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Notarstefano V, Pepe A, Ripanti F, Piccirilli F, Vaccari L, Mariani P. Guanosine hydrogels in focus: A comprehensive analysis through mid-infrared spectroscopy. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2025; 324:124939. [PMID: 39137710 DOI: 10.1016/j.saa.2024.124939] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/29/2024] [Revised: 06/27/2024] [Accepted: 08/05/2024] [Indexed: 08/15/2024]
Abstract
Guanosine nucleosides and nucleotides have the peculiar ability to self-assemble in water to form supramolecular complex architectures from G-quartets to G-quadruplexes. G-quadruplexes exhibit in turn a large liquid crystalline lyotropic polymorphism, but they eventually cross-link or entangle to form a densely connected 3D network (a molecular hydrogel), able to entrap very large amount of water (up to the 99% v/v). This high water content of the hydrogels enables tunable softness, deformability, self-healing, and quasi-liquid properties, making them ideal candidates for different biotechnological and biomedical applications. In order to fully exploit their possible applications, Attenuated Total Reflection-Fourier Transform InfraRed (ATR-FTIR) spectroscopy was used to unravel the vibrational characteristics of supramolecular guanosine structures. First, the characteristic vibrations of the known quadruplex structure of guanosine 5'-monophosphate, potassium salt (GMP/K), were investigated: the identified peaks reflected both the chemical composition of the sample and the formation of quartets, octamers, and quadruplexes. Second, the role of K+ and Na+ cations in promoting the quadruplex formation was assessed: infrared spectra confirmed that both cations induce the formation of G-quadruplexes and that GMP/K is more stable in the G-quadruplex organization. Finally, ATR-FTIR spectroscopy was used to investigate binary mixtures of guanosine (Gua) and GMP/K or GMP/Na, both systems forming G-hydrogels. The same G-quadruplex-based structure was found in both mixtures, but the proportion of Gua and GMP affected some features, like sugar puckering, guanine vibrations, and base stacking, reflecting the known side-to-side aggregation and bundle formation occurring in these binary systems.
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Affiliation(s)
- Valentina Notarstefano
- Università Politecnica delle Marche, Department of Life and Environmental Sciences, Via Brecce Bianche, Ancona, 60131, Italy.
| | - Alessia Pepe
- Università Politecnica delle Marche, Department of Life and Environmental Sciences, Via Brecce Bianche, Ancona, 60131, Italy
| | - Francesca Ripanti
- Università Politecnica delle Marche, Department of Life and Environmental Sciences, Via Brecce Bianche, Ancona, 60131, Italy
| | | | - Lisa Vaccari
- Elettra Sincrotrone Trieste, S.S. 14 - km 163.5, Basovizza, 34149, Italy
| | - Paolo Mariani
- Università Politecnica delle Marche, Department of Life and Environmental Sciences, Via Brecce Bianche, Ancona, 60131, Italy
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2
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Tao YH, Schulke S, Schwaab G, Nealon GL, Pezzotti S, Hodgetts SI, Harvey AR, Havenith M, Wallace VP. Hydration water drives the self-assembly of guanosine monophosphate. Biophys J 2024; 123:931-939. [PMID: 38454599 PMCID: PMC11052693 DOI: 10.1016/j.bpj.2024.03.005] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2023] [Revised: 03/04/2024] [Accepted: 03/04/2024] [Indexed: 03/09/2024] Open
Abstract
Guanosine monophosphate (GMP) is a nucleotide that can self-assemble in aqueous solution under certain conditions. An understanding of the process at the molecular level is an essential step to comprehend the involvement of DNA substructures in transcription and replication, as well as their relationship to genetic diseases such as cancer. We present the temperature-dependent terahertz (1.5-12 THz, 50-400 cm-1) absorptivity spectra of aqueous Na2 GMP solution in comparison with the aqueous solutions of other RNA nucleotides. Distinct absorption features were observed in the spectrum of GMP, which we attribute to the intramolecular modes of the self-assemblies (i.e., G-complexes) that, at 1 M, start to form at 313 K and below. Changes in broad-band features of the terahertz spectrum were also observed, which we associate with the release of hydration water in the temperature-dependent formation of guanine quadruplexes. Using a state-of-the-art THz calorimetry approach correlating spectroscopic to thermodynamic changes, we propose a molecular mechanism of hydrophilic hydration driving GMP self-assembly as a function of temperature. The free energy contribution of hydrophilic hydration is shown as a decisive factor in guanine-quadruplex formation. Our findings spotlight the role of hydration in the formation of macromolecular structures and suggest the potential of hydration tuning for regulating DNA transcription and replication.
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Affiliation(s)
- Yu Heng Tao
- Department of Physics, The University of Western Australia, Crawley, WA, Australia
| | - Simon Schulke
- Department of Physical Chemistry II, Ruhr University Bochum, Bochum, Germany
| | - Gerhard Schwaab
- Department of Physical Chemistry II, Ruhr University Bochum, Bochum, Germany
| | - Gareth L Nealon
- School of Molecular Sciences, The University of Western Australia, Crawley, WA, Australia; Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Crawley, WA, Australia
| | - Simone Pezzotti
- Department of Physical Chemistry II, Ruhr University Bochum, Bochum, Germany
| | - Stuart I Hodgetts
- School of Human Sciences, The University of Western Australia, Crawley, WA, Australia; Perron Institute for Neurological and Translational Science, Nedlands, WA, Australia
| | - Alan R Harvey
- School of Human Sciences, The University of Western Australia, Crawley, WA, Australia; Perron Institute for Neurological and Translational Science, Nedlands, WA, Australia
| | - Martina Havenith
- Department of Physical Chemistry II, Ruhr University Bochum, Bochum, Germany.
| | - Vincent P Wallace
- Department of Physics, The University of Western Australia, Crawley, WA, Australia.
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3
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Wang J, Koduru T, Harish B, McCallum SA, Larsen KP, Patel KS, Peters EV, Gillilan RE, Puglisi EV, Puglisi JD, Makhatadze G, Royer CA. Pressure pushes tRNA Lys3 into excited conformational states. Proc Natl Acad Sci U S A 2023; 120:e2215556120. [PMID: 37339210 PMCID: PMC10293818 DOI: 10.1073/pnas.2215556120] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/11/2022] [Accepted: 05/16/2023] [Indexed: 06/22/2023] Open
Abstract
Conformational dynamics play essential roles in RNA function. However, detailed structural characterization of excited states of RNA remains challenging. Here, we apply high hydrostatic pressure (HP) to populate excited conformational states of tRNALys3, and structurally characterize them using a combination of HP 2D-NMR, HP-SAXS (HP-small-angle X-ray scattering), and computational modeling. HP-NMR revealed that pressure disrupts the interactions of the imino protons of the uridine and guanosine U-A and G-C base pairs of tRNALys3. HP-SAXS profiles showed a change in shape, but no change in overall extension of the transfer RNA (tRNA) at HP. Configurations extracted from computational ensemble modeling of HP-SAXS profiles were consistent with the NMR results, exhibiting significant disruptions to the acceptor stem, the anticodon stem, and the D-stem regions at HP. We propose that initiation of reverse transcription of HIV RNA could make use of one or more of these excited states.
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Affiliation(s)
- Jinqiu Wang
- Graduate Program in Biochemistry and Biophysics, Rensselaer Polytechnic Institute, Troy, NY12180
| | - Tejaswi Koduru
- Department of Biological Sciences, Rensselaer Polytechnic Institute, Troy, NY12180
| | | | - Scott A. McCallum
- Center for Biotechnology and Interdisciplinary Studies, Rensselaer Polytechnic Institute, Troy, NY12180
| | - Kevin P. Larsen
- Department of Structural Biology, Stanford University School of Medicine, Stanford, CA94305
| | - Karishma S. Patel
- Department of Structural Biology, Stanford University School of Medicine, Stanford, CA94305
| | - Edgar V. Peters
- Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, Troy, NY12180
| | | | - Elisabetta V. Puglisi
- Department of Structural Biology, Stanford University School of Medicine, Stanford, CA94305
| | - Joseph D. Puglisi
- Department of Structural Biology, Stanford University School of Medicine, Stanford, CA94305
| | - George Makhatadze
- Department of Biological Sciences, Rensselaer Polytechnic Institute, Troy, NY12180
| | - Catherine A. Royer
- Department of Biological Sciences, Rensselaer Polytechnic Institute, Troy, NY12180
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4
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Caimi F, Zanchetta G. Twisted Structures in Natural and Bioinspired Molecules: Self-Assembly and Propagation of Chirality Across Multiple Length Scales. ACS OMEGA 2023; 8:17350-17361. [PMID: 37251126 PMCID: PMC10210192 DOI: 10.1021/acsomega.3c01822] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 03/17/2023] [Accepted: 04/27/2023] [Indexed: 05/31/2023]
Abstract
Several biomolecules can form dynamic aggregates in water, whose nanometric structures often reflect the chirality of the monomers in unexpected ways. Their twisted organization can be further propagated to the mesoscale, in chiral liquid crystalline phases, and even to the macroscale, where chiral, layered architectures contribute to the chromatic and mechanical properties of various plant, insect, and animal tissues. At all scales, the resulting organization is determined by a subtle balance among chiral and nonchiral interactions, whose understanding and fine-tuning is fundamental also for applications. We present recent advances in the chiral self-assembly and mesoscale ordering of biological and bioinspired molecules in water, focusing on systems based on nucleic acids or related aromatic molecules, oligopeptides, and their hybrid stuctures. We highlight the common features and key mechanisms governing this wide range of phenomena, together with novel characterization approaches.
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5
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Harish B, Wang J, Hayden EJ, Grabe B, Hiller W, Winter R, Royer CA. Hidden intermediates in Mango III RNA aptamer folding revealed by pressure perturbation. Biophys J 2022; 121:421-429. [PMID: 34971617 PMCID: PMC8822612 DOI: 10.1016/j.bpj.2021.12.037] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/06/2021] [Revised: 11/12/2021] [Accepted: 12/23/2021] [Indexed: 02/03/2023] Open
Abstract
Fluorescent RNA aptamers have the potential to enable routine quantitation and localization of RNA molecules and serve as models for understanding biologically active aptamers. In recent years, several fluorescent aptamers have been selected and modified to improve their properties, revealing that small changes to the RNA or the ligands can modify significantly their fluorescent properties. Although structural biology approaches have revealed the bound, ground state of several fluorescent aptamers, characterization of low-abundance, excited states in these systems is crucial to understanding their folding pathways. Here we use pressure as an alternative variable to probe the suboptimal states of the Mango III aptamer with both fluorescence and NMR spectroscopy approaches. At moderate KCl concentrations, increasing pressure disrupted the G-quadruplex structure of the Mango III RNA and led to an intermediate with lower fluorescence. These observations indicate the existence of suboptimal RNA structural states that still bind the TO1-biotin fluorophore and moderately enhance fluorescence. At higher KCl concentration as well, the intermediate fluorescence state was populated at high pressure, but the G-quadruplex remained stable at high pressure, supporting the notion of parallel folding and/or binding pathways. These results demonstrate the usefulness of pressure for characterizing RNA folding intermediates.
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Affiliation(s)
| | - Jinqiu Wang
- Department of Biological Sciences, Rensselaer Polytechnic Institute, Troy
| | - Eric J Hayden
- Department of Biology, Boise State University, Boise
| | - Bastian Grabe
- Department of Chemistry and Chemical Biology, TU Dortmund University, Dortmund, Germany
| | - Wolf Hiller
- Department of Chemistry and Chemical Biology, TU Dortmund University, Dortmund, Germany
| | - Roland Winter
- Physical Chemistry I - Biophysical Chemistry, Department of Chemistry and Chemical Biology, TU Dortmund University, Dortmund, Germany
| | - Catherine A Royer
- Department of Biological Sciences, Rensselaer Polytechnic Institute, Troy.
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6
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Ripanti F, Fasolato C, Mazzarda F, Palleschi S, Ceccarini M, Li C, Bignami M, Bodo E, Bell SEJ, Mazzei F, Postorino P. Advanced Raman Spectroscopy Detection of Oxidative Damage in Nucleic Acid Bases: Probing Chemical Changes and Intermolecular Interactions in Guanosine at Ultralow Concentration. Anal Chem 2021; 93:10825-10833. [PMID: 34324303 PMCID: PMC8382216 DOI: 10.1021/acs.analchem.1c01049] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
DNA/RNA synthesis precursors are especially vulnerable to damage induced by reactive oxygen species occurring following oxidative stress. Guanosine triphosphates are the prevalent oxidized nucleotides, which can be misincorporated during replication, leading to mutations and cell death. Here, we present a novel method based on micro-Raman spectroscopy, combined with ab initio calculations, for the identification, detection, and quantification of oxidized nucleotides at low concentration. We also show that the Raman signature in the terahertz spectral range (<100 cm-1) contains information on the intermolecular assembly of guanine in tetrads, which allows us to further boost the oxidative damage detection limit. Eventually, we provide evidence that similar analyses can be carried out on samples in very small volumes at very low concentrations by exploiting the high sensitivity of surface-enhanced Raman scattering combined with properly designed superhydrophobic substrates. These results pave the way for employing such advanced spectroscopic methods for quantitatively sensing the oxidative damage of nucleotides in the cell.
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Affiliation(s)
- Francesca Ripanti
- Department of Physics, Sapienza University of Rome, P.le A. Moro 5, Rome, Italy
| | - Claudia Fasolato
- Department of Physics and Geology, University of Perugia, Via Alessandro Pascoli, Perugia, Italy
| | - Flavia Mazzarda
- Department of Physics, Sapienza University of Rome, P.le A. Moro 5, Rome, Italy
| | - Simonetta Palleschi
- Department of Environment & Health, Istituto Superiore di Sanità, Viale Regina Elena 299, Rome, Italy
| | - Marina Ceccarini
- National Centre for Rare Diseases, Istituto Superiore di Sanità, Viale Regina Elena 299, Rome, Italy
| | - Chunchun Li
- School of Chemistry and Chemical Engineering, Queen's University of Belfast, Stranmillis Road, Belfast, Northern Ireland
| | - Margherita Bignami
- Department of Environment & Health, Istituto Superiore di Sanità, Viale Regina Elena 299, Rome, Italy
| | - Enrico Bodo
- Department of Chemistry, Sapienza University of Rome, P.le A. Moro, 5, Rome, Italy
| | - Steven E J Bell
- School of Chemistry and Chemical Engineering, Queen's University of Belfast, Stranmillis Road, Belfast, Northern Ireland
| | - Filomena Mazzei
- Department of Environment & Health, Istituto Superiore di Sanità, Viale Regina Elena 299, Rome, Italy
| | - Paolo Postorino
- Department of Physics, Sapienza University of Rome, P.le A. Moro 5, Rome, Italy
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7
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Harish B, Gillilan RE, Zou J, Wang J, Raleigh DP, Royer CA. Protein unfolded states populated at high and ambient pressure are similarly compact. Biophys J 2021; 120:2592-2598. [PMID: 33961866 DOI: 10.1016/j.bpj.2021.04.031] [Citation(s) in RCA: 10] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/19/2021] [Revised: 04/23/2021] [Accepted: 04/29/2021] [Indexed: 10/21/2022] Open
Abstract
The relationship between the dimensions of pressure-unfolded states of proteins compared with those at ambient pressure is controversial; resolving this issue is related directly to the mechanisms of pressure denaturation. Moreover, a significant pressure dependence of the compactness of unfolded states would complicate the interpretation of folding parameters from pressure perturbation and make comparison to those obtained using alternative perturbation approaches difficult. Here, we determined the compactness of the pressure-unfolded state of a small, cooperatively folding model protein, CTL9-I98A, as a function of temperature. This protein undergoes both thermal unfolding and cold denaturation, and the temperature dependence of the compactness at atmospheric pressure is known. High-pressure small angle x-ray scattering studies, yielding the radius of gyration and high-pressure diffusion ordered spectroscopy NMR experiments, yielding the hydrodynamic radius were carried out as a function of temperature at 250 MPa, a pressure at which the protein is unfolded. The radius of gyration values obtained at any given temperature at 250 MPa were similar to those reported previously at ambient pressure, and the trends with temperature are similar as well, although the pressure-unfolded state appears to undergo more pronounced expansion at high temperature than the unfolded state at atmospheric pressure. At 250 MPa, the compaction of the unfolded chain was maximal between 25 and 30°C, and the chain expanded upon both cooling and heating. These results reveal that the pressure-unfolded state of this protein is very similar to that observed at ambient pressure, demonstrating that pressure perturbation represents a powerful approach for observing the unfolded states of proteins under otherwise near-native conditions.
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Affiliation(s)
| | | | - Junjie Zou
- Department of Chemistry, Stony Brook University, Stony Brook, New York; Laufer Center for Physical and Quantitative Biology, Stony Brook University, Stony Brook, New York
| | - Jinqiu Wang
- Department of Biological Sciences, Rensselaer Polytechnic Institute, Troy, New York
| | - Daniel P Raleigh
- Department of Chemistry, Stony Brook University, Stony Brook, New York; Laufer Center for Physical and Quantitative Biology, Stony Brook University, Stony Brook, New York
| | - Catherine A Royer
- Department of Biological Sciences, Rensselaer Polytechnic Institute, Troy, New York.
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8
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Baldassarri EJ, Ortore MG, Spinozzi F, Round A, Ferrero C, Mariani P. K vs. Na Effects on the Self-Assembly of Guanosine 5'-Monophosphate: A Solution SAXS Structural Study +. NANOMATERIALS (BASEL, SWITZERLAND) 2020; 10:E629. [PMID: 32231081 PMCID: PMC7221663 DOI: 10.3390/nano10040629] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 02/28/2020] [Revised: 03/18/2020] [Accepted: 03/20/2020] [Indexed: 12/18/2022]
Abstract
The hierarchical process of guanosine (G) self-assembly, leading in aqueous solution and in the presence of metal cations to the formation of G-quadruplexes, represents an intriguing topic both for the biological correlation with telomerase activity and for the nano-technological applications, as demonstrated by the current measured in a quadruplex wire 100 nm long. Similar to G-rich DNA sequences and G-oligonucleotides, the guanosine 5'-monophosphate (GMP) self-aggregates in water to form quadruplexes. However, due to the absence of a covalent axial backbone, this system can be very useful to understand the chemical-physical conditions that govern the guanosine supramolecular aggregation. We have then investigated by in-solution Synchrotron Small Angle X-ray Scattering technique the role of different cations in promoting the quadruplex formation as a function of concentration and temperature. Results show how potassium, with its peculiar biological traits, favours the G-quadruplex elongation process in respect to other cations (Na + , but also NH 4 + and Li + ), determining the longest particles in solution. Moreover, the formation and the elongation of G-quadruplexes have been demonstrated to be controlled by both GMP concentration and excess cation content, even if they specifically contribute to these processes in different ways. The occurrence of condensed liquid crystalline phases was also detected, proving that excess cations play also unspecific effects on the effective charges on the G-quadruplex surface.
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Affiliation(s)
- Enrico Junior Baldassarri
- Marche Structural Biology Center, Department of Life and Environmental Sciences, Università Politecnica delle Marche, 60131 Ancona, Italy
| | - Maria Grazia Ortore
- Department of Life and Environmental Sciences, Università Politecnica delle Marche, 60131 Ancona, Italy
| | - Francesco Spinozzi
- Department of Life and Environmental Sciences, Università Politecnica delle Marche, 60131 Ancona, Italy
| | - Adam Round
- European XFEL, SPB/SFX Instrument, 22869 Schenefeld, Germany
| | - Claudio Ferrero
- European Synchrotron Radiation Facility-E.S.R.F., 38043 Grenoble, France
| | - Paolo Mariani
- Department of Life and Environmental Sciences, Università Politecnica delle Marche, 60131 Ancona, Italy
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Paterson KA, Arlt J, Jones AC. Dynamic and static quenching of 2-aminopurine fluorescence by the natural DNA nucleotides in solution. Methods Appl Fluoresc 2020; 8:025002. [PMID: 32000159 DOI: 10.1088/2050-6120/ab71c3] [Citation(s) in RCA: 26] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
2-aminopurine (2AP) is a responsive fluorescent base analogue that is used widely as a probe of the local molecular environment in DNA. The ability of 2AP to report changes in local conformation and base-stacking interactions arises from the efficient quenching of its fluorescence by the natural DNA bases. However, the mechanism of this inter-base quenching remains imperfectly understood. Two previous studies of the collisional quenching of 2AP by the natural bases, in different buffer solutions, showed that dynamic quenching efficiency depends on the identity of the natural base, but disagreed on the relative quenching efficiencies of the bases. We report a comprehensive investigation of inter-base quenching of 2AP by the natural nucleoside monophosphates (NMPs), replicating the buffer conditions used in the previous studies. Using time-resolved fluorescence measurements to distinguish between dynamic and static quenching, we find that the dynamic quenching rate constants of the different bases show a consistent trend across both buffers, and this is in line with a charge-transfer mechanism. Time-resolved measurements also provide insight into static quenching, revealing formation of 2AP-NMP ground-state complexes in which 2AP displays a very short fluorescence lifetime, comparable to that seen in oligonucleotides. In these complexes, the dependence of the rate of quenching on the partner base also supports a charge-transfer mechanism.
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Affiliation(s)
- Kyle A Paterson
- EaStCHEM School of Chemistry, University of Edinburgh, Joseph Black Building, David Brewster Road, Edinburgh EH9 3FJ, United Kingdom
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10
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Roche J, Royer CA, Roumestand C. Exploring Protein Conformational Landscapes Using High-Pressure NMR. Methods Enzymol 2019; 614:293-320. [DOI: 10.1016/bs.mie.2018.07.006] [Citation(s) in RCA: 17] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
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11
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Harraz DM, Davis JT. A self-assembled peroxidase from 5'-GMP and heme. Chem Commun (Camb) 2018; 54:1587-1590. [PMID: 29368765 DOI: 10.1039/c7cc09900b] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
Guanosine 5'-monophosphate (5'-GMP) and Fe(iii)-heme form a supramolecular catalyst with peroxidase activity. Catalysis, which depends on self-assembly of 5'-GMP into a G-quadruplex that binds hemin, can be modulated by nucleotide concentration, temperature and the identity of the nucleotide's sugar.
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Affiliation(s)
- Deiaa M Harraz
- Department of Chemistry & Biochemistry, University of Maryland College Park, MD 20742, USA.
| | - Jeffery T Davis
- Department of Chemistry & Biochemistry, University of Maryland College Park, MD 20742, USA.
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