Design of functionalized lipids and evidence for their binding to photosystem II core complex by oxygen evolution measurements, atomic force microscopy, and scanning near-field optical microscopy

Photosystem II core complex (PSII CC) absorbs light energy and triggers a s eries of electron transfer reactions by oxidizing water while producing mol ecular oxygen. Synthetic lipids with different alkyl chains and spacer leng ths bearing functionalized headgroups were specifically designed to bind th e Q, site and to anchor this large photosynthetic complex (240 kDa) in orde r to attempt two-dimensional crystallization. Among the series of different compounds that have been tested, oxygen evolution measurements have shown that dichlorophenyl urea (DCPU) binds very efficiently to the Q, site of PS II CC, and therefore, that moiety has been linked covalently to the headgro up of synthetic lipids. The analysis of the monolayer behavior of these DCP U-lipids has allowed us to select ones bearing long spacers for the anchori ng of PSII CC. Oxygen evolution measurements demonstrated that these long-s pacer DCPU-lipids specifically bind to PSII CC and inhibit electron transfe r. With the use of atomic force microscopy (AFM) and scanning near-field op tical microscopy (SNOM), it was possible to visualize domains of PSII CC bo und to DCPU-lipid monolayers. SNOM imaging has enabled us to confirm that d omains observed by AFM were composed of PSII CC. Indeed, the SNOM topograph y images presented similar domains as those observed by AFM, but in additio n, it allowed us to determine that these domains are fluorescent. Electron microscopy of these domains, however, has shown that the bound PSII CC was not crystalline.