222 J. CHEM. RESEARCH (S), 1997 J. Chem. Research (S), 1997, 222–223† Acid–Base Equilibria of Cysteine in Artiﬁcial Sea Water: Effect of Ionic Strength on the Basis of Speciﬁc Interaction Theory† Teresa Vilari˜ no, Sarah Fiol, Isabel Brandariz, Roberto Herrero and Manuel E. Sastre de Vicente* Dpto de Qu´ ımica Fundamental e Industrial, Facultad de Ciencias, Universidad de La Coru˜ na, 15071-La Coru˜ na, Spain Artiﬁcial sea water, containing NaCl, KCl, CaCl 2 , MgCl 2 and Na 2 SO 4 , was used as a standard medium in potentiometric equilibrium studies of the sulfur-containing amino acid cysteine: the dependence of activity coefﬁcients on ionic strength, and thus salinities, is discussed according to different models based on the Speciﬁc Interaction Theory. Taking into account the relevance of protonation in biologic- ally signiﬁcant cysteine-rich ligands that take part in major metal-complexation processes, we thought it of interest to investigate the effect of ionic strength on the acid–base equi- libria of cysteine in artiﬁcial sea water (ASW), reported as a good approximation in marine chemistry studies regarding the real situation (natural sea water). 1,2 Although the ionization equilibria of cysteine have been studied by several authors, all studies in this context have been carried out at a ﬁxed ionic strength, as noted in a recent review. 3 Lately, our group has started a systematic study of this amino acid and some results obtained in a simple electro- lytic medium (KNO 3 ) have been already reported. 4 The acid– base equilibria can be represented by A 2 H AH (1) AH H AH 2 (2) AH 2 H AH 3 (3) As is shown, the fully protonated form of cysteine con- tains three dissociable protons (CO 2 H, NH 3 , SH). Clearly, the most acidic of these lies on the carboxylic group. By contrast, assigning the donating groups involved in its two most basic equilibria is less straightforward, since it is gener- ally accepted that proton ionization occurs simultaneously from the SH and NH 3 groups and the related constants result from intermingled microscopic processes. 3,5,6 Experimental The L-cysteine used was supplied by Merck (for biochemistry, 99%). The potentiometric apparatus, procedure and conditions used have been described elsewhere. 7–9 The synthetic sea water samples were prepared from NaCl, KCl, CaCl 2 , MgCl 2 and Na 2 SO 4 as electrolytes following the seawater recipe of Millero. 9,10 The salinity, S, is related to the real ionic strength by the equation 11 I = 19.9273S 1000 1.00511S (4) In Table 1 the stoichiometric equilibrium constants (molality scale) are compiled. No pK 1 * values were considered owing to the relatively high errors involved, consistent with previous ﬁndings of other authors. 4 Results and Discussion Even though a great number of interactions might be involved in such a complex medium as seawater, and taking into account the moderate values of the ionic strength in the working range studied, we considered the interactions of cysteine in artiﬁcial seawater to be given by a simple equa- tion. Fig. 1 shows the variation of pK 2 * and pK 3 * with the ionic strength as established from various models (Table 2) based on speciﬁc interaction theories (SIT). 12 The models result from empirical modiﬁcations of the Debye–H¨ uckel equation by inclusion of different terms consisting of powers of the ionic strength. We tried different values for the parameter appearing in the Debye–H¨ uckel term. Guggenheim and Tur- geon 13 proposed taking c = 1 for all solutes and letting the speciﬁc properties of each solute appear in the linear term, whilst later Scatchard 14 showed that a value a J = 1.5 led to better results at ionic strength greater than 0.1 M. As shown in Fig. 1, all the proposed equations ﬁt the log K 2 * I and log K 3 * I plots for cysteine within experimental error and lead to extrapolated values (log K T ) at 25 °C that are consistent with those recently recommended by IUPAC 3 (see Table 3). On the other hand, the plots exhibit a ﬂat minimum over the ionic strength range 0.20–0.72 mol kg 1 . The presence of a minimum is typical of equilibria of the type AHA H or similar, where a separation of electric charge occurs, 15 but not, for example, for an isocoulombic equilibrium of the type BH B H . The most simple way of accounting for the appearance of the minimum is provided by the Guggenheim equation, 13 a model already proposed empirically by Br¨ onsted in previous studies, 16 which includes the electrostatic long-range term and a linear term propor- tional to the ionic strength whose coefﬁcient is generally ascribed to the presence of speciﬁc interactions; competition between the two types of terms (speciﬁc and non-speciﬁc interactions) determines the position and amplitude of the minimum for equilibrium (3) above. In fact, a similar obser- vation, viz. a virtually negligible effect of the ionic strength (a quasi-ﬂat minimum) between 20 mM and 1 M on various deri- vatives of cysteine led Snyder 17 to propose the following rela- tion in studying the use of Br¨ onsted equations for predicting inductive effects on rate constants for the thiol–disulﬁde exchange: log K* = log K T b I 1 c I (5) The value of log K* at inﬁnite ionic strength would lead to a pK  value (log K  = log K T b/c) inﬂuenced by inductive *To receive any correspondence (e-mail: eman@udc.es). †This is a Short Paper as deﬁned in the Instructions for Authors, Section 5.0 [see J. Chem. Research (S), 1997, Issue 1]; there is there- fore no corresponding material in J. Chem. Research (M). Table 1 Stoichiometric equilibrium constants of cysteine in ASW at 25 °C, molal scale, standard deviations in parentheses S (%) I /mol kg 1 log (K 2 /mol kg 1 ) log (K 3 /mol kg 1 ) 0.50 1.05 1.45 2.00 2.50 2.90 3.50 0.10 0.21 0.29 0.40 0.51 0.60 0.72 8.149 (0.001) 8.080 (0.004) 8.073 (0.04) 8.045 (0.009) 8.042 (0.003) 8.086 (0.02) 8.07 (0.02) 10.168 (0.005) 9.945 (0.03) 9.876 (0.03) 9.844 (0.02) 9.842 (0.03) 9.831 (0.06) 9.80 (0.005) Published on 01 January 1997. Downloaded on 28/10/2014 22:04:54. View Article Online / Journal Homepage / Table of Contents for this issue