Shapes of Small Model Peptides

IR-UV hole burning of gramicidin

We reported on the first double resonant IR-UV spectroscopy of isolated gramicidin, providing data on the individual molecules, free of any interactions. We achieved this result by studying these peptides in the gas phase, where they appear to maintain their secondary structure. This finding opens the way to detailed spectroscopic studies of intermediate size isolated peptides and of their controlled interactions with individual solvent and other molecules in mass selected clusters.

The gas phase IR spectrum of isolated gramicidin A shows free and hydrogen bound NH from different parts of the structure.

Charge transfer in phe-gly-gly following conformation selective ionization

The lower trace of Figure 4 shows the resonant two-photon ionization spectrum of laser-desorbed phenyl-glycil-glycene (FGG). The peaks marked as A-D represent origins of different conformations, which we determined in a separate experiment with double resonant hole-burning spectroscopy [1] . Photoionization forms the ions with a local charge by removing the electron from the aromatic ring. We subsequently photodissociated the ions by a pulse from a visible laser with a delay of the order of 50 ns. This excitation deposits energy locally in the charged phenyl ring, the only part of the molecule that absorbs visible photons.
The top traces show resulting fragment signals as a function of the initial ionization wavelength.The branching ratio for this process clearly depends on the original conformation of the neutral molecule. This occurs because the chromophore becomes opaque if rapid charge transfer out of the ring competes with successive absorption.

Leu-enkaphelin changes shape by attachment of one water molecule