselves in the formation of inhomogeneous equilibrium structures of coexisting gel and fluid domains. The interfaces of these domains are found to be dominated by intermediate lipid-chain conformations. The interfacial area is shown to have a pronounced peak at the phase transition. By imposing a simple model for ion diffusion through membranes which assigns a high relative permeation rate to the domain interfaces, the interfacial area is then identified as a membrane property which has the proper temperature variation to account for the peculiar experimental observation of a strongly enhanced passive ion permeability at the phase transition. The excellent agreement with the experimental data for Na+-permeation, taken together with recent experimental results for the phase transition kinetics, provides new insight into the microphysical mechanisms of reversible electric breakdown. This insight indicates that there is no need for aqueous pore-formation to explain the experimental observation of a dramatic increase in ion conductance subsequent to electric pulses.

Intrinsic molecules in lipid membranes change the lipid-domain interfacial area: cholesterol at domain interfaces

A theoretical analysis of the effects of intrinsic molecules on the lateral density fluctuations in lipid bilayer membranes is carried out by means of computer simulations on a microscopic interaction model of the gel-to-fluid chain-melting phase transition. The inhomogeneous equilibrium structures of gel and fluid domains, which in previous work (Cruzeiro-Hansson, L. and Mouritsen, O.G. (1988) Biochim. Biophys. Acta 944, 63-72) were shown to characterize the transition region of pure lipid membranes, are here shown to be enhanced by intrinsic molecules such as cholesterol. Cholesterol is found to increase the interfacial area and to accumulate in the interfaces. The interfacial area, the average cluster size, the lateral compressibility, and the membrane area are calculated as functions of temperature and cholesterol concentration. It is shown that the enhancement by cholesterol of the lateral density fluctuations and the lipid-domain interfacial area is most pronounced away from the transition temperature. The implications of the results are discussed in relation to passive ion permeability and function of interfacially active enzymes such as phospholipase.

Localized versus delocalized ground states of the semiclassical Holstein Hamiltonian

Abstract We carry out a numerical analysis of the semiclassical Holstein Hamiltonian describing the transport of a quasiparticle in a one-dimensional crystal with N sites. The quasiparticle, whose intersite interaction is V , is coupled with a coupling constant g to the lattice vibrations, whose maximum frequency is 2ω. We find that the character of the ground state is determined by two quantities: υ = V / g 2 ω and N . Specifically, there is a threshold value of υ beyond which the single-particle ground state is delocalized. The threshold is dependent on N . As N → ∞, the threshold value of υ becomes infinite signifying that for reasonably sized crystals, the ground state is always localized. Our analysis supports some earlier conjectures.

Effect of long range and anharmonicity in the minimum energy states of the Davydov-Scott model

Abstract The Davydov-Scott model, which describes the states of amide I excitations in a lattice, is extended to include distance dependent nonlocal hopping and/or a nonlinear lattice. Minimum energy one quantum states are determined by numerical minimization. For the parameters which characterize the motion of the amide I excitation in an α-helix it is found that the harmonic terms are a very good approximation of the full Lennard-Jones potential. On the other hand, although the inclusion of distance-independent hopping terms leads to generally broader solitons, surprisingly it also results in the suppression of the transition from localized to delocalized states. A particular way of including of distance dependent terms in the hopping coefficient is also considered which leads to spikes in the lattice distortion and otherwise a rich phase diagram for the corresponding minimum energy states.

Influence of electromagnetic radiation on molecular solitons

The soliton model of charge and energy transport in biological macromolecules is used to suggest one of the possible mechanisms for electromagnetic radiation influence on biological systems. The influence of the electromagnetic field (EMF) on molecular solitons is studied both analytically and numerically. Numerical simulations prove the stability of solitons for fields of large amplitude, and allow the study of emission of phonons. It is shown that in the spectra of biological effects of radiation there are two characteristic frequencies of EMFs, one of which is connected with the most intensive energy absorption and emission of sound waves by the soliton, and the other of which is connected with the soliton photodissociation into a delocalized state.

Electromagnetic Radiation Influence on Nonlinear Charge and Energy Transport in Biosystems

The influence of electromagnetic radiation (EMR) on charge and energy transport processes in biological systems is studied in the light of the soliton model. It is shown that in the spectrum of biological effects of EMR there are two frequency resonances corresponding to qualitatively different frequency dependent effects of EMR on solitons. One of them is connected with the quasiresonance dynamic response of solitons to the EMR. At EMR frequencies close to the dynamic resonance frequency the solitons absorb energy from the field and generate intensive vibrational modes in the macromolecule. The second EMR resonance is connected with soliton decay due to the quantum mechanical transition of the system from the bound soliton state into the excited unbound states.

DNA Structure, Hydration and Dynamics

Although the double helical model of DNA structure is now 40 years old, there is still considerable effort being made to elucidate the range of conformations that can be adopted by this flexible molecule. We review the current state of our knowledge of DNA structure which is available from both experimental and computational approaches.

The Davydov Hamiltonian leads to stochastic energy transfer in proteins

Abstract A new set of equations of motion for the semiclassical Davydov model at finite temperature, which satisfy both the classical statistics of the lattice motion and the quantum statistics of the quasiparticle, is used. The Davydov soliton, defined as the exact one-quantum ground state of the Davydov Hamiltonian, is shown to disappear in the subpicosecond timescale. However, in contrast with previous simulations, the quasiparticle states are essentially localised at 310 K, and the Davydov model remains a possible mechanism for energy transfer in proteins. The dynamics at biological temperatures consists of a stochastic hopping of the quantum quasiparticle from site to site. The biological implications are discussed and in order to stimulate experimental tests of the theory the predicted absorption spectrum of the Davydov model is presented.

Comparison of quantum Monte Carlo and semiclassical Monte Carlo results in investigations of the thermal stability of Davydov solitons

Abstract We study the problem of the thermal stability of the Davydov soliton, in the context of its viability as providing a mechanism for energy transfer in proteins, by making a quantitative comparison between the full quantum model and the semiclassical model which has been used frequently for analytic and numerical calculations. Our goal is to gain insight into the range of validity of the semiclassical model in the particular context of finite temperatures. Our results indicate that, at biologically relevant temperatures, for the parameters chosen, the semiclassical model gives practically the same results as the full quantum mechanical model.

Interplay between dispersive and non-dispersive modes in the polaron problem

Abstract We study the influence of the phonon spectrum on polaron formation and show that three self-trapping regimes can occur. If the lattice and the electron–lattice Hamiltonians are dominated by the same type of phonons, the self-trapping transition is smooth. If there is an imbalance, the transition can either be abrupt or completely eliminated. The binding energies are larger in the case of imbalance. The bandwidth varies linearly with hopping strength, even for strongly localized states.