Vibrational Spectroscopy and Gas-Phase Thermochemistry of the Model Dipeptide N-Acetyl Glycine Methyl Amide International Symposium of Molecular Spectroscopy WI: Comparing Theory and Experiment Christopher M. Leavitt †, Caitlyn Shirley†, Paul Raston‡, Grant Moody†, Kevin B. Moore†, Jay Agarwal†, Henry F. Schaefer†, and Gary E. Douberly † †Department of Chemistry, University of Georgia ‡School of Chemistry and Physics, The University of Adelaide

Identifying the underlying interactions that dictate structure in biological molecules Conformation specific IR spectra obtained using IR-UV double resonance Ions were cooled to ~10 K using a cryogenic ion trap Comparison to (scaled) harmonic frequency calculations facilitates structural assignment What about computed energies? Stearns, et. al. PCCP 2009, 11, 125-132

Neutrals work to! Rotational Spectroscopy IR-UV requires chromophore

(N-acetyl glycine methyl amide) TARGET – NAGMA (N-acetyl glycine methyl amide) GAMEPLAN Obtain IR spectra of peptides isolated in droplets (no UV chromophore necessary) Distinguish between various conformers using linear dichroism spectroscopy Manipulation of the molecular ensemble prior to pick up by the droplets (and is it maintained after pick up) C5 (ΔE= 360 cm-1) C7 (ΔE= 0 cm-1)

h ~17 K 30 bar Trapping Potential Depth ~ 100 K Biomolecule pick-up cell h ~17 K 30 bar T ≈ 0.4 K Trapping Potential Depth ~ 100 K Molecular degrees of freedom are brought into thermal equilibrium with the helium droplet at 0.4 K.

h Infrared cw-OPO Measure depletion on (CH4N)+ channel 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 m/z Geometric / Electron Impact Ionization Cross Sections are Reduced

N-acetyl glycine methyl amide free NH stretches H-bonded NH stretches CH stretches C7 conformer C5 conformer 2800 2850 2900 2950 3000 3050 3350 3400 3450 3500 Wavenumber (cm-1) MP2/def2-TZVPD Scaling factor: 0.961

Biomolecule pick-up cell Orient the molecules with large DC electric field Measure vibrational transition moment angle (α) for each mode p t   Z

Vibrational Transition Moment Determination calc. μp = 3.076 D calc α =36° calc α =78° 10 20 30 40 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 ν= 3383.4 cm-1 // Polarization ^ Polarization I(Field On)/I(Field Off) a=24° a=28° a=32° a=36° a=40° Field Strength (kV/cm) NH stretches Field Strength (kV/cm) 10 20 30 40 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 a=70° a=78° a=80° a=85° a=90° // Polarization I(Field On)/I(Field Off) ^ Polarization ν= 3497.1 cm-1 Depletion (arb.) 3325 3350 3375 3400 3425 3450 3475 3500 3525 Wavenumber (cm-1) MP2/def2-TZVPD

Vibrational Transition Moment Determination calc. μp = 3.382 D calc α =45° calc α =70° 10 20 30 40 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 ν= 3442.2 cm-1 ^ Polarization a=36° a=40° a=45° a=48° I(Field On) / I(Field Off) // Polarization Field Strength (kV/cm) Field Strength (kV/cm) 10 20 30 40 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 a=62° a=66° a=70° a=74° a=78° I(Field On)/I(Field Off) ν= 3494.7 cm-1 ^ Polarization // Polarization Depletion (arb.) 3325 3350 3375 3400 3425 3450 3475 3500 3525 Wavenumber (cm-1) MP2/def2-TZVPD

Rapid cooling in He droplets captures gas-phase equilibrium Establish equilibrium at some temperature kcool>>kr,f kf kcool kcool kr

Use IR intensities to determine T-dependent concentration van‘t Hoff Plot

Probability of dopant “pick-up” (Poisson Statistics) If we raise the temperature of the oven source, the signal from monomers goes away!

Two-stage molecular oven Rough Pump Tsample ~105°C T ~115°C Tss-tube ~125°C Aperture for Droplet Beam Tblock 135-365°C Temperature monitored with K-type thermocouples Tblock and Tsample deviated by no more than 0.3°C for each temperature point

C7 conformer has a statistical weight of 2 A little Thermo anyone? 3492 3494 3496 3498 3500 Depletion (arb.) Wavenumber (cm-1) C7 conformer has a statistical weight of 2

408 K ln(Ic7/Ic5) 638 K 1/T (K-1) 1.6 1.8 2.0 2.2 2.4 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 1/T (K-1) X10-3 408 K ln(Ic7/Ic5) 638 K

Calculation: Method ΔH (kJ/mol) ΔS (J/mol·K) ΔG (kJ/mol) Exp -4.5±0.1 -18.1±0.2 1.6±0.2 Calc @ 500 K -3.64 -21.5 4.23 1.6 1.8 2.0 2.2 2.4 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 1/T (K-1) X10-3 408 K Calculation: Geometry optimization at MP2/aug-cc-pVTZ Focal point analysis brings electronic energy to CCSD(T)/CBS level Harmonic frequencies (MP2/aug-cc-pVTZ) scaled (1.0634 ν<1000 cm-1 0.9557 ν>1000 cm-1) ln(Ic7/Ic5) 638 K

Summary Collected IR spectrum of a model dipeptide embedded in He nanodroplets Used linear dichroism experiment to assign particular bands to the C5 and C7 conformers Determined accurate values of ΔH, ΔS and ΔG, providing a benchmark for theoretical calculations

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