Scholarship from the faculty of the University of Denver's Chemistry and Biochemistry Department.


Submissions from 2022


Foldamers Reveal and Validate Therapeutic Targets Associated with Toxic α-Synuclein Self-Assembly, Jemil Ahmed, Tessa C. Fitch, Courtney M. Donnelly, Johnson A. Joseph, Tyler D. Ball, Mikaela M. Bassil, Ahyun Son, Chen Zhang, Aurélie Ledreux, Scott Horowitz, Yan Qin, Daniel Paredes, and Sunil Kumar


On-road Vehicle Emission Measurements and Their use in Evaluating Emission Control Effectiveness {presentation}, Gary A. Bishop


The Art of Poetry and Its Place in the History of the Coordinating Research Councils Real World Emissions Workshop {poster}, Gary A. Bishop


Utah Wintertime Measurements of Heavy-Duty Vehicle Nitrogen Oxide Emission Factors, Gary A. Bishop, Molly J. Haugen, Brian C. McDonald, and Adam M. Boies

Submissions from 2020


Are Electric Vehicles a Panacea for Reducing Ozone Precursor Emissions?, Gary A. Bishop


Inspection and Maintenance Evaluation using Historical U.S. Remote Sensing Measurements, Gary A. Bishop


On-Road Remote Sensing of Automobile Emissions in the Denver Area: Winter 2020, Gary A. Bishop


On-Road Remote Sensing of Automobile Emissions in the Tulsa Area: Fall 2019, Gary A. Bishop


Vehicle Exhaust Remote Sensing Device Method to Screen Vehicles for Evaporative Running Loss Emissions, Gary A. Bishop, Timothy H. DeFries, James A. Sidebottom, and James M. Kemper


Iron Speciation in PM2.5 from Urban, Agriculture, and Mixed Environments in Colorado, USA, Joseph R. Salazar, David J. Pfotenhauer, Frank Leresche, Fernando L. Rosario-Ortiz, Michael P. Hannigan, Sirine C. Fakra, and Brian Majestic

An Analysis of Real-world Exhaust Emission Control Deterioration in the California Light-duty Gasoline Vehicle Fleet, Tao Zhan, Chris R. Ruehl, Gary A. Bishop, Seyedehsan Hosseini, John F. Collins, Seungju Yoon, and Jorn D. Herner

Submissions from 2019

Characterizing Vortex Beams from a Spatial Light Modulator with Collinear Phase-shifting Holography, Jasmine M. Andersen, Samuel N. Alperin, Andrew A. Voitiv, William G. Holtzmann, Juliet T. Gopinath, and Mark Siemens

Occurrence, Function, and Biosynthesis of Mycofactocin, Richard Ayikpoe, Vishnu Govindarajan, and John A. Latham

Spectroscopic and Electrochemical Characterization of the Mycofactocin Biosynthetic Protein, MftC, Provides Insight into its Redox Flipping Mechanism, Richard Ayikpoe, Thacien Ngendahimana, Michelle Langton, Sheila Bonitatibus, Lindsey M. Walker, Sandra S. Eaton, Gareth R. Eaton, Maria-Eirini Pandelia, Sean J. Elliott, and John A. Latham

MftD Catalyzes the Formation of a Biologically Active Redox Center in the Biosynthesis of the Ribosomally Synthesized and Post-translationally Modified Redox Cofactor Mycofactocin, Richard S. Ayikpoe and John A. Latham


Measuring Emissions from the On-Road Vehicle Fleet in West Los Angeles, Gary A. Bishop


On-Road Remote Sensing of Automobile Emissions in the Chicago Area: Fall 2018, Gary A. Bishop


Three Decades of On-road Mobile Source Emissions Reductions in South Los Angeles, Gary A. Bishop

Diastereoselective Heterocyclization of Geminal Bromo-fluoro Arylcyclopropanes by Nitrosonium Tetrafluoroborate: Access to 4-fluorinated Isoxazolines and Isoxazoles, Oksana B. Bondarenko, Arseniy I. Komarov, George L. Karetnikov, Svetlana N. Nikolaeva, Nikolai V. Zyk, Tina Holt, and Andrei G. Kutateladze

Access to 5-fluoroisoxazoles via the Nitrosation of Geminal Bromo-fluoro Arylcyclopropanes, Oksana B. Bondarenko, Aleksandr A. Vinogradov, Arseniy I. Komarov, Georgy L. Karetnikov, Nikolai V. Zyk, Tina Holt, and Andrei G. Kutateladze

Creating Custom Foldit Puzzles for Teaching Biochemistry, Lorna Dsilva, Shubhi Mittal, Brian Koepnick, Jeff Flatten, Seth Cooper, and Scott Horowitz

Preliminary Results from the FARCE 2015 Campaign: Multidisciplinary Study of the Forest–gas–aerosol–cloud System on the Tropical Island of La Réunion, Valentin Duflot, Pierre Tulet, Olivier Flores, Christelle Barthe, Aurélie Colomb, Laurent Deguillaume, Mickael Vaïtilingom, Anne Perring, J. Alex Huffman, Mark T. Hernandez, Karine Sellegri, Ellis Robinson, David J. O'Connor, Odessa M. Gomez, Frédéric Burnet, Thierry Bourrianne, Dominique Strasberg, Manon Rocco, Allan K. Bertram, Patrick Chazette, Julien Totems, Jacques Fournel, Pierre Stamenoff, Jean-Marc Metzger, Mathilde Chabasset, Clothilde Rousseau, Eric Bourrianne, Martine Sancelme, Anne-Marie Delort, Rachel E. Wegener, Cedric Chou, and Pablo Elizondo

Structural and Functional Characterization of Sulfonium Carbon–Oxygen Hydrogen Bonding in the Deoxyamino Sugar Methyltransferase TylM1, Robert J. Fick, Scott Horowitz, Brandon G. McDole, Mary C. Clay, Ryan A. Mehl, Hashim M. Al-Hashimi, Steve Scheiner, and Raymond C. Trievel

Time-resolved Multirotational Dynamics of Single Solution-phase Tau Proteins Reveals Details of Conformational Variation, Alexander K. Foote, Lydia H. Manger, Michael R. Holden, Martin Margittai, and Randall H. Goldsmith

Depressed Prostaglandins and Leukotrienes in Veterans with Gulf War Illness, Beatrice Alexandra Golomb, Hayley J. Koslik, Uwe Christians, Janis Ritchie, Paul Wilson, Nancy Elkins, Jelena Klawitter, Jost Klawitter, Dwight Smith, and John E. Repine

Role of Polycyclic Aromatic Hydrocarbons on the Photo-catalyzed Solubilization of Simulated Soil-bound Atmospheric Iron, John P. Haynes and Brian Majestic

Diminishing Benefits of Federal Reformulated Gasoline (RFG) Compared to Conventional Gasoline (CG), S. Kent Hoekman, Amber Leland, and Gary A. Bishop

Genomic Tagging of Endogenous Human ESCRT-I Complex Preserves ESCRT-mediated Membrane-remodeling Functions, Huxley K. Hoffman, Melissa V. Fernandez, Nicholas S. Groves, Eric O. Freed, and Schuyler B. van Engelenburg

Discovery of Hydroxylase Activity for PqqB Provides a Missing Link in the Pyrroloquinoline Quinone Biosynthetic Pathway, Eric M. Koehn, John A. Latham, Tara Armand, Robert L. Evans, Xiongying Tu, Carrie M. Wilmot, Anthony T. Iavarone, and Judith P. Klinman

Structure Validation of Complex Natural Products: Time to Change the Paradigm. What did Synthesis of Alstofolinine A Prove?, Andrei G. Kutateladze and Tina Holt

Natural Products Containing the Oxetane and Related Moieties Present Additional Challenges for Structure Elucidation: A DU8+ Computational Case Study, Andrei G. Kutateladze, Tina Holt, and D. Sai Reddy

RNA‐dependent Chaperone (chaperna) as an Engineered Pro‐region for the Folding of Recombinant Microbial Transglutaminase, Jinhee Lee, Ahyun Son, Paul Kim, Soon Bin Kwon, Ji Eun Yu, Gyoonhee Han, and Baik L. Seong

Counterion Influence on Dynamic Spin Properties in a V(iv) Complex, Chun-Yi Lin, Thacien Ngendahimana, Gareth R. Eaton, Sandra S. Eaton, and Joseph M. Zadrozny

Driving Tau into Phase-separated Liquid Droplets, Martin Margittai

A Two-component Protease in Methylorubrum Extorquens with High Activity Toward the Peptide Precursor of the Redox Cofactor Pyrroloquinoline Quinone, Ana M. Martins, John A. Latham, Paulo J. Martel, Ian Barr, Anthony T. Iavarone, and Judith P. Klinman

The Value of Universally Available Raw NMR Data for Transparency, Reproducibility, and Integrity in Natural Product Research, James B. McAlpine, Shao-Nong Chen, Andrei G. Kutateladze, John B. MacMillan, Giovanni Appendino, Andersson Barison, Mehdi A. Beniddir, Maique W. Biavatti, Stefan Bluml, Asmaa Boufridi, Mark S. Butler, Robert J. Capon, Young H. Choi, David Coppage, Phillip Crews, Michael T. Crimmins, Marie Csete, Pradeep Dewapriya, Joseph M. Egan, Mary J. Garson, Grégory Genta-Jouve, William H. Gerwick, Harald Gross, Mary Kay Harper, Precilia Hermanto, James M. Hook, Luke Hunter, Damien Jeannerat, Nai-Yun Ji, Tyler A. Johnson, David G. I. Kingston, Hiroyuki Koshino, Hsiau-Wei Lee, Guy Lewin, Jie Li, Roger G. Linington, Miaomiao Liu, Kerry L. McPhail, Tadeusz F. Molinski, Bradley S. Moore, Joo-Won Nam, Ram P. Neupane, Matthias Niemitz, Jean-Marc Nuzillard, Nicholas H. Oberlies, Fernanda M. M. Ocampos, Guohui Pan, Ronald J. Quinn, D. Sai Reddy, Jean-Hugues Renault, José Rivera-Chávez, Wolfgang Robien, Carla M. Saunders, Thomas J. Schmidt, Christoph Seger, Ben Shen, Christoph Steinbeck, Hermann Stuppner, Sonja Sturm, Orazio Taglialatela-Scafati, Dean J. Tantillo, Robert Verpoorte, Bin-Gui Wang, Craig M. Williams, Philip G. Williams, Julien Wist, Jian-Min Yue, Chen Zhang, Zhengren Xu, Charlotte Simmler, David C. Lankin, Jonathan Bisson, and Guido F. Pauli

An X-band Continuous Wave Saturation Recovery Electron Paramagnetic Resonance Spectrometer Based on an Arbitrary Waveform Generator, Joseph E. McPeak, Richard W. Quine, Sandra S. Eaton, and Gareth R. Eaton

Structural Insights for Vanadium Catecholates and Iron‑sulfur Clusters Obtained from Multiple Data Analysis Methods Applied to Electron Spin Relaxation Data, Thacien Ngendahimana, Richard Ayikpoe, John A. Latham, Gareth R. Eaton, and Sandra S. Eaton

Photoinitiated Cascade for Rapid Access to Pyrroloquinazolinone Core of Vasicinone, Luotonins, and Related Alkaloids, D. Sai Reddy and Andrei G. Kutateladze

Identifying Dynamic, Partially Occupied Residues using Anomalous Scattering, Serena Rocchio, Ramona Duman, Kamel El Omari, Vitaliy Mykhaylyk, Christian Orr, Zhen Yan, Loïc Salmon, Armin Wagner, James C. A. Bardwell, and Scott Horowitz

Thermally Stable Perovskite Solar Cells by Systematic Molecular Design of the Hole-transport Layer, Tracy H. Schloemer, Timothy S. Gehan, Jeffrey A. Christians, Deborah Gale Mitchell, Alex Dixon, Zhen Li, Kai Zhu, Joseph J. Berry, Joseph M. Luther, and Alan Sellinger

Novel π-conjugated Systems Based on Pyrimidine N-Oxide, Kseniya N. Sedenkova, Julia V. Kolodyazhnaya, Dmitry A. Vasilenko, Yulia A. Gracheva, Elena V. Kharitonoshvili, Yuri K. Grishin, Alexey A. Chistov, Victor B. Rybakov, Tina Holt, Andrei G. Kutateladze, Tamara S. Kuznetsova, Elena R. Milaeva, and Elena B. Averina

An Analysis of Real-world Exhaust Emission Control Deterioration in the California Light-duty Gasoline Vehicle Fleet, Tao Zhan, Chris R. Ruehl, Gary A. Bishop, Seyedehsan Hosseini, John F. Collins, and Jorn D. Herner

Submissions from 2018

Cyclopentane Formation from Flexible Precursors Using Samarium(II) Reagents, Christopher D. Aretz, Humberto Escobedo, and Bryan J. Cowen

Mechanism of SmI2 Reduction of 5-Bromo-6-oxo-6-phenylhexyl Methanesulfonate Studied by Spin Trapping with 2-Methyl-2-nitrosopropane, Christopher D. Aretz, Joseph E. McPeak, Gareth R. Eaton, Sandra S. Eaton, and Bryan J. Cowen

Mycofactocin Biosynthesis Proceeds through 3-Amino-5-[(p-hydroxyphenyl)methyl]-4,4-dimethyl-2-pyrrolidinone (AHDP); Direct Observation of MftE Specificity toward MftA*, Richard Ayikpoe, Joe Salazar, Brian Majestic, and John A. Latham

X-ray and EPR Characterization of the Auxiliary Fe–S Clusters in the Radical SAM Enzyme PqqE, Ian Barr, Troy A. Stich, Anthony S. Gizzi, Tyler L. Grove, Jeffrey B. Bonanno, John A. Latham, Tyler Chung, Carrie M. Wilmot, R. David Britt, Steven C. Almo, and Judith P. Klinman

Measurement of T1e, T1N, T1HE, T2e, and T2HE by Pulse EPR at X-Band for Nitroxides at Concentrations Relevant to Solution DNP, Joshua R. Biller, Joseph E. McPeak, Sandra S. Eaton, and Gareth R. Eaton


On-Road Remote Sensing of Automobile Emissions in the Tulsa Area: Fall 2017, Gary A. Bishop


Revisit Lynwood, CA: On-Road Remote Sensing of Automobile Emissions in the Lynwood, CA: 2018, Gary A. Bishop


Investigate the Durability of Diesel Engine Emission Controls, Gary A. Bishop and Molly J. Haugen


On-Road Remote Sensing of Automobile Emissions in the Denver Area: Winter 2017, Gary A. Bishop and Molly J. Haugen


The Story of Ever Diminishing Vehicle Tailpipe Emissions as Observed in the Chicago, IL Area, Gary A. Bishop and Molly J. Haugen


Evaluation of Emissions Benefits of Federal Reformulated Gasoline versus Conventional Gasoline, Gary A. Bishop, S. Kent Hoekman, and Amber Broch

Tabletop 700 MHz Electron Paramagnetic Resonance Imaging Spectrometer, Laura A. Buchanan, George A. Rinard, Richard W. Quine, Sandra S. Eaton, and Gareth R. Eaton

Background Correction in Rapid Scan EPR Spectroscopy, Laura A. Buchanan, Lukas B. Woodcock, Richard W. Quine, George A. Rinard, Sandra S. Eaton, and Gareth R. Eaton

250 MHz Rapid Scan Cross Loop Resonator, Laura A. Buchanan, Lukas B. Woodcock, George A. Rinard, Richard W. Quine, Yilin Shi, Sandra S. Eaton, and Gareth R. Eaton

Accessing 4-oxy-substituted Isoquinolinones via C–H Activation and Regioselective Migratory Insertion with Electronically Biased ynol Ethers, Brandon L. Coles-Taylor, Maximilian S. McCallum, J. Scott Lee, and Brian W. Michel

Electron Paramagnetic Resonance of a 10B-containing Heterocyclic Radical, Sandra S. Eaton, Thacien Ngendahimana, Gareth R. Eaton, Andrew R. Jupp, and Douglas W. Stephan

Continuous Wave Electron Paramagnetic Resonance of Nitroxide Biradicals in Fluid Solution, Sandra S. Eaton, Lukas B. Woodcock, and Gareth R. Eaton

Maximum Caliber can Build and Infer Models of Oscillation in a Three-gene Feedback Network, Taylor Firman, Anar Amgalan, and Kingshuk Ghosh

Long-term Fuel-Specific NOx and Particle Emission Trends for In-use Heavy-duty Vehicles in California, Molly J. Haugen and Gary A. Bishop


Long-Term Fuel-Specific NOx and Particle Emission Trends for In-Use Heavy-Duty Vehicles in California, Molly J. Haugen and Gary A. Bishop


On-Road Remote Sensing of Heavy and Medium-duty Trucks at the Peralta Weigh Station in the South Coast Air Basin: Spring 2017, Molly J. Haugen and Gary A. Bishop


Evaluation of Heavy- and Medium-Duty On-Road Vehicle Emissions in California's South Coast Air Basin, Molly J. Haugen, Gary A. Bishop, Arvind Thiruvengadam, and Daniel K. Carder

Evaluation of Heavy- and medium-duty On-road Vehicle Emissions in California’s South Coast Air Basin, Molly J. Haugen, Gary A. Bishop, Arvind Thiruvengadam, and Daniel K. Carder

Investigation into Photoinduced Auto-oxidation of Polycyclic Aromatic Hydrocarbons Resulting in Brown Carbon Production, John P. Haynes, Keith E. Miller, and Brian Majestic

Addressing the Challenges of Structure Elucidation in Natural Products Possessing the Oxirane Moiety, Andrei G. Kutateladze, Dmitry M. Kuznetsov, Anastasiya A. Beloglazkina, and Tina Holt

Evaluation of Cellular Effects of Fine Particulate Matter from Combustion of Solid Fuels used for Indoor Heating on the Navajo Nation using a Stratified Oxidative Stress Response Model, Ning Li, Wyatt M. Champion, Jemal Imam, Damansher Sidhu, Joseph R. Salazar, Brian Majestic, and Lupita D. Montoya

The Distinct Structural Preferences of Tau Protein Repeat Domains, Xuhua Li, Xuewei Dong, Guanghong Wei, Martin Margittai, Ruth Nussinov, and Buyong Ma

UV Degradation Model for Polymers and Polymer Matrix Composites, T. Lu, E. Solis-Ramos, Y. Yi, and M. Kumosa

Analysis of a Vinculin Homolog in a Sponge (phylum Porifera) Reveals that Vertebrate-like Cell Adhesions Emerged Early in Animal Evolution, Phillip W. Miller, Sabine Pokutta, Jennyfer M. Mitchell, Jayanth V. Chodaparambil, D. Nathaniel Clarke, W. James Nelson, William I. Weis, and Scott A. Nichols

Using Rapid-scan EPR to Improve the Detection Limit of Quantitative EPR by More Than One Order of Magnitude, J. Möser, K. Lips, Mark Tseitlin, Gareth R. Eaton, Sandra S. Eaton, and A. Schnegg

Development and Characterization of an Inexpensive Single-particle Fluorescence Spectrometer for Bioaerosol Monitoring, Benjamin E. Swanson and J. Alex Huffman

Inactivation of Pure Bacterial Biofilms by Impaction of Aerosolized Consumer Products Containing Nanoparticulate Metals, Jennifer Therkorn, Leonardo Calderón, Benton Cartledge, Nirmala Thomas, Brian Majestic, and Gediminas Mainelis

Olefin Metathesis-based Fluorescent Probes for the Selective Detection of Ethylene in Live Cells, Sacha N. W. Toussaint, Ryan T. Calkins, Sumin Lee, and Brian W. Michel

Structural Disorder in Four-repeat Tau Fibrils Reveals a New Mechanism for Barriers to Cross-seeding of Tau Isoforms, Hilary Ann Weismiller, Rachel Murphy, Guanghong Wei, Buyong Ma, Ruth Nussinov, and Martin Margittai

Submissions from 2017

Conformational Changes in C-Reactive Protein Affect Binding to Curved Membranes in a Lipid Bilayer Model of the Apoptotic Cell Surface, Aml Abd Alhamed Alnaas, Carrie Leine Moon, Mitchell Alton, Scott M. Reed, and Michelle K. Knowles

Mechanisms and Barriers in Cancer Nanomedicine: Addressing Challenges, Looking for Solutions, Thomas J. Anchordoquy, Yechezkel Barenholz, Diana Boraschi, Michael Chorny, Paolo Decuzzi, Marina A. Dobrovolskaia, Z. Shadi Farhangrazi, Dorothy Farrell, Alberto Gabizon, Hamidreza Ghandehari, Biana Godin, Ninh M. La-Beck, Julia Ljubimova, S. Moein Moghimi, Len Pagliaro, Ji-Ho Park, Dan Peer, Erkki Ruoslahti, Natalie J. Serkova, and Dmitri Simberg


United States Light and Heavy -Duty Fuel Specific On -Road NO and NO x Emission Factor Trends and Their Importance in Inventory Calculations (Presentation), Gary A. Bishop


On-Road Remote Sensing of Automobile Emissions in the Chicago Area: Fall 2016, Gary A. Bishop and Molly J. Haugen

Adsorption Dynamics of Polyatomic Molecules on Planar Surfaces, Jared T. Burde and M. Mercedes Calbi

Structure Determination and Mechanism of Formation of a seco-Moreliane Derivative Supported by Computational Analysis, Pablo A. Chacón Morales, Juan M. Amaro-Luis, and Andrei G. Kutateladze

Single Lipid Molecule Dynamics on Supported Lipid Bilayers with Membrane Curvature, Philip Cheney, Alan Weisgerber, Alec Feuerbach, and Michelle Knowles

Comparative Measurements of Ambient Atmospheric Concentrations of Ice Nucleating Particles using Multiple Immersion Freezing Methods and a Continuous Flow Diffusion Chamber, Paul J. DeMott, Thomas C. J. Hill, Markus D. Petters, Allan K. Bertram, Yutaka Tobo, Ryan H. Mason, Kaitlyn J. Suski, Christina S. McCluskey, Ezra J. T. Levin, Gregory P. Schill, Yvonne Boose, Anne Marie Rauker, Anna J. Miller, Jake Zaragoza, Katherine Rocci, Nicholas E. Rothfuss, Hans P. Taylor, John D. Hader, Cedric Chou, J. Alex Huffman, Ulrich Pöschl, Anthony J. Prenni, and Sonia M. Kreidenweis

Rapid-scan EPR imaging, Sandra S. Eaton, Yilin Shi, Lukas Woodcock, Laura A. Buchanan, Joseph McPeak, Richard W. Quine, George A. Rinard, Boris Epel, Howard J. Halpern, and Gareth R. Eaton

Repeat Fuel Specific Emission Measurements on Two California Heavy-duty Truck Fleets, Molly J. Haugen and Gary A. Bishop


Repeat Fuel Specific Emission Measurements on Two California Heavy-Duty Truck Fleets, Molly J. Haugen and Gary A. Bishop

Synthesis and Electron Spin Relaxation of Tetracarboxylate Pyrroline Nitroxides, Shengdian Huang, Joseph T. Paletta, Hanan Elajaili, Kirby Huber, Maren Pink, Suchada Rajca, Gareth R. Eaton, Sandra S. Eaton, and Andrzej Rajca

Triquinanes and Related Sesquiterpenes Revisited Computationally: Structure Corrections of Hirsutanols B and D, Hirsutenol E, Cucumin B, Antrodins C–E, Chondroterpenes A and H, Chondrosterins C and E, Dichrocephone A, and Pethybrene, Andrei G. Kutateladze and Dmitry M. Kuznetsov

High-throughput in Silico Structure Validation and Revision of Halogenated Natural Products Is Enabled by Parametric Corrections to DFT-Computed13C NMR Chemical Shifts and Spin–spin Coupling Constants, Andrei G. Kutateladze and D. Sai Reddy

Step-economical Photoassisted Diversity-oriented Synthesis: Sustaining Cascade Photoreactions in Oxalyl Anilides to Access Complex Polyheterocyclic Molecular Architectures, Dmitry M. Kuznetsov and Andrei G. Kutateladze

At the Confluence of Ribosomally Synthesized Peptide Modification and Radical S -adenosylmethionine (SAM) Enzymology, John A. Latham, Ian Barr, and Judith P. Klinman

Catalytic Mechanism of the Hotdog-Fold Thioesterase PA1618 Revealed by X-ray Structure Determination of a Substrate-bound Oxygen Ester Analogue Complex, John A. Latham, Tianyang Ji, Kaila Matthews, Patrick S. Mariano, Karen N. Allen, and Debra Dunaway-Mariano

Chlamydomonas Reinhardtii LFO1 Is an IsdG Family Heme Oxygenase, Lisa J. Lojek, Allison J. Farrand, Jennifer H. Wisecaver, Crysten E. Blaby-Haas, Brian W. Michel, Sabeeha S. Merchant, Antonis Rokas, and Eric P. Skaar