PROTON TRANSFER IN NEUTRAL PEPTIDES EXAMINED BY CONFORMATIONAL SPECIFIC IR/UV SPECTROSCOPY Sander Jaeqx 67th International Symposium on Molecular Spectroscopy 19-06-2012

Motivation Protein (large peptide) Active site: Part of the protein that binds to the substrate and where the chemical reactions takes place Structure of the active site is important for the function of a protein Buried deep in peptide  difficult to study Protein (large peptide)

Motivation Glu Proton Transfer Arg Active site mimics to study intrinsic properties of chemical reaction mediated by active site in the gas phase Amino acids in the active site often exhibit proton transfer Therefore, the mimic also needs to exhibit this proton transfer, however this is not trivial in the gas phase

Can proton transfer occur in the gas phase ??? Motivation Can proton transfer occur in the gas phase ???

Outline Experimental set-up / computational methods Proton transfer in Z-Arg-OH Proton transfer in Z-Glu-Alan-Arg-NHMe (n=0,1,3) Conclusions

Experimental Gas-phase measurements Laser desorption Intrinsic properties Laser desorption Create gas phase molecules Supersonic expansion in a molecular beam (Ar) Cool the translational and vibrational temperature of the molecules desorption laser 1064 nm molecular beam (Ar) sample skimmer

IR-UV spectroscopy Fix laser on electronic transition in REMPI  Constant ion signal belonging to a single conformation IR pulse precedes the UV If resonant with vibrational level depletion of ground state  dip in ion signal Conformation specific IR absorption spectrum Ion signal IR wavelength

Techniques - IR spectroscopy IR spectroscopy gives an direct view on the hydrogen bond network present: Amide I  C=O stretch Amide II NH bend Experimental IR absorption spectra compared with theoretical spectra

Computational approach: Conformational search Simulated annealing Max T: 1300 K Simulation time: 10 – 20 ns # structures: 500 – 1000 ~50 structures optimized on B3LYP/6-31G** level ~25 structures optimized and frequency calculation on B3LYP/6-311+G** level

Proton transfer in Z-Arg-OH ? Arginine most basic amino acid Proton tranfer possible from C-terminus to guanidine side chain of arginine forming a zwitterion Neutral Zwitterion Z-cap Proton Transfer

Proton transfer in Z-Arg-OH ? Gas phase structure arginine still under debate; Canonical form ( Carboxylic acid C=O stretch + 2 x NH bend ) Tautomeric form ( Carboxylic acid C=O stretch + 1 x NH bend ) Zwitterionic form ( 2 x NH bend ) Canonical Tautomeric Zwitterionic

Proton transfer in Z-Arg-OH ? Z-Arg-OH has two dominant gas-phase conformations: Conformation A : 2x NH bend + Carboxylic acid C=O stretch  Canonical structure Conformation B : 1 x NH bend + Carboxylic acid C=O stretch  Tautomeric structure There is no proton transfer in Z-Arg-OH

Z-Arg-OH: no proton transfer Tautomeric form Conformation A Conformation B Canonical form

Proton transfer in Z-Glu-Alan-Arg-NHMe (n = 0,1,3) Proton transfer from carboxylic acid group (Glu) to guanidine group (Arg) Z-cap Glu Ala Arg NHMe

Z-Glu-Arg-NHMe: Side chain interactions B Arg Glu C D Backbone Arg Arg Glu Glu

Z-Glu-Arg-NHMe: Side chain interactions  MATCH!!  No Match  No Match  No Match

Structural assignement Z-Glu-Arg-NHMe

Assigned structures for Z-Glu-Alan-Arg-NHMe Assigned structures for Z-Glu-Ala-Arg-NHMe Z-Glu-Ala3-Arg-NHMe

Type of interactions Two types of interactions: Conventional hydrogen bonding Electrostatic interactions Dispersion interaction Induced dipole – induced dipole interactions Major disadvantage DFT: Dispersion interactions are not included  New dispersion corrected functionals are being developed: DFT-D

B3LYP deficiencies Dispersion No dispersion DFT-D better than DFT for Structure optimization and energy calculations Includes more interactions DFT (B3LYP) does better frequency calculations in Amide I and Amide II region However, it has difficulties in fingerprint region when dispersion interactions are present Ik zou zeggen: both Amide I& II well However when dispersion interactions are present: DFT-D better as soon in Fingerprint region Spectrum arg disp/no disp met DFT en DFT -D Dispersion No dispersion

Conclusions Two tautomers observed in gas-phase Proton transfer does not occur in Z-Arg-OH Z-Glu-Alan-Arg-NHMe (n=0,1 and 3) all show proton transfer All exhibit an electrostatic interaction + dispersion interaction

MOLDYN Group FELIX Group Jos Oomens Anouk Rijs Joost Bakker Vivike Lapoutre FELIX Group Lex van der Meer Britta Redlich Giel Berden Cor Tito Rene van Buuren Wybe Roodhuyzen Joop Stakenborg Michel Riet Josipa Grzetic Denis Kiawi Juehan Gao

Dispersion corrected DFT (DFT-D) Structure optimization and energy calculations with DFT-D are of better quality compared with DFT Includes more interactions Z-Arg-OH Conformation A: With dispersion interaction DFT DFT-D

Dispersion corrected DFT (DFT-D) Structure optimization and energy calculations with DFT-D are of better quality compared with DFT Includes more interactions Z-Arg-OH Conformation B: Without dispersion interaction DFT DFT-D

Intermolecular interactions Two types of interactions: Conventional hydrogen bonding Electrostatic interactions Dispersion interaction Induced dipole – induced dipole interactions Major disadvantage DFT: Dispersion interactions are not included  New dispersion corrected functionals are being developed: DFT-D