Ineffective Orifice Area: Practical Limitations of Accurate EOA Assessment for Low-Gradient Heart Valve Prostheses.

Purpose: The goal of this study was to demonstrate the range in effective orifice area (EOA) values that may be possible given the ISO 5840 definition of EOA and the practical limits in the accurate measurement of pressure differential across large diameter valves.

Methods: A 31 mm mechanical valve was tested on a commercially available pulse duplicator configured for mitral valve testing and tuned to nominal conditions. The experimental data was used as a basis for performing Monte Carlo analyses with published specifications for commonly used pressure sensors as well as measurement equipment accuracy requirements described in ISO 5840.

Directionality of electron-transfer reactions in photosystem I of prokaryotes: universality of the bidirectional electron-transfer model.

The electron-transfer (ET) reactions in photosystem I (PS I) of prokaryotes have been investigated in wild-type cells of the cyanobacterium Synechocystis sp. PCC 6803, and in two site-directed mutants in which the methionine residue of the reaction center subunits PsaA and PsaB, which acts as the axial ligand to the primary electron chlorophyll acceptor A(0), was substituted with histidine. Analysis by pulsed electron paramagnetic resonance spectroscopy at 100 K indicates the presence of two forms of the secondary spin-correlated radical pairs, which are assigned to [P(700)(+)A(1A)(-)] and [P(700)(+)A(1B)(-)], where A(1A) and A(1B) are the phylloquinone molecules bound to the PsaA and the PsaB reaction center subunits, respectively.

The modulus of fibroblast-populated collagen gels is not determined by final collagen and cell concentration: Experiments and an inclusion-based model.

The fibroblast-populated collagen lattice is an attractive model tissue for in vitro studies of cell behavior and as the basis for bioartificial tissues. In spite of its simplicity-containing only collagen and cells-the system is surprisingly difficult to describe mechanically because of the ability of the cells to remodel the matrix, including compaction at short times and synthesis and/or degradation (and cell proliferation) at longer times. The objectives of this work were to measure the equilibrium modulus of fibroblast-populated gels with different collagen and cell concentrations, and to use that characterization as the basis for a theoretical model that could be used to predict gel mechanics based on conditions.

Planar biaxial mechanical behavior of bioartificial tissues possessing prescribed fiber alignment.

Though it is widely accepted that fiber alignment has a great influence on the mechanical anisotropy of tissues, a systematic study of the influence of fiber alignment on the macroscopic mechanical behavior by native tissues is precluded due to their predefined microstructure and heterogeneity. Such a study is possible using collagen-based bioartificial tissues that allow for alignment to be prescribed during their fabrication. To generate a systemic variation of strength of fiber alignment, we made cruciform tissue constructs in Teflon molds that had arms of different aspect ratios.

Functional tissue-engineered valves from cell-remodeled fibrin with commissural alignment of cell-produced collagen.

Heart valve replacements composed of living tissue that can adapt, repair, and grow with a patient would provide a more clinically beneficial option than current inert replacements. Bioartificial valves were produced by entrapping human dermal fibroblasts within a fibrin gel. Using a mold design that presents appropriate mechanical constraints to the cell-induced fibrin gel compaction, gross fiber alignment (commissure-to-commissure alignment in the leaflets and circumferential alignment in the root) and the basic geometry of a native aortic valve were obtained.

An electron paramagnetic resonance investigation of the electron transfer reactions in the chlorophyll d containing photosystem I of Acaryochloris marina.

Electron paramagnetic resonance (EPR) spectroscopy reveals functional and structural similarities between the reaction centres of the chlorophyll d-binding photosystem I (PS I) and chlorophyll a-binding PS I. Continuous wave EPR spectrometry at 12K identifies iron-sulphur centres as terminal electron acceptors of chlorophyll d-binding PS I. A transient light-induced electron spin echo (ESE) signal indicates the presence of a quinone as the secondary electron acceptor (Q) between P(740)(+) and the iron-sulphur centres.

Three-dimensional simulation of anisotropic cell-driven collagen gel compaction.

Tissue equivalents (TEs), formed by entrapping cells in a collagen gel, are an important model system for studying cell behavior. We have previously (Barocas and Tranquillo in J Biomech Eng 117:161-170, 1997a) developed an anisotropic biphasic theory of TE mechanics, which comprises five coupled partial differential equations describing interaction among cells and collagen fibers in the TE. The model equations, previously solved in one or two dimensions, were solved in three dimensions using an adaptive finite-element platform.

Chlorophyll triplet states associated with Photosystem I and Photosystem II in thylakoids of the green alga Chlamydomonas reinhardtii.

The analysis of FDMR spectra, recorded at multiple emission wavelengths, by a global decomposition technique, has allowed us to characterise the triplet populations associated with Photosystem I and Photosystem II of thylakoids in the green alga Chlamydomonas reinhardtii. Three triplet populations are observed at fluorescence emissions characteristic of Photosystem II, and their zero field splitting parameters have been determined. These are similar to the zero field parameters for the three Photosystem II triplets previously reported for spinach thylakoids, suggesting that they have a widespread occurrence in nature.

Bidirectional electron transfer in photosystem I: replacement of the symmetry-breaking tryptophan close to the PsaB-bound phylloquinone A1B with a glycine residue alters the redox properties of A1B and blocks forward electron transfer at cryogenic temperatures.

A conserved tryptophan residue located between the A(1B) and F(X) redox centres on the PsaB side of the Photosystem I reaction centre has been mutated to a glycine in Chlamydomonas reinhardtii, thereby matching the conserved residue found in the equivalent position on the PsaA side. This mutant (PsaB:W669G) was studied using EPR spectroscopy with a view to understanding the molecular basis of the reported kinetic differences in forward electron transfer from the A(1A) and the A(1B) phyllo(semi)quinones. The kinetics of A(1)(-) reoxidation due to forward electron transfer or charge recombination were measured by electron spin echo spectroscopy at 265 K and 100 K, respectively.

Biomechanical and microstructural characteristics of a collagen film-based corneal stroma equivalent.

The growth in refractive surgeries and corneal replacements has fueled interest in the development of a tissue-engineered cornea. This study characterizes the microstructure and biomechanical properties of film-based corneal stroma equivalents over time in culture. The increased collagen density in the films was hypothesized to result in improved mechanical properties both initially and over time.

Analysis of the spin-polarized electron spin echo of the [P700+ A1-] radical pair of photosystem I indicates that both reaction center subunits are competent in electron transfer in cyanobacteria, green algae, and higher plants.

The decay of the light-induced spin-correlated radical pair [P700+ A1-] and the associated electron spin echo envelope modulation (ESEEM) have been studied in either thylakoid membranes, cellular membranes, or purified photosystem I prepared from the wild-type strains of Synechocystis sp. PCC 6803, Chlamydomonas reinhardtii, and Spinaceae oleracea. The decay of the spin-correlated radical pair is described in the wild-type membrane by two exponential components with lifetimes of 2-4 and 16-25 micros.

Modelling of the electron transfer reactions in Photosystem I by electron tunnelling theory: the phylloquinones bound to the PsaA and the PsaB reaction centre subunits of PS I are almost isoenergetic to the iron-sulfur cluster F(X).

Photosystem I is a large macromolecular complex located in the thylakoid membranes of chloroplasts and in cyanobacteria that catalyses the light driven reduction of ferredoxin and oxidation of plastocyanin. Due to the very negative redox potential of the primary electron transfer cofactors accepting electrons, direct estimation by redox titration of the energetics of the system is hampered. However, the rates of electron transfer reactions are related to the thermodynamic properties of the system.

Ammonia displaces methanol bound to the water oxidizing complex of photosystem II in the S2 state.

Ammonia and methanol both bind to the water oxidising complex of photosystem II during its turnover, possibly at sites where water binds during the normal water oxidation process. We have investigated the interaction between these two water analogues at the S2 state of the water oxidising cycle using electron magnetic resonance techniques. We find evidence that ammonia displaces methanol from its binding site.

Photosynthetic water oxidation: the role of tyrosine radicals.

This mini-review outlines the involvement of the tyrosine electron carriers, Y(D) and Y(Z), in the mechanism of electron transfer from water to P680. We discuss our data and put forward our ideas on the role of Y(D) and Y(Z).

Bidirectional electron transfer in photosystem I: electron transfer on the PsaA side is not essential for phototrophic growth in Chlamydomonas.

We have used pulsed electron paramagnetic resonance (EPR) measurements of the electron spin polarised (ESP) signals arising from the geminate radical pair P700(z.rad;+)/A(1)(z.rad;-) to detect electron transfer on both the PsaA and PsaB branches of redox cofactors in the photosystem I (PSI) reaction centre of Chlamydomonas reinhardtii.

Effect of hydroxylamine on photosystem II: reinvestigation of electron paramagnetic resonance characteristics reveals possible S state intermediates.

Previous work in many laboratories has established that hydroxylamine reduces the S(1) state of the water oxidizing complex (WOC) in one-electron steps. Significant levels of what can now be defined as the S(-1)* state are achieved by specific (concentration and incubation length) hydroxylamine treatments. This state has already been studied by electron paramagnetic resonance spectrometry (EPR), and unusual EPR signals were noted (for example, see Sivaraja, M.

Stabilization of iron-sulfur cluster F(X) by intra-subunit interactions unraveled by suppressor and second site-directed mutations in PsaB of Photosystem I.

Intra-subunit interactions in the environment of the iron-sulfur cluster F(X) in Photosystem I (PS I) of Synechocystis sp. PCC 6803 were studied by site-directed and second site suppressor mutations. In subunit PsaB, the cysteine ligand (C565) of F(X) and a conserved aspartate (D566) adjacent to C565 were modified.

Photoaccumulation of the PsaB phyllosemiquinone in photosystem I of Chlamydomonas reinhardtii.

Photoaccumulation of membrane preparations of Chlamydomonas reinhardtii at pH 8 and 220 K reduces the primary and secondary electron acceptors in the Photosystem I (PSI) reaction centre, and produces a maximum of two spins per P700(z.rad;+). Proton electron nuclear double resonance (ENDOR) spectra demonstrate that the phyllosemiquinone produced is that attributed to the PsaA branch of electron transfer.

Electron transfer from the water oxidizing complex at cryogenic temperatures: the S1 to S2 step.

We report the detection of a "split" electron paramagnetic resonance (EPR) signal during illumination of dark-adapted (S(1) state) oxygen-evolving photosystem II (PSII) membranes at <20 K. The characteristics of this signal indicate that it arises from an interaction between an organic radical and the Mn cluster of PSII. The broad radical signal decays in the dark following illumination either by back-reaction with Qa*- or by forward electron transfer from the Mn cluster.

An iron-sulfur cluster in the family 4 uracil-DNA glycosylases.

The 25-kDa Family 4 uracil-DNA glycosylase (UDG) from Pyrobaculum aerophilum has been expressed and purified in large quantities for structural analysis. In the process we observed it to be colored and subsequently found that it contained iron. Here we demonstrate that P.