Effect of pH on fluorescence and absorption of aggregates of chlorophyll a and carotenoids

The fluorescence and UV-Vis absorption spectra of chlorophyll a (Chl-a) and all-trans-beta-carotene (beta-Car) in aqueous ethanol solutions with different pH values were studied. The UV-Vis absorption and Raman spectra of beta-Car and Lutein were also determined under the same conditions. The results show that the geometry of Chl-a is changed with the increase of hydrogen bond under acidic conditions, and the ring of Chl-a is deformed which lead to spectral red shift and fluorescence quenching. The results indicate that with the increase of acidity, Chl-a can form H-type and J-type aggregates in an acidic hydrated ethanol solution. This conclusion can be proved by the division and movement of the spectra. Under the condition of alkaline pH, the Q-band of the absorption spectrum has a small blue shift, while the fluorescence spectrum has a weak blue shift and quenching. By spectral analysis of beta-Car and Lutein, acidic conditions favor the formation of J- and H-aggregates, respectively.