Spatially separating the conformers of the dipeptide Ac-Phe-Cys-NH₂

Atomic-resolution-imaging approaches for single molecules, such as coherent x-ray diffraction at free-electron lasers, require the delivery of high-density beams of identical molecules. However, even very cold beams of biomolecules typically have multiple conformational states populated. We demonstrate the production of very cold ($T_\text{rot}\sim2.3~\text{K}$) molecular beams of intact dipeptide molecules, which we then spatially separate into the individual populated conformational states. This is achieved using the combination of supersonic expansion laser-desorption vaporisation with electrostatic deflection in strong inhomogeneous fields. This represents the first demonstration of a conformer-separated and rotationally-cold molecular beam of a peptide, and will enable future single biomolecule x-ray diffraction measurements.