1
|
Lee HJ, Liu SW, Sulyok-Eiler M, Harmat V, Farkas V, Bánóczi Z, El Khabchi M, Shawn Fan HJ, Hirao K, Song JW. Neighbor effect on conformational spaces of alanine residue in azapeptides. Heliyon 2024; 10:e33159. [PMID: 39021983 PMCID: PMC11253059 DOI: 10.1016/j.heliyon.2024.e33159] [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: 04/25/2024] [Revised: 06/10/2024] [Accepted: 06/14/2024] [Indexed: 07/20/2024] Open
Abstract
The conformational properties of Alanine (Ala) residue have been investigated to understand protein folding and develop force fields. In this work, we examined the neighbor effect on the conformational spaces of Ala residue using model azapeptides, Ac-Ala-azaGly-NHMe (3, AaG), and Ac-azaGly-Ala-NHMe (4, aGA1). Ramachandran energy maps were generated by scanning (φ, ψ) dihedral angles of the Ala residues in models with the fixed dihedral angles (φ = ±90°, ψ = ±0° or ±180°) of azaGly residue using LCgau-BOP and LCgau-BOP + LRD functionals in the gas and water phases. The integral-equation-formalism polarizable continuum model (IEF-PCM) and a solvation model density (SMD) were employed to mimic the solvation effect. The most favorable conformation of Ala residue in azapeptide models is found as the polyproline II (βP), inverse γ-turn (γ'), β-sheet (βS), right-handed helix (αR), or left-handed helix (αL) depending on the conformation of neighbor azaGly residue in isolated form. Solvation methods exhibit that the Ala residue favors the βP, δR, and αR conformations regardless of its position in azapeptides 3 and 4 in water. Azapeptide 5, Ac-azaGly-Ala-NH2 (aGA2), was synthesized to evaluate the theoretical results. The X-ray structure showed that azaGly residue adopts the polyproline II (βP) and Ala residue adopts the right-handed helical (αR) structure in aGA2. The conformational preferences of aGA2 and the dimer structure of aGA2 based on the X-ray structure were examined to assess the performance of DFT functionals. In addition, the local minima of azapeptide 6, Ac-Phe-azaGly-NH2 (FaG), were compared with the previous experimental results. SMD/LCgau-BOP + LRD methods agreed well with the reported experimental results. The results suggest the importance of weak dispersion interactions, neighbor effect, and solvent influence in the conformational preferences of Ala residue in model azapeptides.
Collapse
Affiliation(s)
- Ho-Jin Lee
- Division of Natural and Mathematics Sciences, LeMoyne-Own College, Memphis, TN, 38126, USA
- Department of Natural Sciences, Southwest Tennessee Community College, Memphis, TN, 38015, USA
| | - Shi-Wei Liu
- College of Chemical Engineering, Sichuan University of Science and Engineering, Zigong City, Sichuan Province, 64300, PR China
| | - Máté Sulyok-Eiler
- Laboratory of Structural Biology and Chemistry, Institute of Chemistry, Eötvös Loránd University, Budapest, Hungary
- Hevesy György PhD School of Chemistry, Eötvös Loránd University, Budapest, Hungary
| | - Veronika Harmat
- Laboratory of Structural Biology and Chemistry, Institute of Chemistry, Eötvös Loránd University, Budapest, Hungary
- HUN-REN - ELTE Protein Modeling Research Group, Budapest, Hungary
| | - Viktor Farkas
- Laboratory of Structural Biology and Chemistry, Institute of Chemistry, Eötvös Loránd University, Budapest, Hungary
- HUN-REN - ELTE Protein Modeling Research Group, Budapest, Hungary
| | - Zoltán Bánóczi
- Department of Organic Chemistry, Institute of Chemistry, ELTE Eötvös Loránd University, 1117, Budapest, Hungary
- HUN-REN-ELTE Research Group of Peptide Chemistry, 1117, Budapest, Hungary
| | - Mouna El Khabchi
- LIMAS, Faculty of Sciences Dhar El Mahraz, University Sidi Mohamed Ben Abdallah, Fez, Morocco
| | - Hua-Jun Shawn Fan
- College of Chemical Engineering, Sichuan University of Science and Engineering, Zigong City, Sichuan Province, 64300, PR China
| | - Kimihiko Hirao
- Fukui Institute for Fundamental Chemistry, Kyoto University, Takano, Nishihiraki-cho 34-4, Sakyo-ku, Kyoto, 606-8103, Japan
| | - Jong-Won Song
- Department of Chemistry Education, Daegu University, Daegudae-ro 201, Gyeongsan-si, Gyeongsangbuk-do, 38453, Republic of Korea
| |
Collapse
|
2
|
El Khabchi M, Mcharfi M, Benzakour M, Fitri A, Benjelloun AT, Song JW, Lee KB, Lee HJ. Computational Investigation of Conformational Properties of Short Azapeptides: Insights from DFT Study and NBO Analysis. Molecules 2023; 28:5454. [PMID: 37513326 PMCID: PMC10386235 DOI: 10.3390/molecules28145454] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/09/2023] [Revised: 07/05/2023] [Accepted: 07/12/2023] [Indexed: 07/30/2023] Open
Abstract
Azapeptides have gained much attention due to their ability to enhance the stability and bioavailability of peptide drugs. Their structural preferences, essential to understanding their function and potential application in the peptide drug design, remain largely unknown. In this work, we systematically investigated the conformational preferences of three azaamino acid residues in tripeptide models, Ac-azaXaa-Pro-NHMe [Xaa = Asn (4), Asp (5), Ala (6)], using the popular DFT functionals, B3LYP and B3LYP-D3. A solvation model density (SMD) was used to mimic the solvation effect on the conformational behaviors of azapeptides in water. During the calculation, we considered the impact of the amide bond in the azapeptide models on the conformational preferences of models 4-6. We analyzed the effect of the HB between the side-chain main chain and main-chain main-chain on the conformational behaviors of azapeptides 4-6. We found that the predicted lowest energy conformation for the three models differs depending on the calculation methods. In the gas phase, B3LYP functional indicates that the conformers tttANP-1 and tttADP-1 of azapeptides 4 and 5 correspond to the type I of β-turn, the lowest energy conformation with all-trans amide bonds. Considering the dispersion correction, B3LYP-D3 functional predicts the conformers tctANP-2 and tctADP-3 of azapeptide 4 and 5, which contain the cis amide bond preceding the Pro residue, as the lowest energy conformation in the gas phase. The results imply that azaAsx and Pro residues may involve cis-trans isomerization in the gas phase. In water, the predicted lowest energy conformer of azapeptides 4 and 5 differs from the gas phase results and depends on the calculational method. For azapeptide 6, regardless of calculation methods and phases, tttAAP-1 (β-I turn) is predicted as the lowest energy conformer. The results imply that the effect of the side chain that can form HBs on the conformational preferences of azapeptides 4 and 5 may not be negligible. We compared the theoretical results of azaXaa-Pro models with those of Pro-azaXaa models, showing that incorporating azaamino acid residue in peptides at different positions can significantly impact the folding patterns and stability of azapeptides.
Collapse
Affiliation(s)
- Mouna El Khabchi
- LIMAS, Department of Chemistry, Faculty of Sciences Dhar El Mahraz, Sidi Mohamed Ben Abdallah University, Fez 30000, Morocco
| | - Mohammed Mcharfi
- LIMAS, Department of Chemistry, Faculty of Sciences Dhar El Mahraz, Sidi Mohamed Ben Abdallah University, Fez 30000, Morocco
| | - Mohammed Benzakour
- LIMAS, Department of Chemistry, Faculty of Sciences Dhar El Mahraz, Sidi Mohamed Ben Abdallah University, Fez 30000, Morocco
| | - Asmae Fitri
- LIMAS, Department of Chemistry, Faculty of Sciences Dhar El Mahraz, Sidi Mohamed Ben Abdallah University, Fez 30000, Morocco
| | - Adil Touimi Benjelloun
- LIMAS, Department of Chemistry, Faculty of Sciences Dhar El Mahraz, Sidi Mohamed Ben Abdallah University, Fez 30000, Morocco
| | - Jong-Won Song
- Department of Chemistry Education, Daegu University, Daegudae-ro 201, Gyeongsan-si 38453, Republic of Korea
| | - Kang-Bong Lee
- Climate and Environmental Research Institute, Korea Institute of Science & Technology, Hwarang-ro 14-gil 5, Seoul 02792, Republic of Korea
| | - Ho-Jin Lee
- Department of Natural Sciences, Southwest Tennessee Community College, Memphis, TN 38134, USA
| |
Collapse
|
3
|
Conformational preferences of Ac-Pro-azaXaa-NHMe (Xaa = Asn, Asp, Ala) and the effect of intramolecular hydrogen bonds on their stability in gas phase and solution. J Mol Model 2021; 27:368. [PMID: 34859310 DOI: 10.1007/s00894-021-04992-x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/14/2021] [Accepted: 11/23/2021] [Indexed: 10/19/2022]
Abstract
The conformational preferences of three azadipeptides Ac-Pro-azaXaa-NHMe [Xaa = Asn (1), Asp (2), Ala (3)] have been carried out in gas phase and solution (water) using the density functional method B3LYP/6-311 + + G(d,p) to explore the effect of the change of side chain of azaamino acids at the i + 2 position on the stability of these components. The most stable conformations of compounds (1), (2), and (3) have an amid bond oriented trans, trans, and cis, respectively, in gas phase, whereas the orientation of amid bond in water solvent of compounds (2) and (3) has changed to cis and trans, respectively. We have also noticed the importance of backbone-side chain hydrogen bonds in the stabilization of the β turn motif in gas phase since this motif is more stable in the case of compounds (1) and (2) and less stable in the case of compound (3) in which these hydrogen bonds are absent. Furthermore, the βII(βII') turn structure is more stable than βI turn for all conformations of the three compounds in gas phase, while it is not true in the case of some conformations in solution. Moreover, the stability of β turn increases from azaAsn to azaAsp which could be due to the side chain's basic nature of azaAsn. In general, hydrogen bonds were found to play a key role in the stabilization of these compounds since most of conformers are lower in energy when they have more than two hydrogen bond interactions while conformations with one or no hydrogen bonds are higher in energy and thus less stable.
Collapse
|
4
|
|
5
|
Harris T, Chenoweth DM. Sterics and Stereoelectronics in Aza-Glycine: Impact of Aza-Glycine Preorganization in Triple Helical Collagen. J Am Chem Soc 2019; 141:18021-18029. [DOI: 10.1021/jacs.9b05524] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Affiliation(s)
- Trevor Harris
- Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104-6323, United States
| | - David M. Chenoweth
- Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104-6323, United States
| |
Collapse
|
6
|
Jędrzejewska H, Szumna A. Peptide-based capsules with chirality-controlled functionalized interiors - rational design and amplification from dynamic combinatorial libraries. Chem Sci 2019; 10:4412-4421. [PMID: 31057768 PMCID: PMC6482442 DOI: 10.1039/c8sc05455j] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2018] [Accepted: 03/13/2019] [Indexed: 01/07/2023] Open
Abstract
Self-assembled capsules were designed by two approaches and synthesized using azapeptides of various chiralities attached to resorcin[4]arene by semicarbazone linkers.
Peptides are commonly perceived as inapplicable components for construction of porous structures. Due to their flexibility the design is difficult and shape persistence of such putative structures is diminished. Notwithstanding these limitations, the advantages of peptides as building blocks are numerous: they are functional and functionalizable, widely available, diverse and biocompatible. We aimed at the construction of discrete porous structures that exploit the inherent functionality of peptides by an approach that is inspired by nature: structural pockets are defined by the backbones of peptides while functionality is introduced by their side chains. In this work peptide ribbons were preorganized on a macrocyclic scaffold using azapeptide–aldehyde reactions. The resulting cavitands with semicarbazone linkers arrange the peptide backbones at positions that are suitable for self-assembly of dimeric capsules by formation of binding motifs that resemble eight-stranded β-barrels. Self-assembly properties and inside/outside positions of the side chains depend crucially on the chirality of peptides. By rational optimization of successive generations of capsules we have found that azapeptides containing three amino acids in a (l, d, d) sequence give well-defined dimeric capsules with side chains inside their cavities. Taking advantage of the reversibility of the reaction of semicarbazone formation we have also employed the dynamic covalent chemistry (DCC) for a combinatorial discovery of capsules that could not be rationally designed. Indeed, the results show that stable capsules with side chains positioned internally can be obtained even for shorter sequences but only for combination peptides of (l, l) and (d, l) chirality. The hybrid (l, l)(d, l) capsule is amplified directly from a reaction mixture containing two different peptides. All capsules gain substantial ordering upon self-assembly, which is manifested by a two orders of magnitude increase of the intensity of CD spectra of capsules compared with non-assembled analogs. Temperature-dependent CD measurements indicate that the capsules remain stable over the entire temperature range tested (20–100 °C). Circular dichroism coupled with TD DFT calculations, DOSY measurements and X-ray crystallography allow for elucidation of the structures in the solid state and in solution and guide their iterative evolution for the current goals.
Collapse
Affiliation(s)
- Hanna Jędrzejewska
- Institute of Organic Chemistry , Polish Academy of Sciences , Kasprzaka 44/52 , 01-224 Warsaw , Poland .
| | - Agnieszka Szumna
- Institute of Organic Chemistry , Polish Academy of Sciences , Kasprzaka 44/52 , 01-224 Warsaw , Poland .
| |
Collapse
|
7
|
Comparisons between Crystallography Data and Theoretical Parameters and the Formation of Intramolecular Hydrogen Bonds: Benznidazole. CRYSTALS 2016. [DOI: 10.3390/cryst6050056] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
|
8
|
Yan XS, Wu K, Yuan Y, Zhan Y, Wang JH, Li Z, Jiang YB. β-Turn structure in glycinylphenylalanine dipeptide based N-amidothioureas. Chem Commun (Camb) 2014; 49:8943-5. [PMID: 23964363 DOI: 10.1039/c3cc44336a] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
Transforming the C-terminal amide of a glycinylphenylalanine dipeptide into N-amidothiourea affords a β-turn structure in the formed dipeptide based N-amidothioureas, which can be readily identified by an induced CD signal from the achiral phenylthiourea chromophore.
Collapse
Affiliation(s)
- Xiao-Sheng Yan
- Department of Chemistry, College of Chemistry and Chemical Engineering, MOE Key Laboratory of Analytical Sciences, and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Xiamen University, Xiamen 361005, China.
| | | | | | | | | | | | | |
Collapse
|
9
|
Avan I, Hall CD, Katritzky AR. Peptidomimetics via modifications of amino acids and peptide bonds. Chem Soc Rev 2014; 43:3575-94. [DOI: 10.1039/c3cs60384a] [Citation(s) in RCA: 183] [Impact Index Per Article: 18.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
|
10
|
Abo-Dya NE, Biswas S, Basak A, Avan I, Alamry KA, Katritzky AR. Benzotriazole-Mediated Synthesis of Aza-peptides: En Route to an Aza-Leuenkephalin Analogue. J Org Chem 2013; 78:3541-52. [DOI: 10.1021/jo302251e] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Affiliation(s)
- Nader E. Abo-Dya
- Center for
Heterocyclic Compounds,
Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, United States
- Department of Pharmaceutical Organic
Chemistry, Faculty of Pharmacy, Zagazig University, Zagazig 44519, Egypt
| | - Suvendu Biswas
- Center for
Heterocyclic Compounds,
Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, United States
| | - Akash Basak
- Center for
Heterocyclic Compounds,
Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, United States
| | - Ilker Avan
- Center for
Heterocyclic Compounds,
Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, United States
- Department of Chemistry, Anadolu University, 26470, Eskişehir, Turkey
| | - Khalid A. Alamry
- Department of Chemistry, King Abdulaziz University, Jeddah 21589, Saudi Arabia
| | - Alan R. Katritzky
- Center for
Heterocyclic Compounds,
Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, United States
- Department of Chemistry, King Abdulaziz University, Jeddah 21589, Saudi Arabia
| |
Collapse
|
11
|
Hooman Vahidi S, Morsali A, Ali Beyramabadi S. Quantum mechanical study on the mechanism and kinetics of the hydrolysis of organopalladium Complex [Pd(CNN)P(OMe)3]+ in low acidity range. COMPUT THEOR CHEM 2012. [DOI: 10.1016/j.comptc.2012.06.010] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/28/2022]
|
12
|
Ottersbach PA, Schnakenburg G, Gütschow M. Induction of chirality: experimental evidence of atropisomerism in azapeptides. Chem Commun (Camb) 2012; 48:5772-4. [PMID: 22552375 DOI: 10.1039/c2cc31161e] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/15/2023]
Abstract
Methylation of the peptide bond in model azadipeptides leads to the E configuration and hence to atropisomerism due to a restricted rotation around the N-N axis.
Collapse
Affiliation(s)
- Philipp A Ottersbach
- Pharmaceutical Institute, Pharmaceutical Chemistry I, University of Bonn, An der Immenburg 4, 53121 Bonn, Germany
| | | | | |
Collapse
|
13
|
|
14
|
Abstract
Azapeptides are peptide analogs in which one or more of the amino residues is replaced by a semicarbazide. This substitution of a nitrogen for the α-carbon center results in conformational restrictions, which bend the peptide about the aza-amino acid residue away from a linear geometry. The resulting azapeptide turn conformations have been observed by x-ray crystallography and spectroscopy, as well as predicted based on computational models. In biologically active peptide analogs, the aza-substitution has led to enhanced activity and selectivity as well as improved properties, such as prolonged duration of action and metabolic stability. In light of these characteristics, azapeptides have found important uses as receptor ligands, enzyme inhibitors, drugs, pro-drugs, probes and imaging agents. Recent improvements in synthetic methods for their procurement have ushered in a new era of azapeptide chemistry. This review aims to provide a historical look at the development of azapeptide science along with a focus on recent developments and perspectives on the future of this useful tool for medicinal chemistry.
Collapse
|
15
|
Lee HJ, Park HM, Lee KB. The β-turn scaffold of tripeptide containing an azaphenylalanine residue. Biophys Chem 2007; 125:117-26. [PMID: 16890344 DOI: 10.1016/j.bpc.2006.05.028] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/27/2005] [Revised: 05/10/2006] [Accepted: 05/10/2006] [Indexed: 11/27/2022]
Abstract
The conformational preferences of azaphenylalanine-containing peptide were investigated using a model compound, Ac-azaPhe-NHMe with ab initio method at the HF/3-21G and HF/6-31G(*) levels, and the seven minimum energy conformations with trans orientation of acetyl group and the 4 minimum energy conformations with cis orientation of acetyl group were found at the HF/6-31G(*) level if their mirror images were not considered. An average backbone dihedral angle of the 11 minimum energy conformations is phi=+/-91 degrees +/-24 degrees , psi =+/-18 degrees +/-10 degrees (or +/-169 degrees +/-8 degrees ), corresponding to the i+2 position of beta-turn (delta(R)) or polyproline II (beta(P)) structure, respectively. The chi(1) angle in the aromatic side chain of azaPhe residue adopts preferentially between +/-60 degrees and +/-130 degrees, which reflect a steric hindrance between the N-terminal carbonyl group or the C-terminal amide group and the aromatic side chain with respect to the configuration of the acetyl group. These conformational preferences of Ac-azaPhe-NHMe predicted theoretically were compared with those of For-Phe-NHMe to characterize the structural role of azaPhe residue. Four tripeptides containing azaPhe residue, Boc-Xaa-azaPhe-Ala-OMe [Xaa=Gly(1), Ala(2), Phe(3), Asn(4)] were designed and synthesized to verify whether the backbone torsion angles of azaPhe reside are still the same as compared with theoretical conformations and how the preceding amino acids of azaPhe residue perturb the beta-turn skeleton in solution. The solution conformations of these tripeptide models containing azaPhe residue were determined in CDCl(3) and DMSO solvents using NMR and molecular modeling techniques. The characteristic NOE patterns, the temperature coefficients of amide protons and small solvent accessibility for the azapeptides 1-4 reveal to adopt the beta-turn structure. The structures of azapeptides containing azaPhe residue from a restrained molecular dynamics simulation indicated that average dihedral angles [(phi(1), psi(1)), (phi(2), psi(2))] of Xaa-azaPhe fragment in azapeptide, Boc-Xaa-azaPhe-Ala-OMe were [(-68 degrees, 135 degrees ), (116 degrees, -1 degrees )], and this implies that the intercalation of an azaPhe residue in tripeptide induces the betaII-turn conformation, and the volume change of a preceding amino acid of azaPhe residue in tripeptides would not perturb seriously the backbone dihedral angle of beta-turn conformation. We believe such information could be critical in designing useful molecules containing azaPhe residue for drug discovery and peptide engineering.
Collapse
Affiliation(s)
- Ho-Jin Lee
- Advanced Analysis Center, Korea Institute of Science and Technology, P.O. Box 131 Cheongryangri, Seoul 130-650, Korea
| | | | | |
Collapse
|
16
|
Song JW, Lee HJ, Choi YS, Yoon CJ. Origin of Rotational Barriers of the N−N Bond in Hydrazine: NBO Analysis. J Phys Chem A 2006; 110:2065-71. [PMID: 16451044 DOI: 10.1021/jp055755c] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Hydrazine passes through two transition states, TS1 (phi = 0 degrees ) and TS2 (phi = 180 degrees ), in the course of internal rotation around its N-N bond. The origin of the corresponding rotational barriers in hydrazine has been extensively studied by experimental and theoretical methods. Here, we used natural bond orbital (NBO) analysis and energy decomposition of rotational barrier energy (DeltaE(barrier)) to understand the origin of the torsional potential energy profile of this molecule. DeltaE(barrier) was dissected into structural (DeltaE(struc)), steric exchange (DeltaE(steric)), and hyperconjugative (DeltaE(deloc)) energy contributions. In both transition states, the major barrier-forming contribution is DeltaE(deloc). The TS2 barrier is lowered by pyramidalization of nitrogen atoms through lowering DeltaE(struc), not by N-N bond lengthening through lowering DeltaE(steric). Higher pyramidality of nitrogen atoms of TS2 than that of TS1 explains well why the N-N bond of TS2 is longer than that of TS1. Finally, the steric repulsion between nitrogen lone pairs does not determine the rotational barrier; nuclear-nuclear Coulombic repulsion between outer H/H atoms in TS1 plays an important role in increasing DeltaE(struc). Taken together, we explain the reason for the different TS1 and TS2 barriers. We show that NBO analysis is a useful tool for understanding structures and potential energy surfaces of compounds containing the N-N bond.
Collapse
Affiliation(s)
- Jong-Won Song
- Department of Chemistry, Korea University, 1 Anam-dong, Seoul 136-701, Korea
| | | | | | | |
Collapse
|