Phytol as one of the determinants of chlorophyll interactions in solution

Optical absorption and fluorescence parameters of chlorophyll a and the phytol-free chlorophyllide a, as well as of their Mg-depleted derivatives, were compared in a series of organic solvents. In contrast to prevailing opinion, the spectral properties of chlorophyll are not indifferent to the removal of phytol. The electronic absorption spectra of chlorophyll a and chlorophyllide a differ and display a different dependence on the nature of the solvent, which cannot be explained solely by the location of a charged carboxylic group in the proximity of the pi- electron system. In fact, measurements in media of varying basicity show that deprotonation of the free carboxylic group in chlorophyllide, i.e., the presence of a negative point charge near the macrocycle, has no effect on pigment absorption spectra. Analysis of the solvent effect on the Q(Y) energies in terms of solvent polarity reveals that the phytyl moiety perturbs the spectral features of chlorophyll, mainly due to its interactions with the pigment solvation shell. The phytyl residue might also be thus partly involved in controlling the central metal ligation in chlorophylls. This influence of phytol on the spectral features of chlorophyll should be taken into account when comparing the spectra in solution with various spectral forms of chlorophyll in vivo.