The polarized UV-absorption spectra and the crystal structure of two different monoclinic crystal forms of the retinal homologue beta-8'-apocarotenal.

During the visual process, light absorption in the 11-cis retinylidene chromophore leads to a rapid cis-trans-isomerization which initiates the phototransduction step. Important spectroscopic properties of this chromophore can be derived from polarized UV-absorption spectra of crystalline 11-cis-retinal if a parallel X-ray structure analysis is performed. Several questions about the relation between molecular geometry and spectroscopic behavior could not be answered from these spectra. All crystal forms of 11-cis-retinal contain this molecule in its 6-s-cis-ring conformation. For the retinal homologue, beta-8'-apocarotenal (APC), however, two crystal forms with different ring conformation can be grown. The spectrum of alpha-APC (6-s-cis) shows a vibronic structure whereas that of beta-APC (6-s-trans) is diffuse but has a distinct shoulder on the low energy side of the main band. This S-band is typical for retinal spectra and has been ascribed to a transition into a 1Ag-*-state. The appearance of the S-band is not correlated with a 6-s-cis-conformation as suggested by the retinal spectra but is due to intermolecular interactions: beta-APC has a dense dimer packing and a strong electrostatic interaction between the pi-electron systems. This might cause the "forbidden" 1Ag-*-transition. On the other hand, this interaction is missing in the loose and polar packing of alpha-APC which favors vibration in the polyene chain. This finding is remarkable in view of the photodynamic behavior of the visual chromophore for which strong electrostatic interactions with the protein helices of its binding site have to be postulated.