1
|
Damjanovic J, Miao J, Huang H, Lin YS. Elucidating Solution Structures of Cyclic Peptides Using Molecular Dynamics Simulations. Chem Rev 2021; 121:2292-2324. [PMID: 33426882 DOI: 10.1021/acs.chemrev.0c01087] [Citation(s) in RCA: 32] [Impact Index Per Article: 10.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
Protein-protein interactions are vital to biological processes, but the shape and size of their interfaces make them hard to target using small molecules. Cyclic peptides have shown promise as protein-protein interaction modulators, as they can bind protein surfaces with high affinity and specificity. Dozens of cyclic peptides are already FDA approved, and many more are in various stages of development as immunosuppressants, antibiotics, antivirals, or anticancer drugs. However, most cyclic peptide drugs so far have been natural products or derivatives thereof, with de novo design having proven challenging. A key obstacle is structural characterization: cyclic peptides frequently adopt multiple conformations in solution, which are difficult to resolve using techniques like NMR spectroscopy. The lack of solution structural information prevents a thorough understanding of cyclic peptides' sequence-structure-function relationship. Here we review recent development and application of molecular dynamics simulations with enhanced sampling to studying the solution structures of cyclic peptides. We describe novel computational methods capable of sampling cyclic peptides' conformational space and provide examples of computational studies that relate peptides' sequence and structure to biological activity. We demonstrate that molecular dynamics simulations have grown from an explanatory technique to a full-fledged tool for systematic studies at the forefront of cyclic peptide therapeutic design.
Collapse
Affiliation(s)
- Jovan Damjanovic
- Department of Chemistry, Tufts University, Medford, Massachusetts 02155, United States
| | - Jiayuan Miao
- Department of Chemistry, Tufts University, Medford, Massachusetts 02155, United States
| | - He Huang
- Department of Chemistry, Tufts University, Medford, Massachusetts 02155, United States
| | - Yu-Shan Lin
- Department of Chemistry, Tufts University, Medford, Massachusetts 02155, United States
| |
Collapse
|
2
|
Slough DP, McHugh SM, Lin YS. Understanding and designing head-to-tail cyclic peptides. Biopolymers 2018; 109:e23113. [PMID: 29528114 PMCID: PMC6135719 DOI: 10.1002/bip.23113] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2017] [Revised: 02/23/2018] [Accepted: 02/26/2018] [Indexed: 01/30/2023]
Abstract
Cyclic peptides (CPs) are an exciting class of molecules with a variety of applications. However, design strategies for CP therapeutics, for example, are generally limited by a poor understanding of their sequence-structure relationships. This knowledge gap often leads to a trial-and-error approach for designing CPs for a specific purpose, which is both costly and time-consuming. Herein, we describe the current experimental and computational efforts in understanding and designing head-to-tail CPs along with their respective challenges. In addition, we provide several future directions in the field of computational CP design to improve its accuracy, efficiency and applicability. These advances, combined with experimental techniques, shall ultimately provide a better understanding of these interesting molecules and a reliable working platform to rationally design CPs with desired characteristics.
Collapse
Affiliation(s)
| | | | - Yu-Shan Lin
- Department of Chemistry, Tufts University, Medford, Massachusetts, 02155, United States
| |
Collapse
|
3
|
Toniolo C, Crisma M, Formaggio F, Alemán C, Ramakrishnan C, Kalmankar N, Balaram P. Intramolecular backbone···backbone hydrogen bonds in polypeptide conformations. The other way around: ɛ-turn. Biopolymers 2017; 108. [DOI: 10.1002/bip.22911] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/24/2016] [Revised: 06/17/2016] [Accepted: 06/29/2016] [Indexed: 01/27/2023]
Affiliation(s)
- Claudio Toniolo
- Department of Chemistry; University of Padova; Padova 35131 Italy
- Institute of Biomolecular Chemistry, Padova Unit, CNR; Padova 35131 Italy
| | - Marco Crisma
- Institute of Biomolecular Chemistry, Padova Unit, CNR; Padova 35131 Italy
| | - Fernando Formaggio
- Department of Chemistry; University of Padova; Padova 35131 Italy
- Institute of Biomolecular Chemistry, Padova Unit, CNR; Padova 35131 Italy
| | - Carlos Alemán
- Departament d'Enginyeria Quimica; ETSEIB, Universitat Politècnica de Catalunya; Barcelona 08028 Spain
| | | | - Neha Kalmankar
- National Centre for Biological Sciences (TIFR); GKVK Campus Bangalore 560065 India
| | - Padmanabhan Balaram
- Molecular Biophysics Unit; Indian Institute of Science; Bangalore 560012 India
| |
Collapse
|
4
|
Arnhold FS, Linden A, Heimgartner H. Synthesis of Aib- and Phe(2Me)-Containing Cyclopentapeptides. Helv Chim Acta 2015. [DOI: 10.1002/hlca.201400323] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
|
5
|
Culf AS, Čuperlović-Culf M, Léger DA, Decken A. Small head-to-tail macrocyclic α-peptoids. Org Lett 2014; 16:2780-3. [PMID: 24797336 DOI: 10.1021/ol501102b] [Citation(s) in RCA: 38] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
A convenient and efficient methodology for the head-to-tail macrocyclization of small 3-mer, 4-mer, and 5-mer α-peptoid acids (9-, 12-, and 15-atom N-substituted glycine oligomers) is described. The cyclic trimer has a ccc amide sequence in the crystal structure, whereas the tetramer has ctct and the pentamer has ttccc stereochemistry. NMR analysis reveals rigid structures in solution. These synthetic macrocycles may prove useful in medicinal and materials applications.
Collapse
Affiliation(s)
- Adrian S Culf
- Atlantic Cancer Research Institute , 35 Providence Street, Moncton, NB E1C 8X3, Canada
| | | | | | | |
Collapse
|
6
|
Shi S, Yan L, Yang Y, Fisher-Shaulsky J, Thacher T. An extensible and systematic force field, ESFF, for molecular modeling of organic, inorganic, and organometallic systems. J Comput Chem 2003; 24:1059-76. [PMID: 12759906 DOI: 10.1002/jcc.10171] [Citation(s) in RCA: 63] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
ESFF is a rule-based force field designed for modeling organic, inorganic, and organometallic systems. To cover this broad range of molecular systems, ESFF was developed in an extensible and systematic manner. Several unique features were introduced including pseudoangle and a dot product function representing torsion energy terms. The partial atomic charges that are topology-dependent are determined from ab initio (DFT) calculated electronegativity and hardness for valence orbitals. The van der Waals parameters are charge-dependent, and correlated with the ionization potential for atoms in various valence states. To obtain a set of well-defined and physically meaningful parameters, ESFF employs semiempirical rules to translate atomic-based parameters to parameters typically associated with a covalent valence force field. The atomic parameters depend not only on atom type, but also on internal type, thus resulting in a more accurate force field. This article presents the theory and the method used to develop the force field. The force field has been applied to molecular simulations of a wide variety of systems including nucleic acids, peptides, hydrocarbons, porphyrins, transition metal complexes, zeolites, and organometallic compounds. Agreement with the experimental results indicates that ESFF is a valuable tool in molecular simulations for understanding and predicting both crystal and gas phase molecular structures.
Collapse
Affiliation(s)
- Shenghua Shi
- Pfizer Global Research & Development, La Jolla Laboratories/Agouron Pharmaceuticals Inc, 3550 General Atomics Court, San Diego, California 92121, USA
| | | | | | | | | |
Collapse
|
7
|
Nikiforovich GV, Kövér KE, Zhang WJ, Marshall GR. Cyclopentapeptides as Flexible Conformational Templates. J Am Chem Soc 2000. [DOI: 10.1021/ja991728m] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Gregory V. Nikiforovich
- Contribution from the Center for Molecular Design and Department of Molecular Biology and Pharmacology, Washington University, St. Louis, Missouri 63130, and L. Kossuth University, H-4010 Debrecen, Hungary
| | - Katalin E. Kövér
- Contribution from the Center for Molecular Design and Department of Molecular Biology and Pharmacology, Washington University, St. Louis, Missouri 63130, and L. Kossuth University, H-4010 Debrecen, Hungary
| | - Wei-Jun Zhang
- Contribution from the Center for Molecular Design and Department of Molecular Biology and Pharmacology, Washington University, St. Louis, Missouri 63130, and L. Kossuth University, H-4010 Debrecen, Hungary
| | - Garland R. Marshall
- Contribution from the Center for Molecular Design and Department of Molecular Biology and Pharmacology, Washington University, St. Louis, Missouri 63130, and L. Kossuth University, H-4010 Debrecen, Hungary
| |
Collapse
|
8
|
Ito H. Linear response polarizability bandshape calculations of vibrational circular dichroism, vibrational absorption, and electronic circular dichroism of cyclo(Gly-Pro-Gly-D-Ala-Pro): A small cyclic pentapeptide having β- and γ-turns. ACTA ACUST UNITED AC 1998. [DOI: 10.1002/(sici)1520-6343(1996)2:1<17::aid-bspy3>3.0.co;2-p] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
|
9
|
Zanotti G, Saviano M, Saviano G, Tancredi T, Rossi F, Pedone C, Benedetti E. Structure of cyclic peptides: the crystal and solution conformation of cyclo(Phe-Phe-Aib-Leu-Pro). THE JOURNAL OF PEPTIDE RESEARCH : OFFICIAL JOURNAL OF THE AMERICAN PEPTIDE SOCIETY 1998; 51:460-6. [PMID: 9650721 DOI: 10.1111/j.1399-3011.1998.tb00645.x] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
A solid-state and solution conformation analyses of the cyclopentapeptide cyclo(Phe-Phe-Aib-Leu-Pro) has been carried out by X-ray diffraction and nuclear magnetic resonance techniques. The structure of the hexagonal crystals, grown from a methanol solution [a = b = 16.530(4) A, c = 21.356(9) A, space group P6(5), Z = 6], shows the presence of one intramolecular N-H ..O=C hydrogen bond with the formation of a gamma-turn (C7). The Aib3 residue, at the center of the gamma-turn, presents unexpected values of the torsion angles [phi = 70.5 degrees and psi = -73.8 degrees], which have been observed only once before for this helicogenic residue. A cis peptide bond occurs between Leu4 and Pro5; all other peptide bonds are trans. The overall conformation for the cyclopentapeptide with one cis-peptide bond on one side and an intramolecular gamma-turn on the opposite side results in an equatorial topology of the side-chains of the Phe1, Phe2 and Leu4 residues. Indeed, the Calpha-Cbeta and Cbeta-Cgamma bonds of these residues lie approximately in the mean plane of the cyclic ring system. The structure is compared with data in the literature on cyclic pentapeptides. In addition the Pro-Phe-Phe moiety shows a conformation similar to that observed in other larger cyclic bioactive peptides, which indicates a reduced number of conformations for this sequence. The solution study was carried out in three different solvent systems: chloroform, acetonitrile and methanol in the temperature interval 220-300 K. In all three solvents the room temperature spectra show that the peptide is conformationally nonhomogeneous. In acetonitrile at low temperatures it is possible to reduce the conformational equilibrium to two predominant conformers which differ for the cis-trans isomerism of the Leu4-Pro5 peptide bond.
Collapse
Affiliation(s)
- G Zanotti
- Centro di Studio sulla Chimica del Farmaco del C.N.R, Università di Roma La Sapienza, Italy
| | | | | | | | | | | | | |
Collapse
|
10
|
Nagarajaram HA, Ramakrishnan C. Stereochemical studies on cyclic peptides: Detailed energy minimization studies on hydrogen bonded all-trans cyclic pentapeptide backbones. J Biosci 1995. [DOI: 10.1007/bf02703300] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
|
11
|
|
12
|
Pavone V, Lombardi A, Saviano M, Di Blasio B, Nastri F, Fattorusso R, Maglio O, Isernia C. Beta-alanine containing cyclic peptides with predetermined turned structure. V. Biopolymers 1994; 34:1505-15. [PMID: 7827262 DOI: 10.1002/bip.360341108] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
In the present paper we describe the synthesis, purification, single crystal x-ray analysis, and solution structural characterization by nmr spectroscopy, combined with restrained molecular dynamic simulations, of the cyclic hexapeptide cyclo-(Pro-Phe-beta-Ala-Phe-Phe-beta-Ala). The peptide was synthesized by classical solution methods and the cyclization of the free hexapeptide was accomplished in good yields in diluted methylenechloride solution using N,N-dicyclohexyl-carbodiimide. The compound crystallizes in the monoclinic space group P2(1) from methanol/ethyl acetate. The molecule adopts in the solid state a conformation characterized by cis beta-Ala6-Pro1 peptide bond. The alpha-amino acid residues are at the corner positions of turned structures. The Pro1-Phe2 segment is incorporated in a pseudo type I beta-turn, while Phe4-Phe5 is in a typical type I beta-turn. Assignment of all 1H and 13C resonances was achieved by homo- and heteronuclear two-dimensional techniques in dimethylsulfoxide (DMSO) solutions. The conformational analysis was based on interproton distances derived from rotating frame nuclear Overhauser effect spectroscopy spectra and homonuclear coupling constants. Restrained molecular dynamic simulation in vacuo was also performed to built refined molecular models. The molecule is present in DMSO solution as two slowly interconverting conformers, characterized by a cis-trans isomerism around the beta-Ala6-Pro1 peptide bond. This work confirms our expectations on the low propensity of beta-alanyl residues to be positioned at the corners of turned structure.
Collapse
Affiliation(s)
- V Pavone
- Centro Interdipartimentale di Ricerca su Peptidi Bioattivi and CEINGE-Biotecnologie Avanzate, University of Naples, Federico II, Italy
| | | | | | | | | | | | | | | |
Collapse
|
13
|
Liu ZP, Gierasch LM. Combined use of molecular dynamics simulations and NMR to explore peptide bond isomerization and multiple intramolecular hydrogen-bonding possibilities in a cyclic pentapeptide, cyclo(Gly-Pro-D-Phe-Gly-Val). Biopolymers 1992; 32:1727-39. [PMID: 1472655 DOI: 10.1002/bip.360321214] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
The conformational behavior of a model cyclic pentapeptide--cyclo(Gly-L-Pro-D-Phe-Gly-L-Val)--has been explored through the combined use of in vacuo molecular dynamics simulations and a range of nmr experiments (preceding paper). The molecular dynamics analysis suggests that, despite the conformational constraints imposed by formation of the pentapeptide cycle, this pentapeptide undergoes conformational transitions between various hydrogen-bonded conformations, characterized by low energy barriers. An inverse gamma turn with Pro in position i + 1 and a gamma turn with D-Phe in position i + 1 are two alternatives occurring frequently. Like other DLDDL cyclic pentapeptides, cyclo(Gly-Pro-D-Phe-Gly-Val) is also stabilized by an inverse gamma-turn structure with the beta-branched Val residue in position i + 1, and this hydrogen bond is retained in the different conformational families. The gamma-turn around D-Phe3 and the inverse gamma turn around Val5 are consistent with the nmr observations. 3JNH-CH alpha coupling constants of the all-trans forms were calculated from one of the molecular dynamics trajectories and are comparable to nmr experimental data, suggesting that the conformational states visited during the simulation are representative of the conformational distribution in solution. In addition to the equilibrium among various hydrogen-bonded all-trans conformers, the observation in nmr spectra of two sets of resonances for all peptide protons indicated a slow conformational interconversion of the Gly-Pro peptide bond between trans and cis isomers. The activation energy between these two conformers was determined experimentally by magnetization transfer and was calculated by high temperature constrained molecular dynamics simulation. Both methods yield a free energy of activation of ca. 20 kcal/mol. Furthermore, the free energy of activation is dependent on the direction of rotation of the Gly-Pro peptide bond.
Collapse
Affiliation(s)
- Z P Liu
- Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas 75235-9041
| | | |
Collapse
|
14
|
Bruch MD, Rizo J, Gierasch LM. Impact of a micellar environment on the conformations of two cyclic pentapeptides. Biopolymers 1992; 32:1741-54. [PMID: 1472656 DOI: 10.1002/bip.360321215] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
In an effort to explore the influence of interfacial environments on reverse turns, we have performed a detailed analysis by nmr of the solution conformations of two cyclic pentapeptides in sodium dodecyl sulfate (SDS) micelles. The first peptide, cyclo (D-Phe1-Pro2-Gly3-D-Ala4-Pro5), adopts a single rigid conformation in solution (either chloroform or dimethylsulfoxide) and in crystals, whereas the second, cyclo (Gly1-Pro2-D-Phe3-Gly4-Val5), is much more flexible and adopts different conformations in the crystal and in solution. Both of these peptides are solubilized by SDS micelles, and nmr relaxation rates indicate that they are both partially immobilized by interaction with the micelles. Furthermore, some amide protons in both peptides participate in hydrogen bonds with water. In the presence of micelles, the former peptide retains a conformation essentially the same as that found in crystals and in solution, which consists of a beta turn and an inverse gamma turn. However, the micellar environment has a significant effect on the latter peptide. In particular, the population of a conformer containing a cis Gly-Pro peptide bond is increased significantly. The most likely conformation of the cis isomer, determined by a combination of nmr and restrained molecular dynamics, contains a Gly1-Pro2 delta turn and a gamma turn about D-Phe3. The nmr data on the trans isomer indicate that this isomer is averaging between two conformations that differ mainly in the orientation of the D-Phe3-Gly4 peptide bond.
Collapse
Affiliation(s)
- M D Bruch
- Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas 75235-9041
| | | | | |
Collapse
|
15
|
Stroup AN, Rockwell AL, Gierasch LM. Solution conformations of two flexible cyclic pentapeptides: cyclo(Gly-Pro-D-Phe-Gly-Ala) and cyclo(Gly-Pro-D-Phe-Gly-Val). Biopolymers 1992; 32:1713-25. [PMID: 1472654 DOI: 10.1002/bip.360321213] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
In an effort to explore the residue preferences in three-residue reverse turns (so-called gamma-turns), two cyclic pentapeptides--cyclo(Gly1-Pro2-D-Phe3-Gly4-Ala5) (I) and cyclo(Gly1-Pro2-D-Phe3-Gly4-Val5) (II)--have been synthesized and analyzed by nmr. It was anticipated that the Gly-Pro-D-Phe-Gly portions of these molecules would favor a beta-turn conformation, leaving the remainder of the molecule to adopt a gamma turn, as seen in several previously studied model cyclic pentapeptides. The nmr data for both peptides in CDCl3 (5% DMSO-d6) and in neat DMSO-d6 indicate that the most populated conformation contains a distorted beta turn around Pro2-D-Phe3, which includes a gamma turn around D-Phe3. The distortion in the beta turn does not impede the formation of an inverse gamma turn around residue 5, and indeed, this conformation is observed in both peptides. Both the alanine and the bulkier valine residues are therefore found to be compatible with an inverse gamma turn. Molecular dynamics simulations on the title peptides are reported in the following paper. These simulations indicate that there is conformational flexibility around the D-Phe3-Gly4 peptide bond, which enables the formation of the gamma turn around D-Phe3. The third paper in this series explores the impact of a micellar environment on conformational equilibria in II.
Collapse
Affiliation(s)
- A N Stroup
- Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas 75235-9041
| | | | | |
Collapse
|
16
|
Stradley SJ, Rizo J, Bruch MD, Stroup AN, Gierasch LM. Cyclic pentapeptides as models for reverse turns: determination of the equilibrium distribution between type I and type II conformations of Pro-Asn and Pro-Ala beta-turns. Biopolymers 1990; 29:263-87. [PMID: 2328290 DOI: 10.1002/bip.360290130] [Citation(s) in RCA: 83] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
Cyclic pentapeptides are excellent models for reverse turns and have been used extensively in our laboratory to explore the influence of different amino acid sequences on turn preference. This paper is divided into two parts: In the first, we review our previous studies of cyclic pentapeptides. We summarize work that demonstrates the range of conformations possible within the cyclic pentapeptide backbone, the importance of sequence chirality in determining the backbone fold, and the utility of these cyclic pentapeptides as models for various turns. In the second, we present new results on two cyclic pentapeptides that contain beta-turns with Pro-Ala or Pro-Asn sequences in the i + 1 and i + 2 positions. By stereochemical criteria, a type I beta-turn is expected to be preferred by such L-L sequences. On the other hand, in proteins Asn occurs frequently in the i + 2 position of type II turns. We asked whether the same propensity would be manifest in an isolated model peptide, and if so, what the interactions were that influenced the relative stability of the type I and type II turns. To address these questions we have compared the conformational behavior of two peptides: cyclo(Gly-Pro-Ala-D-Phe-Pro) and cyclo(D-Ala-Pro-Asn-Gly-Pro). From previous studies, we anticipated that both peptides would contain an inverse gamma-turn and a beta-turn which consisted of either Gly-Pro-Ala-D-Phe or D-Ala-Pro-Asn-Gly in positions i to i + 3, respectively. Nuclear magnetic resonance analysis confirms this overall backbone conformation. Furthermore, quantitative nuclear Overhauser effect measurements in combination with molecular dynamics simulations and torsionally-forced energy minimizations have enabled us to determine that both type I and type II beta-turns are present in equilibrium in these peptides. The introduction of Asn in position i + 2 shifts this equilibrium significantly towards type II. We have done preliminary assessment of the possible side-chain/backbone conformations that contribute to the shift in populations.
Collapse
Affiliation(s)
- S J Stradley
- Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas 75235-9041
| | | | | | | | | |
Collapse
|