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Spectroscopic investigation of β-peptides: Ac-β3-Phe-NHMe, Ac-β3-Phe-β3-Ala-NHMe and Ac-β3-Ala-β3-Phe-NHMe.
Soo Hyuk Choi and Samuel H. Gellman Department of Chemistry, University of Wisconsin-Madison, Madison, WI
53706
Esteban E. Baquero, William H. James, III, , Timothy S. Zwier, Department of Chemistry, Purdue University, West
Lafayette, IN 47907
Peptide Containing Systems in the Gas Phase • α-peptides have been extensively studied in the jet by several groups.
• Gas phase studies are advantageous in probing the conformational preferences of isolated molecules, and give the best connection to theory.
Michel Mons
• β-peptides differ from α-peptides by an extra carbon linking the peptide groups.
• The extra linkage provides extra flexibility and a different set of conformational preferences.
• Conformational preferences of β-peptides are not as well known or understood.
Angew. Chem. Int. Ed. 2007, 46, 2463-2466Angew. Chem. Int. Ed. 2006, 45, 5166-5169
Mattanjah S. deVriesM. GerhardsMolecular Physics, Vol. 103, No. 11–12, 10–20 June 2005, 1521–1529
C
N1
C1
O1
C
C2
N2
O2
C
CC3
N3
O3
H1
H2 H3
NH
NH
NH
O OO
Ac-β3-Phe-NHMe
Ac-β3-Phe- β3-Ala-NHMe Ac-β3-Ala-β3-Phe-NHMe
Cß
HN1 C
C1
H3C
O1
C2
N2
CH3
O2
H1 H2
The conformational preferences and H-bonding properties of synthetic foldamers: Beta peptides
Collaboration with Samuel Gellman and Soo-Hyuk ChoiUniv. of Wisconsin-Madison
C6C8C10C12
ExperimentalExperimental
Resonant Two-Photon Ionization spectroscopy (R2PI)
Biomolecule* (S1)
Biomolecule (S0)
Biomolecule+ + e- - Heat to raise vapor pressure of molecules
- Molecules entrained in gas pulse (neon backing gas at 1-5 bar for these studies)
- Collisional cooling to zero point vibrational levels
- Heating (190 -250 °C) leads to concerns about thermal decomposition. The R2PI method gives mass analysis to confirm the spectrum is due to the molecule of interest.
UV-UV Hole-burning SpectroscopyUV-UV Hole-burning Spectroscopy
- Records the UV spectrum of a single conformation free from interference from others present in the expansion
Biomolecule* (S1)
Biomolecule (S0)
Biomolecule+ + e- H
ole-
burn
Pro
be
Conformer A
Hol
e-bu
rn
Pro
be
Conformer B
UV Hole-burn laser fixed: Provides selectivity UV probe laser tuned
Boltzmann distributionof conformers in the pre-expansion
Collisional cooling to zero-point vibrational level
B*
B*
B*CA
B*
C
CB
A
A
A
CA
ABC C
AAB BBB B
B B
UV
UV
C
Laser Timing
50-200nsec
UVHole-burn
UVprobe
Ion
In
ten
sity
(a
.u.)
3756037520374803744037400Wavenumbers (cm-1)
A
B
*
Resonant 2 Photon Ionization (R2PI) and Hole Burning Spectrum of Betapeptide
Ac--Phe-NHMe
Cluster BandMasses Observed:
Dimer and M+149 g/mol
Cluster BandMasses Observed:
Water 1 and Water 2
NH
O
NH
O
One dominant conformerMinor
Conformer
So Resonant Ion-dip Infrared Spectroscopy (RIDIRS)
Biomolecule *(S1)
Biomolecule+ + e-
Biomolecule (A) NH or OH stretch
(S0, v=1)
UV Source fixed: Provides selectivity IR Source tuned
Laser Timing
50-200nsec
IRHole-burn
UVprobe
Active Baseline Subtraction
3000 3200 3400 3600 3800Wavenumbers (cm-1)
SubtractedSignal
UV only
UV +IR
Difference
Cß
HN1 C
C1
H3C
O1
C2
N2
CH3
O2
H1 H2
C6C8
RIDIR Spectrum of Conformer A and B Ac-β3-Phe-NHMe
1.m
ol-1.c
m-1
Cß
H2N1 C
C1
H3C
O1
C2
N2
CH3
O2
H1 H2
A. Aubry, M.T. Cung and M. Marraud J. Am. Chem. Soc. 1985, 107, 7640-7647
Ion
in
te
ns
ity
(a
.u.)
3500345034003350 Wavenumber (cm
-1)
A
B
N1H1 …O2=C2
3400 cm-1
N2H2
3488 cm-1
N2H2…O1=C1
3416 cm-1
N1 H1
3454 cm-1
FT-IR Spectrum of beta-peptide chain L-Pro-L-Ala
N1H1 3437-3440 cm-1 N2H2 3466-3469 cm-1
C6
C8?
C6(c)
C6(b)
C6(a) C8(a)
C8(b)
C8(c)
12 3
Chromophore Position
φ1θ1
ψ1HN N
H
O O
Betatripeptide H-bonding architectures
•3 types of C6 rings•5 types of C8 rings•3 chromophore positions
C6(a)(1)
C6(b)(1)
C8(a)(3)
C6(c)(3)
C8(b)(1)
C8(c)(3)C8(b)(2)C8(d)(3)C8(a)(2)C6(b)(1)
C6(a)(2)
C8(a)(1)
C8(e)(3)
C6(a)(1)
C6(b)(1)
C8(a)(3)
C8(b)(1)
C8(b)(2)
C8(a)(2)
-2.00
3.00
8.00
13.00
18.00
23.00
28.00
Re
lati
ve
En
erg
y (
kJ
/mo
l)
DFT (OPT) MP2 (SP) RIMP2 (OPT)
C6(c)(1)
C6(b)(1)
C6(a)(1) C8(a)(3)
C8(b)(3)
C8(c)(3)
Calculated Relative Conformer Energies Ac-3-Phe-NHMe
RepresentativeC6 structures
RepresentativeC8 structures
•C6(a)(1) most stable•Many C8 structures in next tier
Ion
inte
nsi
ty (
a.u
.)
3500345034003350 Wavenumber (cm
-1)
C6(a)(1) 0.00 kJ/mol
C6(b)(1) 5.86 kJ/mol
C8(a)(2) 10.05 kJ/mol
C8(b)(3) 8.66 kJ/mol
C8(d)(3) 9.88 kJ/mol
RepresentativeC8 H-bonded NH
Unusual C8’s withweak H-bond
• Dominant conformer = C6• Minor conformer = unusual C8 (return to after consider tripeptides)
C6 (a)(1)
C6 (b)(1)
C8(b)(3)
C8(d)(3)
C8(a)(2)
Comparing Expt. and Calculated IR spectra Ac-3-Phe-NHMe
Acute angle
Long distance
Ion
Inte
sity
(a.
u.)
3760037550375003745037400Wavenumbers (cm
-1)
D
A B
C
E
A
B
C
D
E
*
*
*
*
*
Ion
In
tesi
ty (
a.u
.)
3480344034003360Wavenumbers (cm
-1)
A
B
C
D
E
C6C8C10C12
C
N1
C1
O1
C
C2
N2
O2
C
CC3
N3
O3
H1
H2 H3
RIDIR Spectrum of Conformer A - E ofAc-β3-Phe-β3-Ala-NHMe
R2PI and HB Spectra of Betapeptide Ac-β3-Phe-β3-Ala-NHMe
• 5 conformations, 2 major, 3 minor• Several unique H-bonding architectures
UV
HB
Spe
ctra
C6/C6
C6(a) C6(b) C8(a) C8(b)
C8/C8C10 C12
C8/C122→1. 3→1
Double ring / double acceptor
C6/C81→2, 3→2
C6C8
1→22→3
3→22→1
1→3 3→1
Single ring structures
12
3
Chromophore Position
Double ring structures
Betatripeptide H-bonding architectures
C6a/C6a
C6b/C6a
C6a/C6b
C8a/C8b
C6b/C6a
C10
C10
C6a/C8a
C8a/C8aC6b/C6b
C8a/C8bC8a/C12C10C8a/C8aC6b/C6b
C10/C6
C12
C12
-2.0
3.0
8.0
13.0
18.0
23.0
28.0
33.0
Rel
ativ
e E
nerg
y (k
J/m
ol)
C6a/C6a (1)
C6a/C8a (1)
C8a/C8a (2) C12 (1)
C10 (1)
C8/C12 (2)
DFT B3LYP/6-31+G* relative energies Ac-β3-Phe-β3-Ala-NHMe
C6a/C6a Double ring lowest
C6/C6C6/C8
C8/C8 Double rings
Double ring/Double acceptor
Single ring
Single ring
Double ring
Double ring
Double ring
348034403400336033203280Wavenumber (cm
-1)
C6(a)/C6(a) (1) 0.00 kJ/mol
C6(b)/C6(a) (1) 4.89 kJ/mol
C8(a)/C8(b) (3) 7.37kJ/mol
C8(a)/C8(a) (3) 11.05 kJ/mol
C6a/C6a (1)C6b/C6a (1)
C8a/C8b (3)C8a/C8a (3)
Conformer B
Conformer D
Conformer C
Comparing IR spectra with calculations
Ac-β3-Phe-β3-Ala-NHMe
•Conformers B,D = C6/C6 double rings (B=one of major conformers)•Conformer C = C8/C8 double ring
Assignments:
3480344034003360Wavenumber (cm
-1)
C10(b) (3) 28.44kJ/mol
C10 (a) (3) 14.23 kJ/mol
C6(a)/C8(a) (1) 8.65 kJ/mol
C8(a)/C12 (3) 14.02 kJ/mol
C8/C12 (2)
C10(a)(1)C10(b)(3)
C6a/C8a (1)
Conformer A
Conformer E
Comparing IR spectra with calculations Ac-β3-Phe-β3-Ala-NHMe
•C8 H-bonded NH now lower in frequency than C6
Ion
inte
nsity
(a.
u.)
3765037600375503750037450Wavenumbers (cm
-1)
A
B
CD
E
*
*
*
Ion
In
ten
sity
(a
.u.)
348034403400336033203280Wavenumber (cm
-1)
C6C8C10C12
A
NH
NH
NH
O OO
R2PI and HB Spectra of Betapeptide Ac-β3-Ala-β3-Phe-NHMe
RIDIR Spectrum of Conformer A - E ofAc-β3-Ala-β3-Phe-NHMe
B
C
A
B
C
D
E
•5 conformations: 2 major (A,B), 3 minor (C-E)
Ion
Inte
nsi
ty (
a.u
.)
3480344034003360Wavenumber (cm
-1)
Phe-AlaC6/C6double
ring
Phe-AlaC8/C8double
ring
Ion
Inte
nsity
(a.u
.)
3480344034003360Wavenumber (cm
-1)
Phe-AlaC10
Phe-AlaC6/C8
C8/C12
A B
D E
Comparing single-conformation IR spectra
Ac-β3-Ala-β3-Phe-NHMe
•A=C6/C6 double ring•B=C10 single ring
2 major conformers•D=C8/C8 double ring•E= Double ring/double acceptor•C= TBD (C8/C8 double ring???)
3 minor conformers
Same as Ac-β3-Phe-β3-Ala-NHMe Similar to Ac-β3-Ala-β3-Phe-NHMe
ConclusionsAc-β3-Phe-NHMe• 2 conformations 1 major (C6) and 1 minor (Unusual C8) •Unusual C8 creates a weaker than normal H-bonded structure •This has to be due to a long H--O distance or an acute angle between peptides.
CßH
N1 C
C1
H3C
O1
C2
N2CH3
O2
H1 H2
• 5 conformations, 2 major, 3 minor
• 2 major conformations are C6/C6 and C10
• 3 minor conformations are C8/C8, C6/C6 and the possibility of a double acceptor conformer C6/C8 or C8/C12. C
N1
C1
O1
C
C2
N2
O2
C
CC3
N3
O3
H1
H2 H3
Ac-β3-Phe- β3-Ala-NHMe
NH
NH
NH
O OO
Ac-β3-Ala-β3-Phe-NHMe• 5 conformations, 2 major, 3 minor
• Assignments can be done by comparison to Phe-Ala arrengement.
• 2 major conformations C10 and C6/C6
• 3 minor C8/C8 , C8/C12 or C6/C8 and one unusual conformer not yet assignned.
AcknowledgementsAcknowledgements
Professor Timothy S. Zwier
Current Group Members:Dr. Ching-Ping LiuDr. Christian MüllerWilliam H. James III *V. Alvin Shubert Tracy LeGreveNathan PillsburyJoshua Newby Chirantha Rodrigo Joshua Sebree
Former Group Members:Dr. Jaime StearnsDr. Talitha SelbyDr. Jasper Clarkson
Professor Samuel H. GellmanSoo-Hyuk Choi
Funding
Professor Kenneth JordanDr. Daniel Schofield