The absorption spectrum of the all-trans retinal chromophore in the protonated Schiff-base form, that is, the biologically relevant form, has been measured in vacuo, and a maximum is found at 610 nm. The absorption of retinal proteins has hitherto been compared to that of protonated retinal in methanol, where the absorption maximum is at 440 nm. In contrast, the new gas-phase absorption data constitute a well-defined reference for spectral tuning in rhodopsins in an environment devoid of charges and dipoles. They replace the misleading comparison with absorption properties in solvents and lay the basis for reconsidering the molecular mechanisms of color tuning in the large family of retinal proteins. Indeed, our measurement directly shows that protein environments in rhodopsins are blue- rather than red shifting the absorption. The absorption of a retinal model chromophore with a neutral Schiff base is also studied. The data explain the significant blue shift that occurs when metharhodopsin I becomes deprotonated as well as the purple-to-blue transition of bacteriorhodopsin upon acidification.

The absorption spectra of two photoactive yellow protein model chromophores have been measured in vacuum using an electrostatic ion storage ring. The absorption spectrum of the isolated chromophore is an important reference for deducing the influence of the protein environment on the electronic energy levels of the chromophore and separating the intrinsic properties of the chromophore from properties induced by the protein environment. In vacuum the deprotonated trans-thiophenyl-p-coumarate model chromophore has an absorption maximum at 460 nm, whereas the photoactive yellow protein absorbs maximally at 446 nm. The protein environment thus only slightly blue-shifts the absorption. In contrast, the absorption of the model chromophore in aqueous solution is significantly blue-shifted (lambda(max) = 395 nm). A deprotonated trans-p-coumaric acid has also been studied to elucidate the effect of thioester formation and phenol deprotonation. The sum of these two changes on the chromophore induces a red shift both in vacuum and in aqueous solution.

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Absolute total cross sections have been measured for electron impact dissociative excitation and dissociative ionization of H+2 and D+2 in the energy range 5–3000 eV. The vibrational population of the primary H+2 beam has been analysed by dissociative charge exchange on a potassium target, and is in good agreement with the measurements of von Busch and Dunn (1972 Phys. Rev. A 5 1726). Kinetic energy release (KER) distributions have been extracted from momentum analysis of the released protons and deuterons at selected impact energies. A model calculation has been performed to interpret the different spectra. Below 100 eV, the distributions exhibit a sharp peak in the range 0–1 eV that is attributed to the dissociative excitation of high vibrational levels to the 2pσu repulsive state in the vicinity of their outer turning point. This observation is consistent with the measured vibrational population extending up to v = 13, as confirmed by the appearance threshold of the dissociative ionization (DI) channel. The KER distributions exhibit a second contribution peaking between 1 and 5 eV, resulting from the admixture of the (1sσg → 2pσu), (1sσg → 2pπu) and (1sσg → 2sσg) electronic transitions. A distinctive hump is also present around 9 eV, that coincides both with the maximum of the DI contribution, and with the high-energy shoulder of the 2pπu and 2sσg contributions. The present measurements are in qualitative agreement with the previous results of Caudano and Delfosse, and are fairly well reproduced by our first-order model.