Excitation of crystalline all-trans retinal under pressure

Polycrystalline all-trans retinal (ATR) has been excited at 647.1 nm under pressure up to 9 GPa at room temperature and the resulting Raman spectrum has been measured. The large majority of Raman bands in the 700-1700 cm(-1) range have positive pressure coefficients, i.e. bonds stiffen upon compression. The Raman spectrum is recovered after the compression-decompression cycle indicating that the molecular unit does not undergo an irreversible transformation under our experimental conditions. Model density functional (DF) calculations with the B3-LYP exchange-correlation functional and the 6-31G* basis set have been performed on 1-cis, 3-trans,5-trans, 7-trans,9-trans,1,5,9-trimethyldecapentaenal (TMDPL) mimicking the polyenic moiety of ATR. By shortening the single and double CC bonds of TMDPL up to 0.0015 Angstrom it was possible to fit the frequency dependence on pressure of the most intense polyenic bands with single and double bond compressibilities approximate to0.00292 and 0.00117 Angstrom/GPa, respectively. The trans --> cis isomerization in the crystal state under pressure and in the presence of the 647.1 nm excitation light is suggested by the Raman spectrum in the fingerprint region. A simplified model is proposed for the occurrence of this process in the crystal phase.