ORIGINAL RESEARCH Structure and solvation properties of aqueous sulfobetaine micelles in the presence of organic spin probes: a Molecular Dynamics simulation study Alessia Di Giampaolo • Giorgio Cerichelli • Marco Chiarini • Isabella Daidone • Massimiliano Aschi Received: 25 October 2012 / Accepted: 7 January 2013 / Published online: 2 February 2013 Ó Springer Science+Business Media New York 2013 Abstract Structural and mechanical-dynamical information on a micelle formed by 72 monomers of N-dodecyl-N,N- dimethyl-3-ammonio-1-propanesulfonate (SB 3–12) in water solution have been obtained through Molecular Dynamics simulations. Additional simulations have been carried out to evaluate whether and to what extent the structural features of the micelle are affected by the pres- ence of a spin probe, typically utilized for the experimental characterization of these, rather complex, systems. Results indicate that the micelle shows a spherical shape not heavily influenced by the presence of the probe which only produces a slight volume increase. Further analysis also indicates that the probe, although inducing scarce altera- tions at the micelle-solvent interface, heavily influences the micelle core which appears less stable (more fluctuating) and less hydrated. Finally, analysis of residence times of water indicate—in line with many previous studies—the presence of a dual population characterized by different interactions with micelle interior. Keywords Soft matter  Micelles structure  Molecular Dynamics simulations Introduction N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (Fig. 1), also termed as sulfobetaine 3–12 (SB 3–12) is a zwitterionic detergent extensively utilized for solubilizing membrane proteins in their native state. SB 3–12 tends to form micelles which, although for- mally neutral, are able to incorporate anions [1–3] and because of the absence of a net charge, have been recog- nized as especially stable against external variations par- ticularly high electrolyte formulations and temperature [4]. Furthermore, these micelles have demonstrated excellent compatibility with other surfactants, mixtures of which show interesting synergistic effects [5–8]. They are also characterized for having chemical stability both in acids and alkalis [9]. These surfactants have recently attracted increasing attention owing to their wide applicability and their increased commercial uses. 1 Among water-soluble surfactants, zwitterionics, as surfactant class, are charac- terized by having excellent dermatological properties and low eye irritation, therefore well suited for use in shampoos and other personal care products [10, 11]; in addition, they are also used as antistatic agents, textile softeners, corro- sion inhibitors, and foam depressants. Similarly to any other aggregate of amphiphilic mole- cules in solution, there is a huge interest in determining the actual size and structure of these micelles, which may sometimes represent a rather difficult task. As a matter of fact the structures of these aggregates are the result of a This study is dedicated to Prof. Aldo Domenicano distinguished scientist, incomparable teacher and dear friend on the occasion of his 75th birthday. Electronic supplementary material The online version of this article (doi:10.1007/s11224-013-0207-3) contains supplementary material, which is available to authorized users. A. D. Giampaolo  G. Cerichelli  I. Daidone  M. Aschi (&) Dipartimento Di Scienze Fisiche e Chimiche Universita’ l’Aquila, Via Vetoio s.n.c., 67100 L’Aquila, Italy e-mail: aschi@caspur.it M. Chiarini Dipartimento di Scienze degli Alimenti Universita’ di Teramo, Viale Crucioli 122, 64100 Teramo, Italy 1 See the series of papers Langmuir 1991, 7, 842. 123 Struct Chem (2013) 24:945–953 DOI 10.1007/s11224-013-0207-3