Single-conformation ultraviolet and infrared spectroscopy of model synthetic foldamers: β-peptides Ac-β3-hPhe- β3-hAla-NHMe and Ac-β3-hAla-β3- hPhe-NHMe

Esteban E. Baquero, William H. James, Hyuk Choi Soo, Samuel H. Gellman, Timothy S. Zwier

Research output: Contribution to journalArticle

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Abstract

The conformational preferences and infrared and ultraviolet spectral signatures of two model β-peptides, Ac-β3-hPhe- β3-hAla-NHMe (1) and Ac-β3-hAla- β3-hPhe-NHMe (2), have been explored under jet-cooled, isolated-molecule conditions. The mass-resolved, resonant two-photon ionization spectra of the two molecules were recorded in the region of the S 0-S1 origin of the phenyl substituents (37200-37800 cm-1). UV-UV hole-burning spectroscopy was used to determine the ultraviolet spectral signatures of five conformational isomers of both 1 and 2. Transitions due to two conformers (labeled A and B) dominate the R2PI spectra of each molecule, while the other three are minor conformers (C-E) with transitions a factor of 3-5 smaller. Resonant ion-dip infrared spectroscopy was used to obtain single-conformation infrared spectra in the 3300-3700 cm -1 region. The infrared spectra showed patterns of NH stretch transitions characteristic of the number and type of intramolecular H-bonds present in the β-peptide backbone. For comparison with experiment, full optimizations of low-lying minima of both molecules were carried out at DFT B3LYP/6-31+G*, followed by single point MP2/6-31+G* and selected MP2/aug-cc-pVDZ calculations at the DFT optimized geometries. Calculated harmonic vibrational frequencies and infrared intensities for the amide NH stretch vibrations were used to determine the β-peptide backbone structures for nine of the ten observed conformers. Conformers 1B, 1D, and 2A were assigned to double ring structures containing two C6 H-bonded rings (C6a/C6a), conformers 1A and 2B are C10 single H-bonded rings, conformers 1C and 2D are double ring structures composed of two C8 H-bonded rings (C8/C8), and conformers 1E and 2E are double ring/double acceptor structures in which two NH groups H-bond to the same C=O group, thereby weakening both H-bonds. Both 1E and 2E are tentatively assigned to C6/C8 double ring/double acceptor structures, although C8/C12 structures cannot be ruled out unequivocally. Finally, no firm conformational assignment has been made for conformer 2C whose unusual infrared spectrum contains one very strong H-bond with NH stretch frequency at 3309 cm-1, a second H-bonded NH stretch fundamental of more typical value (3399 cm-1), and a third fundamental at 3440 cm-1, below that typical of a branched-chain free NH. The single conformation spectra provide characteristic wavenumber ranges for the amide NH stretch fundamentals ascribed to C6 (3378-3415 cm-1), C8 (3339-3369 cm-1), and C10 (3381-3390 cm-1) H-bonded rings.

Original languageEnglish
Pages (from-to)4795-4807
Number of pages13
JournalJournal of the American Chemical Society
Volume130
Issue number14
DOIs
Publication statusPublished - 2008 Apr 9

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Ultraviolet spectroscopy
Peptides
Conformations
Infrared spectroscopy
Spectrum Analysis
Infrared radiation
Amides
Molecules
Discrete Fourier transforms
Vibration
Photons
Ions
Vibrational spectra
Isomers
Ionization
Spectroscopy
Geometry
Experiments

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

@article{873e401d330644d99aec0237797c0c00,
title = "Single-conformation ultraviolet and infrared spectroscopy of model synthetic foldamers: β-peptides Ac-β3-hPhe- β3-hAla-NHMe and Ac-β3-hAla-β3- hPhe-NHMe",
abstract = "The conformational preferences and infrared and ultraviolet spectral signatures of two model β-peptides, Ac-β3-hPhe- β3-hAla-NHMe (1) and Ac-β3-hAla- β3-hPhe-NHMe (2), have been explored under jet-cooled, isolated-molecule conditions. The mass-resolved, resonant two-photon ionization spectra of the two molecules were recorded in the region of the S 0-S1 origin of the phenyl substituents (37200-37800 cm-1). UV-UV hole-burning spectroscopy was used to determine the ultraviolet spectral signatures of five conformational isomers of both 1 and 2. Transitions due to two conformers (labeled A and B) dominate the R2PI spectra of each molecule, while the other three are minor conformers (C-E) with transitions a factor of 3-5 smaller. Resonant ion-dip infrared spectroscopy was used to obtain single-conformation infrared spectra in the 3300-3700 cm -1 region. The infrared spectra showed patterns of NH stretch transitions characteristic of the number and type of intramolecular H-bonds present in the β-peptide backbone. For comparison with experiment, full optimizations of low-lying minima of both molecules were carried out at DFT B3LYP/6-31+G*, followed by single point MP2/6-31+G* and selected MP2/aug-cc-pVDZ calculations at the DFT optimized geometries. Calculated harmonic vibrational frequencies and infrared intensities for the amide NH stretch vibrations were used to determine the β-peptide backbone structures for nine of the ten observed conformers. Conformers 1B, 1D, and 2A were assigned to double ring structures containing two C6 H-bonded rings (C6a/C6a), conformers 1A and 2B are C10 single H-bonded rings, conformers 1C and 2D are double ring structures composed of two C8 H-bonded rings (C8/C8), and conformers 1E and 2E are double ring/double acceptor structures in which two NH groups H-bond to the same C=O group, thereby weakening both H-bonds. Both 1E and 2E are tentatively assigned to C6/C8 double ring/double acceptor structures, although C8/C12 structures cannot be ruled out unequivocally. Finally, no firm conformational assignment has been made for conformer 2C whose unusual infrared spectrum contains one very strong H-bond with NH stretch frequency at 3309 cm-1, a second H-bonded NH stretch fundamental of more typical value (3399 cm-1), and a third fundamental at 3440 cm-1, below that typical of a branched-chain free NH. The single conformation spectra provide characteristic wavenumber ranges for the amide NH stretch fundamentals ascribed to C6 (3378-3415 cm-1), C8 (3339-3369 cm-1), and C10 (3381-3390 cm-1) H-bonded rings.",
author = "Baquero, {Esteban E.} and James, {William H.} and Soo, {Hyuk Choi} and Gellman, {Samuel H.} and Zwier, {Timothy S.}",
year = "2008",
month = "4",
day = "9",
doi = "10.1021/ja078272q",
language = "English",
volume = "130",
pages = "4795--4807",
journal = "Journal of the American Chemical Society",
issn = "0002-7863",
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}

Single-conformation ultraviolet and infrared spectroscopy of model synthetic foldamers : β-peptides Ac-β3-hPhe- β3-hAla-NHMe and Ac-β3-hAla-β3- hPhe-NHMe. / Baquero, Esteban E.; James, William H.; Soo, Hyuk Choi; Gellman, Samuel H.; Zwier, Timothy S.

In: Journal of the American Chemical Society, Vol. 130, No. 14, 09.04.2008, p. 4795-4807.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Single-conformation ultraviolet and infrared spectroscopy of model synthetic foldamers

T2 - β-peptides Ac-β3-hPhe- β3-hAla-NHMe and Ac-β3-hAla-β3- hPhe-NHMe

AU - Baquero, Esteban E.

AU - James, William H.

AU - Soo, Hyuk Choi

AU - Gellman, Samuel H.

AU - Zwier, Timothy S.

PY - 2008/4/9

Y1 - 2008/4/9

N2 - The conformational preferences and infrared and ultraviolet spectral signatures of two model β-peptides, Ac-β3-hPhe- β3-hAla-NHMe (1) and Ac-β3-hAla- β3-hPhe-NHMe (2), have been explored under jet-cooled, isolated-molecule conditions. The mass-resolved, resonant two-photon ionization spectra of the two molecules were recorded in the region of the S 0-S1 origin of the phenyl substituents (37200-37800 cm-1). UV-UV hole-burning spectroscopy was used to determine the ultraviolet spectral signatures of five conformational isomers of both 1 and 2. Transitions due to two conformers (labeled A and B) dominate the R2PI spectra of each molecule, while the other three are minor conformers (C-E) with transitions a factor of 3-5 smaller. Resonant ion-dip infrared spectroscopy was used to obtain single-conformation infrared spectra in the 3300-3700 cm -1 region. The infrared spectra showed patterns of NH stretch transitions characteristic of the number and type of intramolecular H-bonds present in the β-peptide backbone. For comparison with experiment, full optimizations of low-lying minima of both molecules were carried out at DFT B3LYP/6-31+G*, followed by single point MP2/6-31+G* and selected MP2/aug-cc-pVDZ calculations at the DFT optimized geometries. Calculated harmonic vibrational frequencies and infrared intensities for the amide NH stretch vibrations were used to determine the β-peptide backbone structures for nine of the ten observed conformers. Conformers 1B, 1D, and 2A were assigned to double ring structures containing two C6 H-bonded rings (C6a/C6a), conformers 1A and 2B are C10 single H-bonded rings, conformers 1C and 2D are double ring structures composed of two C8 H-bonded rings (C8/C8), and conformers 1E and 2E are double ring/double acceptor structures in which two NH groups H-bond to the same C=O group, thereby weakening both H-bonds. Both 1E and 2E are tentatively assigned to C6/C8 double ring/double acceptor structures, although C8/C12 structures cannot be ruled out unequivocally. Finally, no firm conformational assignment has been made for conformer 2C whose unusual infrared spectrum contains one very strong H-bond with NH stretch frequency at 3309 cm-1, a second H-bonded NH stretch fundamental of more typical value (3399 cm-1), and a third fundamental at 3440 cm-1, below that typical of a branched-chain free NH. The single conformation spectra provide characteristic wavenumber ranges for the amide NH stretch fundamentals ascribed to C6 (3378-3415 cm-1), C8 (3339-3369 cm-1), and C10 (3381-3390 cm-1) H-bonded rings.

AB - The conformational preferences and infrared and ultraviolet spectral signatures of two model β-peptides, Ac-β3-hPhe- β3-hAla-NHMe (1) and Ac-β3-hAla- β3-hPhe-NHMe (2), have been explored under jet-cooled, isolated-molecule conditions. The mass-resolved, resonant two-photon ionization spectra of the two molecules were recorded in the region of the S 0-S1 origin of the phenyl substituents (37200-37800 cm-1). UV-UV hole-burning spectroscopy was used to determine the ultraviolet spectral signatures of five conformational isomers of both 1 and 2. Transitions due to two conformers (labeled A and B) dominate the R2PI spectra of each molecule, while the other three are minor conformers (C-E) with transitions a factor of 3-5 smaller. Resonant ion-dip infrared spectroscopy was used to obtain single-conformation infrared spectra in the 3300-3700 cm -1 region. The infrared spectra showed patterns of NH stretch transitions characteristic of the number and type of intramolecular H-bonds present in the β-peptide backbone. For comparison with experiment, full optimizations of low-lying minima of both molecules were carried out at DFT B3LYP/6-31+G*, followed by single point MP2/6-31+G* and selected MP2/aug-cc-pVDZ calculations at the DFT optimized geometries. Calculated harmonic vibrational frequencies and infrared intensities for the amide NH stretch vibrations were used to determine the β-peptide backbone structures for nine of the ten observed conformers. Conformers 1B, 1D, and 2A were assigned to double ring structures containing two C6 H-bonded rings (C6a/C6a), conformers 1A and 2B are C10 single H-bonded rings, conformers 1C and 2D are double ring structures composed of two C8 H-bonded rings (C8/C8), and conformers 1E and 2E are double ring/double acceptor structures in which two NH groups H-bond to the same C=O group, thereby weakening both H-bonds. Both 1E and 2E are tentatively assigned to C6/C8 double ring/double acceptor structures, although C8/C12 structures cannot be ruled out unequivocally. Finally, no firm conformational assignment has been made for conformer 2C whose unusual infrared spectrum contains one very strong H-bond with NH stretch frequency at 3309 cm-1, a second H-bonded NH stretch fundamental of more typical value (3399 cm-1), and a third fundamental at 3440 cm-1, below that typical of a branched-chain free NH. The single conformation spectra provide characteristic wavenumber ranges for the amide NH stretch fundamentals ascribed to C6 (3378-3415 cm-1), C8 (3339-3369 cm-1), and C10 (3381-3390 cm-1) H-bonded rings.

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