Ternary mutual diffusion coefficients of aqueous {L-dopa (1) + b-CD (2)} solutions at T = 298.15 K Marisa C.F. Barros a,b , M. Luísa Ramos a , Hugh D. Burrows a , Miguel A. Esteso b , Derek G. Leaist c , Ana C.F. Ribeiro a,⇑ a Department of Chemistry and Coimbra Chemistry Centre, University of Coimbra, 3004-535 Coimbra, Portugal b U.D. Química Física, Universidad de Alcalá, 28871 Alcalá de Henares, Madrid, Spain c Department of Chemistry, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada article info Article history: Received 22 May 2015 Received in revised form 16 June 2015 Accepted 18 June 2015 Available online 26 June 2015 Keywords: Levodopa b-Cyclodextrin Diffusion coefficient NMR spectroscopy Aqueous solutions abstract Ternary mutual diffusion coefficients (D 11 , D 22 , D 12 and D 21 ) measured by the Taylor dispersion method are reported for aqueous solutions of {levodopa (L-dopa) + b-cyclodextrin (b-CD)} solutions at T = 298.15 K and concentrations up to 0.007 mol  dm 3 . Significant effects on the diffusion were observed, suggesting interactions between this carbohydrate and L-dopa. Support for this came from 1 H NMR spectroscopy, which shows that these effects are due to formation of 1:1 (b-CD:L-dopa) complexes. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Levodopa, also called L-dopa (l-3,4-dihydroxyphenylalanine), is a precursor of the neurotransmitter dopamine, and one of the most effective neurological drugs used in the treatment of patients with Parkinson’s disease [1–4] (scheme 1). Electrochemical sensors have been developed for quantification of this drug in recent years, which use a b-cyclodextrin/poly (N-acetylaniline) [5]. For example, Aslanoglu et al. [6] proposed a b-cyclodextrin doped poly(2,5-diami nobenzenesulfonic acid) modified glassy carbon electrode for detection of the L-dopa/cyclodextrin species. However, although a considerable amount of work has been done on the development of systems containing this drug [7–10], its transport behavior is still poorly understood. Because this infor- mation is essential for the design of those systems, we have carried out a comprehensive study of the diffusion of this drug in aqueous solutions containing b-cyclodextrin as a carrier (scheme 2). Specifically, we have measured multicomponent chemical ternary diffusion coefficients (D 11 , D 22 , D 12 and D 21 ) for aqueous solutions {L-dopa (1) + b-CD (2)} determined in the concentration range (0.50 to 7.00) 10 3 (mol  dm 3 ) for each component at T = 298.15 K, using the Taylor dispersion technique [11–14]. Results from 1 H NMR spectroscopy show interactions between this carbohydrate and L-dopa due to formation of 1:1 (b-CD/:L-dopa) complexes. Relevant information was obtained about the influence of the macromolecular solute b-CD on the diffusion of L-dopa from a comparison between the main (D 11 and D 22 ) and binary diffusion coefficients (D 1 and D 2 ), and from the values of the cross diffusion coefficients (D 12 and D 21 ). Coupled diffusion of these components, indicated by cross-diffusion coefficients D 12 and D 21 , is analyzed using a reliable literature model [15–18]. 2. Experimental 2.1. Materials L-dopa (Fluka purum mass fraction purity P 99%) and b-cyclodextrin (b-CD) (Sigma Aldrich; water mass fraction 0.131; molar mass M = 1135.00 g  mol 1 ) were purchased from Sigma, Germany, with water mass fraction of 0.131, were used as received (table 1). Aqueous solutions were prepared using Millipore-Q water (diffusion measurements) and D 2 O (NMR spectroscopy). All solutions were freshly prepared at t = 25.0 °C before each experi- ment. Water content in b-CD (i.e., mass fraction 0.131) was accounted upon solution preparation. pH of the solutions are those of dissolution. That is, pH 6.3 and 6.8 for solute fraction of L-dopa X 1 = 0 and X 1 = 1, respectively. Under these conditions and from the pK values [19] pK a1 = 2.3, pK a2 = 8.11, pK a3 = 9.92, we can consider that this drug is present mostly in the zwitterionic form. http://dx.doi.org/10.1016/j.jct.2015.06.022 0021-9614/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +351 239854460; fax: +351 239 827703. E-mail address: anacfrib@ci.uc.pt (A.C.F. Ribeiro). J. Chem. Thermodynamics 90 (2015) 169–173 Contents lists available at ScienceDirect J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct