Conformation Specific Spectroscopic Investigation of β- and α/β-peptides: Insight Into The Amide I and Amide II Spectral Signatures William H. James III, Christian W. Müller, Esteban E. Baquero, and Timothy S. Zwier Purdue University, West Lafayette IN Soo Hyuk Choi and Samuel H. Gellman University of Wisconsin-Madison, Madison WI RG07 June 19, rd OSU International Symposium on Molecular Spectroscopy

What Questions Can We Answer? Linear and 2D IR Spectra of d(GC) 8 A.T. Krummel, M.T. Zanni J. Phys. Chem. B (2006) Solution Phase and Conformation Specific Gas Phase IR Spectra of Ac-β 3 -Phe-NHMe E. Baquero, et al., J. Am. Chem. Soc (2008). Conformation Specific Spectra Connect Experiment to Theory to Gain Insight into the Coupling of Modes of a Particular Structure.

What Can Be Learned From Our Methods? UV Source fixed: Provides l selectivity IR Source tuned Resonant ion dip infrared spectroscopy (RIDIRS): Conformation specific IR spectrum AgGaSe 2 crystal provides tunability form ~ cm -1. Supersonic Expansion cools gas phase molecules to effective temperatures of a few degrees Kelvin Boltzmann distribution of the vibrational population prior to expansion Collisional cooling to zero-point vibrational level B* C E C C B A D A A A B C C A A E B B B B B D UV C A Mass selective, conformation specific IR spectroscopy.

C8 and C6 Ac-β 3 -hPhe-NHMe C10, C6/C6, C8/C8, C6/C8 Ac-β 3 -hAla-β 3 -hPhe-NHMe C10, C6/C6, C8/C8, C6/C8 Ac-β 3 -hPhe-β 3 -Ala-NHMe Ac-Phe-ACPC-NHMe C5/C8 Ac-ACPC-(L)-Phe-NHMe C7/C8 and C11 Peptide Containing Systems E. Baquero et al., JACS (2008) E. Baquero et al., JACS (2008) Ac-β 3 -hAla-(D)Phe-NHMe Ac-β 3 -hAla-(L)Phe-NHMe Ac-Phe-β 3 -hAla-NHMe C7/C8, C5/C6, C7 C7/C8, C11, C5/C6 C5/C8,C7/C8, C11, C5/C6, C7/C11 Ac-ACPC-(D)-Phe-NHMe C7/C8 and C11

Two Conformers Resolved. Firm Assignments Based on NH Stretch Spectral Region. Ac-β 3 -Phe-NHMe C6 C8 C6 The “Simplest” System NH Stretch Spectral Region Proven to be Exceptionally Diagnostic! How Diagnostic Is the cm -1 Region? B3LYP/6-31+G(D) Frequencies Scaled 0.96

Ac-β 3 -Phe-NHMe C6 C8 The “Simplest” System Very Similar Signatures in C=O Stretch Region NH Bend Appears to be Slightly More Diagnostic NH bend shows increase in intensity for C6 ring, both experimentally and computationally. B3LYP/6-31+G(D) Scaled 0.98

Ac-β 3 -hPhe-NHMe Ac-β 3 -hTyr-NHMe Diagnostic NH Bend: Is There A Trend? Only one conformer observed for Ac-β 3 -hTyr-NHMe, a C6. C=O Stretch and NH bend Similar to C6 of Ac-β 3 -hPhe-NHMe. NH Bend Enhancement in C6 H-bonded Conformer. C6 C8 C6

Ac-β 3 -hAla-β 3 -hPhe-NHMe Ac-β 3 -hPhe-β 3 -Ala-NHMe

C8 and C6 Ac-β 3 -hPhe-NHMe C10, C6/C6, C8/C8, C6/C8 Ac-β 3 -hAla-β 3 -hPhe-NHMe C10, C6/C6, C8/C8, C6/C8 Ac-β 3 -hPhe-β 3 -Ala-NHMe Ac-Phe-ACPC-NHMe C5/C8 Ac-ACPC-(L)-Phe-NHMe C7/C8 and C11 Peptide Containing Systems E. Baquero et al., JACS (2008) E. Baquero et al., JACS (2008) Ac-β 3 -hAla-(D)Phe-NHMe Ac-β 3 -hAla-(L)Phe-NHMe Ac-Phe-β 3 -hAla-NHMe C7/C8, C5/C6, C7 C7/C8, C11, C5/C6 C5/C8,C7/C8, C11, C5/C6, C7/C11 Ac-ACPC-(D)-Phe-NHMe C7/C8 and C11

Ac-Phe-β 3 -hAla-NHMe C5/C8 C5/C6 C5/C8 C5/C6 A C5/C8b(1) B C5/C6a(1) Reasonable representation for C5/C6, but not for C5/C8 in NH Stretch Region. MID-IR Region Shows Much Better Agreement for C5/C8.

O N H O N H O H N Ac-β 3 -hAla-(D)Phe-NHMe O N H O N H O H N Ac-β 3 -hAla-(L)Phe-NHMe Conformation and Diastereomeric Specific Mid-IR Spectra

O N H O N H O H N Ac-β 3 -hAla-(D)Phe-NHMe O N H O N H O H N Ac-β 3 -hAla-(L)Phe-NHMe C7/C8 Conformational Family C7L/C8c(1) C7D/C8c(3) C7L/C8h(3) C7L/C8d(3) C7L/C8a(1)

B3LYP/6-31+G(D) Transition Dipole Coupling Model A C7L/C8c(1) C C7D/C8c(3) D C7L/C8h(3) Transition Dipole Coupling (TDC) Model TDC does not accurately describe experimental spectra. Poor description anticipated due to carbonyls being nearest neighbors and being covalently linked. O N H O N H O H N Ac-β 3 -hAla-(L)Phe-NHMe

B3LYP/6-31+G(D) Transition Dipole Coupling Model TDC does not accurately describe experimental spectra. Poor description anticipated due to carbonyls being nearest neighbors and being covalently linked. O N H O N H O H N Ac-β 3 -hAla-(L)Phe-NHMe Transition Dipole Coupling (TDC) Model E C5/C6a(1) G C5/C6b(2)

C6 C8 C6 C10 C6/C6 C8/C8 C6/C6 C10 unassigned β-peptide Spectral Signatures Both Amide I and II Regions are Diagnostic!

α/β-peptide Spectral Signatures C5/C8 C5/C6 C7/C11 C7/C8 C5/C6 C7/C8 C11 C7/C8

α/β-peptide Spectral Signatures C5/C8 C7/C8 C11 C7/C8

Conclusions Resolved 26 Conformation Specific Spectra of 10 Peptide Containing Systems. Both C=O Stretch and NH Bend are fairly diagnostic. B3LYP/6-31+G(D) Computed Spectra are Modestly Representing Experimental Spectra Future Work Continue and Expand Coupling Analysis to Other Theoretical Methods. Record Spectral Signatures of Gamma Peptide Systems in the Amide I Region.

Acknowledgements Professor Timothy S. Zwier Current Group Members: Dr. Christian Müller Dr. Mike Nix Tracy LeGreve Nathan Pillsbury Joshua Newby Chirantha Rodrigo Joshua Sebree Evan Buchanan Former Group Members: Dr. Jaime Stearns Dr. Talitha Selby Dr. Jasper Clarkson Dr. Ching-Ping Liu Dr. Esteban Baquero Dr. V. Alvin Shubert Professor Samuel H. Gellman Soo Hyuk Choi Professor Martin Zanni Computational Resources FUNDING