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Hryc J, Markiewicz M, Pasenkiewicz-Gierula M. Stacks of monogalactolipid bilayers can transform into a lattice of water channels. iScience 2023; 26:107863. [PMID: 37766978 PMCID: PMC10520361 DOI: 10.1016/j.isci.2023.107863] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/01/2023] [Revised: 08/17/2023] [Accepted: 09/06/2023] [Indexed: 09/29/2023] Open
Abstract
The lipid matrix of thylakoid membranes is a lamellar bilayer, but under a certain condition it can convert locally into a nonlamellar structure. This is possible because one of the main membrane lipids, MGDG, promotes the formation of an inverse hexagonal phase. Here, the spontaneous transformation of aligned hydrated MGDG bilayers into nonlamellar structures is investigated using all-atom molecular dynamics simulation. Previous studies have demonstrated that MGDG polar head groups connect vertically across the interface. In this study, the evolution of the system's initial structure into a lattice of water channels and contacted surfaces created by numerous vertical MGDG connections depended on the width of the hydrating water layers. These widths controlled the bilayers' ability to bend, which was a prerequisite for channel formation. Locally, an intensive exchange of MGDG molecules between apposing bilayer leaflets occurred, although a stable semi-toroidal stalk did not develop.
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Affiliation(s)
- Jakub Hryc
- Department of Computational Biophysics and Bioinformatics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Krakow, Poland
| | - Michal Markiewicz
- Department of Computational Biophysics and Bioinformatics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Krakow, Poland
| | - Marta Pasenkiewicz-Gierula
- Department of Computational Biophysics and Bioinformatics, Faculty of Biochemistry, Biophysics and Biotechnology, Jagiellonian University, 30-387 Krakow, Poland
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2
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Anggara K, Sršan L, Jaroentomeechai T, Wu X, Rauschenbach S, Narimatsu Y, Clausen H, Ziegler T, Miller RL, Kern K. Direct observation of glycans bonded to proteins and lipids at the single-molecule level. Science 2023; 382:219-223. [PMID: 37824645 PMCID: PMC7615228 DOI: 10.1126/science.adh3856] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/28/2023] [Accepted: 08/31/2023] [Indexed: 10/14/2023]
Abstract
Proteins and lipids decorated with glycans are found throughout biological entities, playing roles in biological functions and dysfunctions. Current analytical strategies for these glycan-decorated biomolecules, termed glycoconjugates, rely on ensemble-averaged methods that do not provide a full view of positions and structures of glycans attached at individual sites in a given molecule, especially for glycoproteins. We show single-molecule analysis of glycoconjugates by direct imaging of individual glycoconjugate molecules using low-temperature scanning tunneling microscopy. Intact glycoconjugate ions from electrospray are soft-landed on a surface for their direct single-molecule imaging. The submolecular imaging resolution corroborated by quantum mechanical modeling unveils whole structures and attachment sites of glycans in glycopeptides, glycolipids, N-glycoproteins, and O-glycoproteins densely decorated with glycans.
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Affiliation(s)
- Kelvin Anggara
- Max-Planck Institute for Solid-State Research; Stuttgart, DE-70569, Germany
| | - Laura Sršan
- Institute of Organic Chemistry, University of Tübingen; Tübingen, DE-72076, Germany
| | - Thapakorn Jaroentomeechai
- Copenhagen Center for Glycomics, Department of Cellular & Molecular Medicine, University of Copenhagen; Copenhagen, DK-2200, Denmark
| | - Xu Wu
- Max-Planck Institute for Solid-State Research; Stuttgart, DE-70569, Germany
| | - Stephan Rauschenbach
- Max-Planck Institute for Solid-State Research; Stuttgart, DE-70569, Germany
- Chemistry Research Laboratory, Department of Chemistry, University of Oxford; Oxford, OX1 3TA, United Kingdom
| | - Yoshiki Narimatsu
- Copenhagen Center for Glycomics, Department of Cellular & Molecular Medicine, University of Copenhagen; Copenhagen, DK-2200, Denmark
- GlycoDisplay ApS, Copenhagen, DK-2200, Denmark
| | - Henrik Clausen
- Copenhagen Center for Glycomics, Department of Cellular & Molecular Medicine, University of Copenhagen; Copenhagen, DK-2200, Denmark
| | - Thomas Ziegler
- Institute of Organic Chemistry, University of Tübingen; Tübingen, DE-72076, Germany
| | - Rebecca L. Miller
- Copenhagen Center for Glycomics, Department of Cellular & Molecular Medicine, University of Copenhagen; Copenhagen, DK-2200, Denmark
| | - Klaus Kern
- Max-Planck Institute for Solid-State Research; Stuttgart, DE-70569, Germany
- Institut de Physique, École Polytechnique Fédérale de Lausanne; Lausanne, CH-1015, Switzerland
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3
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González-Ramírez EJ, Etxaniz A, Alonso A, Goñi FM. Phase behaviour of C18-N-acyl sphingolipids, the prevalent species in human brain. Colloids Surf B Biointerfaces 2022; 219:112855. [PMID: 36137336 DOI: 10.1016/j.colsurfb.2022.112855] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/13/2022] [Revised: 09/10/2022] [Accepted: 09/14/2022] [Indexed: 10/31/2022]
Abstract
Lipidomic analysis of the N-acyl components of sphingolipids in different mammalian tissues had revealed that brain tissue differed from all the other samples in that SM contained mainly C18:0 and C24:1N-acyl chains, and that the most abundant Cer species was C18:0. Only in the nervous system was C18:0 found in sizable proportions. The high levels of C18:0 and C16:0, respectively in brain and non-brain SM, were important because SM is by far the most abundant sphingolipid in the plasma membrane. In view of these observations, the present paper is devoted to a comparative study of the properties of C16:0 and C18:0 sphingolipids (SM and Cer) pure and in mixtures of increasing complexities, using differential scanning calorimetry, confocal microscopy of giant unilamellar vesicles, and correlative fluorescence microscopy and atomic force microscopy of supported lipid bilayers. Membrane rigidity was measured by force spectroscopy. It was found that in mixtures containing dioleoyl phosphatidylcholine, sphingomyelin and cholesterol, i.e. representing the lipids predominant in the outer monolayer of cell membranes, lateral inhomogeneities occurred, with the formation of rigid domains within a continuous fluid phase. Inclusion of saturated Cer in the system was always found to increase the rigidity of the segregated domains. C18:0-based sphingolipids exhibit hydrocarbon chain-length asymmetry, and some singularities observed with this N-acyl chain, e.g. complex calorimetric endotherms, could be attributed to this property. Moreover, C18:0-based sphingolipids, that are typical of the excitable cells, were less miscible with the fluid phase than their C16:0 counterparts. The results could be interpreted as suggesting that the predominance of C18:0 Cer in the nervous system would contribute to the tightness of its plasma membranes, thus facilitating maintenance of the ion gradients.
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Affiliation(s)
- Emilio J González-Ramírez
- Instituto Biofisika (CSIC, UPV/EHU) and Department of Biochemistry, University of the Basque Country, 48940 Leioa, Spain
| | - Asier Etxaniz
- Instituto Biofisika (CSIC, UPV/EHU) and Department of Biochemistry, University of the Basque Country, 48940 Leioa, Spain
| | - Alicia Alonso
- Instituto Biofisika (CSIC, UPV/EHU) and Department of Biochemistry, University of the Basque Country, 48940 Leioa, Spain.
| | - Félix M Goñi
- Instituto Biofisika (CSIC, UPV/EHU) and Department of Biochemistry, University of the Basque Country, 48940 Leioa, Spain
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4
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Buslaev P, Mustafin K, Gushchin I. Principal component analysis highlights the influence of temperature, curvature and cholesterol on conformational dynamics of lipids. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2020; 1862:183253. [PMID: 32142820 DOI: 10.1016/j.bbamem.2020.183253] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/13/2019] [Revised: 02/14/2020] [Accepted: 02/27/2020] [Indexed: 01/06/2023]
Abstract
Membrane lipids are inherently highly dynamic molecules. Currently, it is difficult to probe the structures of individual lipids experimentally at the timescales corresponding to atomic motions, and consequently molecular dynamics simulations are used widely. In our previous work, we have introduced the principal component analysis (PCA) as a convenient framework for comprehensive quantitative description of lipid motions. Here, we present a newly developed open source script, PCAlipids, which automates the analysis and allows us to refine the approach and test its limitations. We use PCAlipids to determine the influence of temperature, cholesterol and curvature on individual lipids, and show that the most prominent lipid tail scissoring motion is strongly affected by these factors and allows tracking of phase transition. Addition of cholesterol affects the conformations and selectively changes the dynamics of lipid molecules, impacting the large-amplitude motions. Introduction of curvature biases the conformational ensembles towards more extended structures. We hope that the developed approach will be useful for understanding the molecular basis of different processes occurring in lipid membrane systems and will stimulate development of complementary experimental techniques probing the conformations of individual lipid molecules.
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Affiliation(s)
- P Buslaev
- Moscow Institute of Physics and Technology, Dolgoprudny, Russia.
| | - K Mustafin
- Moscow Institute of Physics and Technology, Dolgoprudny, Russia
| | - I Gushchin
- Moscow Institute of Physics and Technology, Dolgoprudny, Russia.
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5
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Abstract
Abstract
The skin barrier, which is essential for human survival on dry land, is located in the uppermost skin layer, the stratum corneum. The stratum corneum consists of corneocytes surrounded by multilamellar lipid membranes that prevent excessive water loss from the body and entrance of undesired substances from the environment. To ensure this protective function, the composition and organization of the lipid membranes is highly specialized. The major skin barrier lipids are ceramides, fatty acids and cholesterol in an approximately equimolar ratio. With hundreds of molecular species of ceramide, skin barrier lipids are a highly complex mixture that complicate the investigation of its behaviour. In this minireview, the structures of the major skin barrier lipids, formation of the stratum corneum lipid membranes and their molecular organization are described.
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Affiliation(s)
- K. Vávrová
- Charles University, Faculty of Pharmacy in Hradec Králové, Skin Barrier Research Group, Hradec Králové , Czech Republic
| | - A. Kováčik
- Charles University, Faculty of Pharmacy in Hradec Králové, Skin Barrier Research Group, Hradec Králové , Czech Republic
| | - L. Opálka
- Charles University, Faculty of Pharmacy in Hradec Králové, Skin Barrier Research Group, Hradec Králové , Czech Republic
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6
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Školová B, Hudská K, Pullmannová P, Kováčik A, Palát K, Roh J, Fleddermann J, Estrela-Lopis I, Vávrová K. Different Phase Behavior and Packing of Ceramides with Long (C16) and Very Long (C24) Acyls in Model Membranes: Infrared Spectroscopy Using Deuterated Lipids. J Phys Chem B 2014; 118:10460-70. [DOI: 10.1021/jp506407r] [Citation(s) in RCA: 60] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Affiliation(s)
- Barbora Školová
- Skin
Barrier Research Group, Faculty of Pharmacy in Hradec Králové, Charles University in Prague, Heyrovského 1203, 50005 Hradec Králové, Czech Republic
- Institute
of Medical Physics and Biophysics, University of Leipzig, Härtelstrasse
16-18, 04275 Leipzig, Germany
| | - Klára Hudská
- Skin
Barrier Research Group, Faculty of Pharmacy in Hradec Králové, Charles University in Prague, Heyrovského 1203, 50005 Hradec Králové, Czech Republic
| | - Petra Pullmannová
- Skin
Barrier Research Group, Faculty of Pharmacy in Hradec Králové, Charles University in Prague, Heyrovského 1203, 50005 Hradec Králové, Czech Republic
| | - Andrej Kováčik
- Skin
Barrier Research Group, Faculty of Pharmacy in Hradec Králové, Charles University in Prague, Heyrovského 1203, 50005 Hradec Králové, Czech Republic
| | - Karel Palát
- Skin
Barrier Research Group, Faculty of Pharmacy in Hradec Králové, Charles University in Prague, Heyrovského 1203, 50005 Hradec Králové, Czech Republic
| | - Jaroslav Roh
- Skin
Barrier Research Group, Faculty of Pharmacy in Hradec Králové, Charles University in Prague, Heyrovského 1203, 50005 Hradec Králové, Czech Republic
| | - Jana Fleddermann
- Institute
of Medical Physics and Biophysics, University of Leipzig, Härtelstrasse
16-18, 04275 Leipzig, Germany
| | - Irina Estrela-Lopis
- Institute
of Medical Physics and Biophysics, University of Leipzig, Härtelstrasse
16-18, 04275 Leipzig, Germany
| | - Kateřina Vávrová
- Skin
Barrier Research Group, Faculty of Pharmacy in Hradec Králové, Charles University in Prague, Heyrovského 1203, 50005 Hradec Králové, Czech Republic
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7
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Kováčik A, Roh J, Vávrová K. The chemistry and biology of 6-hydroxyceramide, the youngest member of the human sphingolipid family. Chembiochem 2014; 15:1555-62. [PMID: 24990520 DOI: 10.1002/cbic.201402153] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/04/2014] [Indexed: 11/08/2022]
Abstract
Sphingolipids are crucial for the life of the cell. In land-dwelling mammals, they are equally important outside the cell-in the extracellular space of the skin barrier-because they prevent loss of water. Although a large body of research has elucidated many of the functions of sphingolipids, their extensive structural diversity remains intriguing. A new class of sphingolipids based on 6-hydroxylated sphingosine has recently been identified in human skin. Abnormal levels of these 6-hydroxylated ceramides have repeatedly been observed in atopic dermatitis; however, neither the biosynthesis nor the roles of these unique ceramide subclasses have been established in the human body. In this Minireview, we summarize the current knowledge of 6-hydroxyceramides, including their discovery, structure, stereochemistry, occurrence in healthy and diseased human epidermis, and synthetic approaches to 6-hydroxysphingosine and related ceramides.
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Affiliation(s)
- Andrej Kováčik
- Skin Barrier Research Group, Charles University in Prague, Faculty of Pharmacy in Hradec Králové, Heyrovského 1203, 50005 Hradec Králové (Czech Republic)
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8
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Pullmannová P, Staňková K, Pospíšilová M, Skolová B, Zbytovská J, Vávrová K. Effects of sphingomyelin/ceramide ratio on the permeability and microstructure of model stratum corneum lipid membranes. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2014; 1838:2115-26. [PMID: 24824073 DOI: 10.1016/j.bbamem.2014.05.001] [Citation(s) in RCA: 42] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/04/2014] [Revised: 04/10/2014] [Accepted: 05/03/2014] [Indexed: 11/27/2022]
Abstract
The conversion of sphingomyelin (SM) to a ceramide (Cer) by acid sphingomyelinase (aSMase) is an important event in skin barrier development. A deficiency in aSMase in diseases such as Niemann-Pick disease and atopic dermatitis coincides with impaired skin barrier recovery after disruption. We studied how an increased SM/Cer ratio influences the barrier function and microstructure of model stratum corneum (SC) lipid membranes. In the membranes composed of isolated human SC Cer (hCer)/cholesterol/free fatty acids/cholesteryl sulfate, partial or full replacement of hCer by SM increased water loss. Partial replacement of 25% and 50% of hCer by SM also increased the membrane permeability to theophylline and alternating electric current, while a higher SM content either did not alter or even decreased the membrane permeability. In contrast, in a simple membrane model with only one type of Cer (nonhydroxyacyl sphingosine, CerNS), an increased SM/Cer ratio provided a similar or better barrier against the permeation of various markers. X-ray powder diffraction revealed that the replacement of hCer by SM interferes with the formation of the long periodicity lamellar phase with a repeat distance of d=12.7nm. Our results suggest that SM-to-Cer processing in the human epidermis is essential for preventing excessive water loss, while the permeability barrier to exogenous compounds is less sensitive to the presence of sphingomyelin.
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Affiliation(s)
- Petra Pullmannová
- Skin Barrier Research Group, Charles University in Prague, Faculty of Pharmacy, Heyrovského 1203, 500 05 Hradec Králové, Czech Republic.
| | - Klára Staňková
- Skin Barrier Research Group, Charles University in Prague, Faculty of Pharmacy, Heyrovského 1203, 500 05 Hradec Králové, Czech Republic
| | - Markéta Pospíšilová
- Skin Barrier Research Group, Charles University in Prague, Faculty of Pharmacy, Heyrovského 1203, 500 05 Hradec Králové, Czech Republic
| | - Barbora Skolová
- Skin Barrier Research Group, Charles University in Prague, Faculty of Pharmacy, Heyrovského 1203, 500 05 Hradec Králové, Czech Republic
| | - Jarmila Zbytovská
- Institute of Chemical Technology Prague, Technická 5, 166 28 Prague, Czech Republic
| | - Kateřina Vávrová
- Skin Barrier Research Group, Charles University in Prague, Faculty of Pharmacy, Heyrovského 1203, 500 05 Hradec Králové, Czech Republic
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9
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Rupp C, Steckel H, Müller BW. Mixed micelle formation with phosphatidylcholines: The influence of surfactants with different molecule structures. Int J Pharm 2010; 387:120-8. [DOI: 10.1016/j.ijpharm.2009.12.018] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/07/2009] [Revised: 12/02/2009] [Accepted: 12/04/2009] [Indexed: 10/20/2022]
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10
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Abstract
Many different hypotheses on the molecular mechanisms of vesicle fusion exist. Because these mechanisms cannot be readily asserted experimentally, we address the problem by a coarse-grained molecular dynamics simulations study and compare the results with the results of other techniques. The simulations performed include the fusion of small and large vesicles and exocytosis, i.e., the fusion of small vesicles with flat bilayers. We demonstrate that the stalk, the initial contact between two fusing vesicles, is initiated by lipid tails that extend spontaneously. The stalk is revealed to be composed of the contacting monolayers only, yet without hydrophobic voids. Anisotropic and radial expansion of the stalk have been theorized; we show that stalk evolution can proceed via both pathways starting from similar setups and that water triggers the transition from elongated stalk to hemifusion diaphragm.
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Affiliation(s)
- A F Smeijers
- Department of Biomedical Engineering, Technische Universiteit Eindhoven, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.
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11
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Dahlberg M. Polymorphic phase behavior of cardiolipin derivatives studied by coarse-grained molecular dynamics. J Phys Chem B 2007; 111:7194-200. [PMID: 17542632 DOI: 10.1021/jp071954f] [Citation(s) in RCA: 56] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Cardiolipin (CL) is a negatively charged four acyl chain lipid, associated with energy production in bacterial and mitochondrial membranes. Due to the shape of CL, negative curvatures of aggregates are favorable if the charges in the head group can be reduced. The phase polymorphism of CL, and of associated derivatives with 2, 3, 4, or 5 chains, has been determined previously and offers a model system in which micellar, lamellar, and inverse hexagonal phases can be observed. We present an extension to a previously established coarse-grained molecular dynamics model with the aim of reproducing the different CL phases with two adjustable parameters: the number of acyl chains and the effective head group charge. With molecular dynamics simulations of large lipid systems, we observed transitions between different phases on the nanosecond to microsecond time scale. Charge screening by high salt or low pH was successfully modeled by a reduction of phosphate charge, which led to the adoption of aggregates with more negative curvature. Although specific ion binding at the interface and other atomistic details are sacrificed in the coarse-grained model, we found that it captures general phase features over a large range of aggregate geometries.
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Affiliation(s)
- Martin Dahlberg
- Division of Physical Chemistry, Arrhenius Laboratory, Stockholm University, Stockholm, Sweden.
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12
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Vávrová K, Zbytovská J, Palát K, Holas T, Klimentová J, Hrabálek A, Dolezal P. Ceramide analogue 14S24 ((S)-2-tetracosanoylamino-3-hydroxypropionic acid tetradecyl ester) is effective in skin barrier repair in vitro. Eur J Pharm Sci 2004; 21:581-7. [PMID: 15066658 DOI: 10.1016/j.ejps.2003.12.009] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/12/2003] [Revised: 12/03/2003] [Accepted: 12/10/2003] [Indexed: 11/23/2022]
Abstract
Stratum corneum ceramides are fundamental for maintaining the skin barrier properties. Their content is decreased in some skin diseases, e.g. atopic dermatitis, and ceramide supplementation is one of the therapeutic approaches. In the present study we have designed novel ceramide analogue 14S24 ((S)-2-tetracosanoylamino-3-hydroxypropionic acid tetradecyl ester) as a potential barrier-repairing agent. We report a convenient two-step synthesis of this analogue with high yields. The ability of 14S24 to repair the disturbed skin barrier was evaluated in vitro on the porcine skin. After 2h application of 14S24 on the skin disrupted by lipid extraction, the permeability decreased significantly almost to the values of the native skin. The compound is effective in 0.1% aqueous suspension and its effect is comparable with physiological skin lipids under the same condition. The comparison of 14S24 and skin ceramides was made via computer modelling and the in silico physico-chemical parameters are reported. We suggest that allylic hydroxyl, that is essential for the apoptogenic activity of ceramides, is not a necessary component of the skin barrier-forming ceramides. The main result of this study is to demonstrate that simpler and easier-to-synthesise ceramide analogues could be effective in the skin barrier repair.
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Affiliation(s)
- Katerina Vávrová
- Department of Inorganic and Organic Chemistry, Faculty of Pharmacy, Charles University, Heyrovského 1203, 50005 Hradec Králové, Czech Republic.
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13
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Stevens MJ, Hoh JH, Woolf TB. Insights into the molecular mechanism of membrane fusion from simulation: evidence for the association of splayed tails. PHYSICAL REVIEW LETTERS 2003; 91:188102. [PMID: 14611319 DOI: 10.1103/physrevlett.91.188102] [Citation(s) in RCA: 118] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/08/2003] [Indexed: 05/24/2023]
Abstract
We present coarse-grained simulations of fusion between two liposomes from which a detailed picture of lipid movements emerges. In these simulations the bilayers dilate at the contact edge, and the resulting increase in the area per lipid produces a tilting of the individual molecules as predicted. Fusion is initiated when some of these tilted lipids splay their aliphatic tails, such that the molecules are shared between the opposing leaflets. Multiple splayed lipids subsequently associate with their aliphatic tails in contact, which produces a new hydrophobic core. As the tails extend into a more parallel conformation the two outer leaflets become contiguous to produce a hemifused structure. The results have interesting implications for biological membrane fusion and suggest new possibilities for designing molecules that control fusion.
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Affiliation(s)
- Mark J Stevens
- Sandia National Laboratories, P.O. Box 5800, MS 1411, Albuquerque, New Mexico 87185-1411, USA
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