Investigation on the structure of liquid N-methylformamide–dimethylsulfoxide mixtures João M.M. Cordeiro ⇑ , Alan K. Soper ISIS Facility, STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot, Oxon OX11 0QX, United Kingdom article info Article history: Received 16 November 2010 In final form 6 January 2011 Available online 12 January 2011 Keywords: Neutron diffraction Peptide bonds Hydrogen bonds Non-aqueous solvents abstract The structures of liquid mixtures of N-methylformamide (NMF) and dimethyl sulfoxide (DMSO) at two concentrations (80% and 50% NMF) are investigated using a combination of neutron diffraction aug- mented with isotopic substitution and empirical potential structure refinement simulations. The results indicate that the NMF and DMSO molecules are hydrogen-bonded to one another with a preference for NMF–DMSO hydrogen bonding, compared to the NMF–NMF ones. The liquid is orientationally structured as a consequence of these hydrogen bonds between molecules. NMF–DMSO dimers are very stable spe- cies in the bulk of the mixture. The structure of the dimers is such that the angle between the molecular dipole moments is around 60°. The NMF molecules are well solvated in DMSO with potential implications for peptides solvation in this solvent. Published by Elsevier B.V. 1. Introduction Understanding the interactions between molecules in liquid mixtures is essential to understand their molecular behavior. By looking into mixtures consisting of liquids that have properties of biological interest, the behavior of equivalent structures in relevant biological systems may be captured. One of the most important structures related to those systems is the peptide bond, common in peptides, proteins and other biological molecules. As water is the natural biological solvent, many functions of biological molecules are related to its interactions with water molecules, and water plays an important role in many biological processes, as for instance, the enzymatic activity. However, in recent years it has been recognized that significant biological enzymatic activity may exist in media other than the aqueous environment [1]. Because of that, enzymology in non-aqueous media has received a great deal of attention on account of the substantial implications regarding basic protein science as well as biotechnology [2,3]. As a consequence, many of the facts influencing catalysis in non- aqueous solvents are now known [4–11]. Notwithstanding, as this is a new field of study, there are still many points to be clarified about the molecular events that occur in enzymes when perform- ing catalysis under non-physiological conditions. Molecular simulations methods are powerful tools to investi- gate molecular interactions in liquid systems [12–22]. From these studies knowledge about the structure, solvent effects, making and breaking structure effects, hydrogen bonds (H-bonds) and other details of the liquids have emerged. However, in the case of biolog- ical molecules, as for instance enzymes, it is difficult to start by looking at the real molecules, due to the great computational effort required by the simulation of such a system [23]. Because of this, computationally simpler systems are useful models in order to investigate the behavior of a particular feature of an interesting system, and the acquired knowledge can then be extended to the actual system. Liquid N-methylformamide (NMF–HCONHCH 3 –MW = 59 g/mol), is one of the simplest molecules that includes the OCNH peptide bond in its structure. NMF, pure or in aqueous solutions, has been the subject of study in recent years, theoretically [24–34] as well as experimentally [24,32–38]. These molecules act as proton donors and acceptors via their C@O and NAH groups and consequently form C@OHAN H-bonds with each other, the same type of H- bond that is known to play an important role in the stabilization of the ordered intramolecular structure of peptides and proteins in aqueous solutions [39]. Moreover, these molecules could also form the so called CAHO@C weak H-bonds, which might be of importance in the stabilization of many systems, as it has been extensively reported [40]. Some of the most important properties of NMF, which make it a popular organic solvent, are its very high dielectric constant (e = 182 at 25 °C) [41], dipole moment l = 3.86 D [42] or 3.78 D [43], boiling point 199.5 °C, and heat of vaporiza- tion of 13.77 kcal/mol [44]. Such large values for these properties, for a molecular liquid, have been attributed to the H-bonded net- work throughout the liquid. On the other hand, dimethylsulfoxide (DMSO–(CH 3 ) 2 SO– MW = 78 g/mol) is a polar aprotic liquid widely used as organic 0301-0104/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.chemphys.2011.01.003 ⇑ Corresponding author. Address: Unesp – Univ Estadual Paulista, Depto de Física e Química, Av. Brasil, 56, 15385-000 Ilha Solteira, SP, Brazil. Tel.: +55 18 3743 1064, fax: +55 18 3742 4868. E-mail address: cordeiro@dfq.feis.unesp.br (J.M.M. Cordeiro). Chemical Physics 381 (2011) 21–28 Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys