Study of the conformational changes of chlorophyll a (Chl a) colloids withthe atomic force microscope

Atomic force microscopic (AFM) images of chlorophyll a (Chl a) colloids dep osited onto mica and Au(111) present two different shapes. The colloids app ear as clouds on mica and as large chains on Au(111). This difference in to pography is attributed to different interactions between the colloids and t he substrate. The real-time changes occurring during the electrodeposition of the colloids are also monitored. For an applied electric field intensity varying between 5 x 10(3) and 25 x 10(4) V/m, the colloids are opened and flattened relative to a deposition in the absence of an electric field and the drop method. However, when these films are exposed to ethanol vapors, d isaggregation occurrs. These changes indicate that the association of Chi a dimers can form nanocrystals with large size distribution: 45-100 nm. Arra ngement of nanocrystals in colloids is a characteristic feature of microcry stalline Chi a. The interaction between the colloids and H2O can also provo ke aggregate dissociation. (C) 1999 Academic Press.