Aggregation of chlorophylls in monolayers. Part IV. The reorganisation of chlorophyll a in multilayer array.

The nature of the interaction between the chlorophyll a molecules in multilayer arrays obtained by the Langmuir-Blodgett technique is examined by electronic and infrared spectroscopies. Following the deposition of the multilayers, we observed a blue shift with time in the electronic spectra. This effect is monitored by infrared spectroscopy. The intensity of the coordinated ketone band is decreased while the intensity of the free ketone band is increased. These modifications are explained by the reorganization of the chlorophyll a molecules from an organized to a less organized one. The influence of H2O, D2O and SO2 vapors on the chlorophyll a multilayers give some informations on the role of water molecules in the aggregation of chlorophyll a in this ordered system. From these observations, a model is proposed for the multilayer arrangement implying two molecules of water per molecule of chlorophyll a.