Abstract. In the present work, the conformational equilibrium for the herbicide diuron (DCMU) has been investigated using high level ab initio calculations. The solvent eect was included through two dierent con- tinuum models: (1) the real cavity IPCM method and (2) the standard dipole Onsager model SCRF. The eect due to solute-solvent hydrogen-bond interactions was analyzed considering a hybrid discreet-continuum mod- el. At the Hartree-Fock level, the gas phase results showed that only the trans forms (A and B) are present in the equilibrium mixture, with the relative concentra- tions found to be 33% (A) and 67% (B) (HF/ 6-311+G**//6-31G**). When the electronic correlation eect is included (MP2/6-31G*//HF/6-31G*), a relative stabilization of the cis forms was observed, with the conformational distribution calculated as 38% (A), 50% (B), 6% (C) and 6% (D). The trans conformations were found to be completely planar, which has been consid- ered to be a prerequisite for the herbicide binding. In water solution, the trans conformation A should be the most abundant conformer, the IPCM and SCRF values being ca. 100% and ca. 85% respectively. The IPCM calculations with the isodensity level set to 0.0005 present a conformational distribution close to that obtained from the hybrid model [92% (A) and 8% (B)], which has been considered our best solvent approach. Regarding the biological action of urea-type herbicides, the results presented here are important, because some QSAR studies have suggested that the partition coecient is related to the herbicide activity, so the conformational equilibrium may play a role in the biological action. Key words: DCMU ± Diuron ± Conformational analysis ± Solvent eect ± Ab initio calculation 1 Introduction The electron transport involved in the light phase of photosynthesis in plants occurs through the action of two photosynthetic reaction centers named photosystem I (PSI) and photosystem II (PSII). These systems are constituted of protein complexes associated with the thylakoid membrane and are connected by another transmembrane protein called cytochrome b 6 f complex (cyt-b 6 f). In this mechanistic model (known as Z- scheme), the electrons are transferred between these protein complexes via a mobile electron carrier [1]. Many common herbicides interfere in the electron ¯ow from PSII to cyt-b 6 f. They compete with the secondary electron acceptor plastoquinone (Q B ) for the binding site on PSII, blocking the reduction process of Q B by the reduced primary quinone Q A and consequently inter- rupting the electron-transport chain [1]. The structural information obtained from purple bacteria has contributed to the elucidation of the constitution of the PSII reaction center (RC) [2]. From homology studies between protein sequences of photo- synthetic bacteria and PSII, the so-called Q B site has been identi®ed as being located on a 32 kDa protein known as the D1 subunit [2]. The use of inhibitor and mutants of the protein subunits has allowed the identi- ®cation of the exact Q B and the herbicide binding domain [3]. The decisive step in the elucidation of the structure of PSII was the resolution of the crystalline structure of the protein subunits in the photosynthetic RC of Rhodopseudomonas viridis [4], which has been useful for modeling the PSII reaction center [5]. Belonging to the class of classical photosynthetic inhibitors, the substituted aryl-ureas were the ®rst group of highly eective herbicides and were introduced in 1956 [6]. As triazines and phenolics, the urea-type inhibitors act by blocking the oxidation of the reduced primary quinone Q A by Q B . The parent compound of the urea-type derivatives (DCMU, [3-(3,4-dichlorophe- nyl)-1,1-dimethylurea]) has been used as probe in some Correspondence to: H.F. Dos Santos Regular article Gas phase and water solution conformational analysis of the herbicide diuron (DCMU): an ab initio study HeÂlio F. Dos Santos 1 , Patrick J. O'Malley 2 , Wagner B. De Almeida 1 1 LaboratoÂrio de QuõÂmica Computacional e Modelagem Molecular (LQC-MM), Departamento de QuõÂmica, ICEx, U.F.M.G., Belo Horizonte, MG, 31270-901, Brazil 2 Department of Chemistry, U.M.I.S.T., P O Box 88, Manchester M60 1QD, UK Received: 23 February 1998 / Accepted: 28 May 1998 / Published online: 19 August 1998 Theor Chem Acc (1998) 99:301±311 DOI 10.1007/s002149800m16