1. ASSESSING THE IMPACT OF BACKBONE LENGTH AND CAPPING AGENT ON THE CONFORMATIONAL PREFERENCES OF A MODEL PEPTIDE: CONFORMATION SPECIFIC IR AND UV SPECTROSCOPY OF 2- AMINOISOBUTYRIC ACID MG13
Adjust title?
Joseph R. Gord, Daniel M. Hewett, Alicia O. Hernandez-Castillo, Karl N. Blodgett, Matthew A. Kubasik, and Timothy S. Zwier

2. Introduction Aib (U) vs Alanine (A)φ ψ
(n)
Aib (U) vs Alanine (A)
Single methyl group
a-aminoisobutyric Acid (Aib)n forms 310-helices
(i → i+3)
3 residues/turn
f and y angles are typically -60º and -30º, respectively
Point out the group numbering…
Timothy Zeko; Steven F. Hannigan; Timothy Jacisin; Matthew J. Guberman-Pfeffer; Eric R. Falcone; Melissa J. Guildford; Christopher Szabo; Kathryn E. Cole; Jessica Placido; Erin Daly; Matthew A. Kubasik; J. Phys. Chem. B  2014, 118,
DOI: /jp408818g
Copyright © 2013 American Chemical Society

3. Motivation
Consider competition between a(3.613)- and 310- helices under solvent-free conditions
Competing structures
Look for earliest onset of 310-helix formation
Crystal structure shows 310 with n=6
Identify spectroscopic signatures
Clean up..

4. Methods R2PI RIDIRS IR-UV HB s0 sn Ion s0 sn Ion s0 sn Ion S0 A B C
1064 nm
Graphite rod B C C A B A s0 sn
Ion
R2PI s0 sn
Ion
RIDIRS
200 ns
IR-UV HB s0 sn
Ion
200 ns B C A B C A B C A B B A A C C S0 A B C

5. Previous Data: Major Conformations
Z-U-OH
Z-U2-OH
Z-U2-OMe C5 C5 C5
Add Dihedral angles….
Phi: -180
Psi: -180
Phi: 58, 172
Psi: 32, -178
Phi: -64, -174
Psi: -31, -174

6. Z-(Aib)4-OMe
R2PI A B C A C B
Fix the colors….

7. Z-U4-OMe A:
C10/C10
NH (3)···O=C (0)
NH (4)···O=C (1)
C10
C10
Free (3)
C10 (2, 3 in)
C10 (0)
C10 (1)
Free (4)
C10 (4)
C10 (3)
Free (2)
Free (1)
Fix the labels for the mid IR… lots of coupling.
+ 3 kJ/mol
Phi: -61, -57, -57, -48
Psi: -27, -22, -35, -40
Clear 310-Helical structure is seen with only n = 4!

8. Z-U4-OMe B: C10/C7 Competing C10/C7 motif NH (3)···O=C (0)
Freq Grp
OMe cap C=O
Z-cap C=O
C=O(1)
C=O(3)
C=O(2)
Free (4)
C10 (0)
Free (1)
Free (3, 7-2 out)
C7 (2, F3 in)
C7 (4)
C10 (3)
Free (1 ,2) C7
C10
Note that the amide I fundamentals are spread over 90 cm-1. You can learn a lot from this. My guess is that the overtone of C=O(2) couples with NH(4) which is in a C7 H-bond with C=O(2), and that is why we have a doublet down in this region.
+ 4 kJ/mol
Phi: 63, 58, -75, 52
Psi: 25, 30, 74, -143
Competing C10/C7 motif

9. Z-U4-OMe C Competing C7/C7/C14 motif + 24 kJ/mol C7
NH (1)···O=C (3, 4)
NH (2)···O=C (0)
NH (3)···O=C (1)
C11 (3)
C14 (4)
C14/C11 (1)
C7 (1)
C7 (0)
Free (2)
Free (4)
C7 (3)
C7 (2)
What makes C7(3) so low in frequency?
The NH is bent out-of-plane a bit, which is interesting.
Is this induced by the extra methyl group?
The weak C7(2) is to the Z-cap C=O.
Is it that C7(2) is so weak or the C7(3) is so strong? C7
+ 24 kJ/mol
Phi: -82, 68, -53, 43
Psi: 34, -79, 151, 49
Competing C7/C7/C14 motif
C14 C7

10. Z-U4-OMe Assignments Major Conformer Minor Conformer Minor Conformer
Conformer A
+3 kJ/mol
Phi: -61, -57, -57, -48
Psi: -27, -22, -35, -40
Conformer B
+4 kJ/mol
Phi: 63, 58, -75, 52
Psi: 25, 30, 74, -143
Conformer C
+24 kJ/mol
Phi: -82, 68, -53, 43
Psi: 34, -79, 151, 49

11. Z-(Aib)6-OMe A B
Largest Structure Ever…in the Zwier Lab!

12. Z-(Aib)6-OMe B Clear 310 structure: Four C10s! + 0.0 kJ/mol
Free (5)
C10
C10 (1)
C10 (3 & 4)
C10 (6 & 5)
C10 (3)
C10 (0)
Free (4)
Free (6)
Free (2)
Free (1)
C10
In the Z-U4’s, the only thing that gets us down well below 3300 is the low frequency C7 in conformers B and C of Z-U4-OMe.
Remove the Fit from A and focus on B. Maybe don’t show A at all on this slide..
Clear 310 structure: Four C10s!
+ 0.0 kJ/mol
Phi: -60, -54, -53, -55, -59, 50
Psi: -28, -28, -30, -30, -28, 40

13. Z-(Aib)6-OMe A

14. Z-(Aib)6-OH -OH is a more intrusive, natural cap +1 kJ/mol
The assignments for this one are kinda funky. We should talk through them. I’ll put some additional slides together.
Remove stick spectra
Talk about regions
Change the color scheme on this slide. Add shading for h-bonding regions.
+1 kJ/mol
Phi: 96, 53, 54, 49, 73, -57
Psi: 28, 29, 28, 39, -51, -39
-OH is a more intrusive, natural cap

15. Conclusions 310-helix seen at the earliest stages
Full turn formed with only 4 residues
More fully formed 310 seen with n =6
Multiple competing structures observed
Aib is a clear 310 former!
C10

16. Alicia O. Hernandez-Castillo
Acknowledgements
Prof. Timothy S. Zwier
Prof. Matthew A. Kubasik
Zwier Group Members
Daniel Hewett
Alicia O. Hernandez-Castillo
Karl N. Blodgett