The nitrogen Is core level photoionization spectra of the amino acids glycine, methionine and proline in the gas phase have been measured. The spectra of proline show two peaks separated by 0.5 eV, although the molecule contains only one nitrogen atom. Electronic structure calculations demonstrate that the peaks are due to two different types of conformer of proline, but the electronic structure differences giving rise to the core level shifts are complex. The relative intensities of the peaks at 413 K and 438 K set limits to the number of conformers and the values of their relative free energy. (c) 2007 Elsevier B.V. All rights reserved.