EXPLORING THE CONFORMATIONAL BEHAVIOR OF RIGID PORPHYRIN-QUINONE SYSTEMS BY HIGH-TEMPERATURE MD SIMULATIONS AND TEMPERATURE-DEPENDENT H-1-NMR EXPERIMENTS

Photoinduced electron transfer reactions play an important role in the primary step of the biological photosynthesis process. In an attempt to understand better the mechanism of the charge separation organic do nor-acceptor molecules containing porphyrins and quinones were designe d as photosynthesis models. In order to study the structure dependence of the photoinduced electron transfer twofold and fourfold bridged po rphyrin-quinone systems with increasing donor-acceptor distance were s ynthesized (Figure 1) [1, 2, 3]. It was assumed that in these molecule s the porphyrin and quinone should be linked in a rigid and well-defin ed orientation. To verify this assumption the conformational behavior of these systems was studied by high-temperature MD simulations in com bination with conformational analysis of selected minimized structures [4, 5]. As an example we describe the dynamical behavior of the quadr uply bridged porphyrin-quinone with naphthyl groups as aryl spacer ('' naphthyl cage'') (Figure 2) which was investigated by the following ge nerally used method: The initial structure used for MD simulation was constructed with the builder modul of INSIGHT II [6]. To stimulate the dynamics in such rigid molecules as the ''porphyrinquinone cages'' du ring a simulation time of 1000 ps we had to choose 1000 K as simulatio n temperature (Figure 3). The CFF91-forcefield [7] was used because it had been proven to reproduce well the Xray-structures of our porphyri n-quinone systems [4]. To analyse the dynamic behavior we defined inte rnal coordinates by which the characteristic motions of the molecule c an be described efficiently (''characteristical coordinates'') (Figure 4). These ''characteristical coordinates'' were evaluated by statisti cal methods (frequency plots) (Figure 5). In addition we carried out c onformational analysis on selected minimized structures with respect t o the same internal coordinates (Figure 6). In contrast to the basic a ssumption we found that the ''naphthyl cage'' is not completely rigid. There is a characteristic dynamic process in the ''naphthyl cage'' (F igure 7). Nevertheless, the conformational mobility of the fourfold br idged porphyrin-quinone system is significantly decreased compared to the twofold bridged porphyrin-quinone system [4]. These calculated res ults exhibit good agreement with experimental results derived from tem perature dependent H-1-NMR experiments [3].