Abstract. Molecular mechanics minimizations based on the CVFF force ®eld and molecular dynamics simulation for a time of 2.5 ns were performed to examine the conformational behaviour and the molec- ular motion of acetylcholine in vacuo and in aqueous solution. Five low-lying conformations, namely the TT, TG, GG, G*G and GT, were obtained from molecular mechanics computations with the GT structure as the absolute minimum. Molecular dynamics trajectories in vacuo and in water show that only four GT, GG, G*G and TG) and three TG, TT and GT) conforma- tions are present in the simulation time, respectively. Density functional B3LYP and second-order Mùller± Plesset MP2) methods were then used to study all the ®ve lowest-lying conformers of acetylcholine neuro- transmitter in vacuo and in water by the polarizable continuum model approach. The B3LYP and MP2 computations show that in the gas phase all minima lie in a narrow range of energy with the G*G conformer as the most stable one. The relative minima GG, GT, TG and TT are located at 1.1 3.3), 1.8 4.2), 2.1 4.5) and 4.3 7.3) kcal/mol above the absolute one at the B3LYP MP2) level. The preferred conformation in water is the TG. Solvation reduces the relative energy dierences between the ®ve minima in both computa- tions. Key words: Acetylcholine ± Density functional theory ± Second-order Mùller±Plesset ± Molecular dynamics ± Solvent eects 1 Introduction The biological action of acetylcholine ACh + ) in the transmission processes of nerve impulses is carried on all the synapsis of the autonomic central nervous system. The knowledge of its conformational behaviour is a topic of central interest in biological structure± activity relationships relevant to molecular pharmacol- ogy because the structure of cholinergic receptors has not been well established yet [1, 2]. The activity of ACh + on both nicotinic and muscarinic sites was related to its conformational ¯exibility [3±6]. In fact, this small molecule governs dierent important func- tions, including the binding to receptors of the postsynaptic membrane and its degradation by acety- lcholinesterase AChase), two processes highly depen- dent upon overall molecular conformations. For this reason, the structure and the conformational properties of ACh + have been the subject of many theoretical and experimental studies in the gas, liquid and solid states [4, 7±30]. The dierent conformations of ACh + derive from the rotation of the four internal torsional angles Scheme 1). Because two of these angles s 0 and s 3 ) were found to be near the trans value 180°) in most of the previous ex- perimental [7±9] and theoretical [4, 11, 16, 22, 25±30] investigations, the conformational ¯exibility of ACh + can be ascribed to the remaining C±C±O±C s 1 ) and N + ±C±C±O s 2 ) dihedral angles Scheme 1). Starting from an early X-ray study of Canepa et al. [5] and So- rum [31], several techniques, including NMR [10, 12±15, 19, 21], X-ray [5, 7, 8, 31±38], electron diraction [39] and Raman spectroscopy [20], were used for the deter- mination of the ACh + structure. From a theoretical point of view, many computational methods i.e. ab initio, semiempirical, empirical and molecular dynamics, MD, methods) were employed [4, 9, 11, 16±18, 22±30, 40, 41] in order to ascertain its conformational beha- viour. However, notwithstanding this impressive num- ber of works, many problems remain to be clari®ed and Correspondence to: N. Russo e-mail: nrusso@unical.it Regular article Molecular dynamics, density functional and second-order Mùller±Plesset theory study of the structure and conformation of acetylcholine in vacuo and in solution T. Marino 1 , N. Russo 1 , E. Tocci 2 , M. Toscano 1 1 Dipartimento di Chimica and Centro di Calcolo ad Alte Prestazioni per Elaborazioni Parallele e Distribuite-Centro d'Eccellenza MURST, UniversitaÁ della Calabria, 87030 Arcavacata di Rende, Italy 2 IRMERC-CNR-UniversitaÁ della Calabria, 87030 Arcavacata di Rende, Italy Received: 4 April 2001 / Accepted: 5 July 2001 /Published online: 30 October 2001 Ó Springer-Verlag 2001 Theor Chem Acc 2001) 107:8±14 DOI 10.1007/s002140100291