Integral weak diffusion and diffusion approximations applied to ion transport through biological ion channels

In this article a theory is presented to calculate integral properties of biological ion channels (like currentvoltage and conductance-concentration relations). The qualitative form of these relations predicted by the theory agrees well with data measured in experiments. For instance, the saturation of the channel conductance with increasing external ion concentration is predicted for a class of ion channels (as, for instance, found for the gramicidin A, acetylcholine receptors, NMDA, and sarcoplasmic reticulum channels). In contrast to commonly used approaches such as the Eyring rate theory, this method is directly related to physical parameters of the ion channel such as the channel length and diameter, dielectric constant, ionic mobility, and minimal ionic concentration inside the channel. The theory starts from Nernst-Planck and Poisson equations. Using the method of phase trajectory (as proposed by Schottky) and the regional approximation, rather general expressions can be derived for integral channel quantities in the drift limit (|Vl > kBT/eo) in the presence of multiple ionic species. The theory predicts two typical types of conductance-concentration relations found experimentally: a monotone saturating conductance and a maximum in the conductance. The realized type of relation depends on the minimal ionic concentration inside the channel. In the present form the theory is restricted to narrow ion channels where the length exceeds its diameter. The ions are assumed to behave like structureless point charges at not too high ionic concentration

Integral weak diffusion and diffusion approximations applied to ion transport through biological ion channels

In this article a theory is presented to calculate integral properties of biological ion channels (like currentvoltage and conductance-concentration relations). The qualitative form of these relations predicted by the theory agrees well with data measured in experiments. For instance, the saturation of the channel conductance with increasing external ion concentration is predicted for a class of ion channels (as, for instance, found for the gramicidin A, acetylcholine receptors, NMDA, and sarcoplasmic reticulum channels). In contrast to commonly used approaches such as the Eyring rate theory, this method is directly related to physical parameters of the ion channel such as the channel length and diameter, dielectric constant, ionic mobility, and minimal ionic concentration inside the channel. The theory starts from Nernst-Planck and Poisson equations. Using the method of phase trajectory (as proposed by Schottky) and the regional approximation, rather general expressions can be derived for integral channel quantities in the drift limit (|Vl > kBT/eo) in the presence of multiple ionic species. The theory predicts two typical types of conductance-concentration relations found experimentally: a monotone saturating conductance and a maximum in the conductance. The realized type of relation depends on the minimal ionic concentration inside the channel. In the present form the theory is restricted to narrow ion channels where the length exceeds its diameter. The ions are assumed to behave like structureless point charges at not too high ionic concentration

GaInN/GaN-Heterostructures and Quantum Wells Grown by Metalorganic Vapor-Phase Epitaxy

The dependence of the In-incorporation efficiency and the optical properties of MOVPE-grown GaInN/GaN-heterostructures on various growth parameters has been investigated. A significant improvement of the In-incorporation rate could be obtained by increasing the growth rate and reducing the H2-partial pressure in the MOVPE reactor. However, GaInN layers with a high In-content typically show an additional low energy photoluminescence peak, whose distance to the band-edge increases with increasing In-content. For GaInN/GaN quantum wells with an In-content of approximately 12%, an increase of the well thickness is accompanied by a significant line broadening and a large increase of the Stokes shift between the emission peak and the band edge determined by photothermal deflection spectroscopy. With a further increase of the thickness of the GaInN layer, a second GaInN-correlated emission peak emerges. To elucidate the nature of these optical transitions, power-dependent as well as time-resolved photoluminescence measurements have been performed and compared to the results of scanning transmission electron microscopy.</jats:p

Metalorganic vapor phase epitaxial growth of GaInN/GaN hetero structures and quantum wells

ABSTRACTGaInN/GaN heterostructures and quantum wells have been grown by low pressure metalorganic vapor phase epitaxy on sapphire using an AIN nucleation layer. We found a significant In incorporation only for growth temperatures of 700°C, although still very high In/Ga ratios in the gas phase had to be adjusted. The In content could be increased by reducing the H2/N2 flow ratio in the main carrier gas. GaInN layers typically show two lines in low temperature photoluminescence which are identified as excitonic-like (high energy peak) and impurity-related-like (low energy) by time-resolved spectroscopy. Quantum wells with a thickness between 8 and 0.5 nm showed only one emission line. The peak of the thinnest wells shows excitonic-like behaviour, whereas we found a smooth transition to an impurity-related-like type with increasing thickness. By scanning transmission electron microscopy studies we found indications for composition fluctuations in these thicker quantum wells which may cause localization effects for the excitons and thus be responsible for the observed optical spectra.</jats:p