The Hofmeister Series

This concept was established by Hofmeister [93], pertaining to precipitation of egg albumin by a series of aqueous sodium salts with various anions, of which increasing concentrations were required with diminishing effectiveness along the series  

When a surface is present as in colloidal solutions, to which essentially all physiological systems belong, it is justified to deal with the observed phenomenology in terms of the Hofmeister series. This concept was established in the late 19th century by Hofmeister (1888) as the series of aqueous sodium salts with various anions of which increasing concentrations were required in order to precipitate egg albumin from the solution. The series he presented was  

---

Note Some of Grahame s values for and included in this table. For a common cation, the sequence of anions in order of increasing adsorption is similar to that of the Hofmeister series in coagulation studies, and it is evident that specific adsorption properties are involved. 

Cacace M G, Landau E M and Ramsden J J 1997 The Hofmeister series salt and solvent effects on interfacial phenomena Q. Rev. Biophys. 30 241-78... 

S ts can be used to precipitate proteins by salting out effects. The effectiveness of various salts is determined by the Hofmeister series, with anions being effective in the order citrate > PO4" > SO4" > CH3COO > Cl > NO3 , and cations according to NH4 > > Na ... 

J.M. Roberts, A.R. Diaz, D.T. Fortin, J.M. Friedle and S.D. Piper, Influence of the Hofmeister series on the retention of amines in reversed-phase liquid chromatography, Anal. Chem., 74 (2002) 4927-4932. 

With the help of selectivity coefficients, such as in Eqs. (4.51) and (4.52) a general order of affinity can be given. For most clays the Hofmeister series (the same series as was given in Eq. (2.17) for the affinity of ions to oxide surfaces)... 

A number of studies have reported the abilities of different eluting salts to produce both different resolution and different selectivities.17 23 As expected, polyvalent ions are stronger eluters than monovalent ions. Some studies have suggested a correlation between eluting ability and ranking in the Hofmeister series. Others fail to observe this correlation, but nevertheless note differences in elution behavior. Overall, it is impossible to predict which eluting ions, if any, may produce a... 

An empirically derived relationships that describes the systematic effects of different neutral salts on the solubility of proteins. Collins and Washabaugh indicate that the order of ionic species eluding from a Sephadex G-10 column corresponds to the known order of effectiveness ions in the Hofmeister series on protein solubility ... 

Substances destroying the water structure, fhe so-called chao-tropic substances such as urea, guanidinium hydrochlorid, or some organic solvents such as mefhanol, efhanol, or acetonitrile, suppress hydrophobic interactions. Ions also alter the water structure. The power of destruction of water structure is given by the Hofmeister series ... 

Differences in both the charge density and the hydration of ions determine the Hofmeister series (lyotropic series).91... 

The retention caused by different salts in equal concentration is in accordance with the Hofmeister series. The higher the molar surface tension increment of a salt, the greater is its effect on protein retention 55). Higher column temperatures produce prolonged retention in HIC, in contrast to RPC where retention decreases with increasing temperature56). 

A summary of the transition temperatures of RNase in the presence of a variety of electrolytes is given in Table XX 361, 362). The effects of the usual salts follow the Hofmeister series. The remarkable difference in denaturing power of the various guanidinium salts is quite general. The sulfate actually tends to stabilize slightly while the thiocyanate is a more powerful denaturant than the chloride. Irreversible denaturation is markedly inhibited by spermine 363). There is no clear correlation between stabilizing effects and the complex inhibition curves obtained by Wold (see Section VI,E,2). 

Stability of an enzyme is usually understood to mean temperature stability, although inhibitors, oxygen, an unsuitable pH value, or other factors such as mechanical stress or shear can decisively influence stability (Chapter 17). The thermal stability of a protein, often employed in protein biochemistry, is characterized by the melting temperature Tm, the temperature at which a protein in equilibrium between native (N) and unfolded (U) species, N U, is half unfolded (Chapter 17, Section 17.2). The melting temperature of a protein is influenced on one hand by its amino acid sequence and the number of disulfide bridges and salt pairs, and on the other hand by solvent, added salt type, and added salt concentration. Protein structural stability was found to correlate also with the Hofmeister series (Chapter 3, Section 3.4 Hofmeister, 1888 von Hippel, 1964 Kaushik, 1999) [Eq. (2.18)]. 

Building Quantitative Models for the Hofmeister Series and Cohn-Edsall and Setschenow Equations... 

While the Hofmeister series and the Cohn-Edsall and Setschenow equations are useful tools for the estimation of protein stability and precipitation behavior, their usefulness is limited because of the lack of a quantitative relationship to molecular or solution properties. The goal of past and current efforts is to quantify the Hofmeister series and to predict the constants Ks and /3 (or Ks and log [E]0) in the Cohn-Edsall or Setschenow equations, respectively. Some of the most relevant efforts focus on ... 

A. Aroti, E. Leontidis, M. Dubois, and T. Zemb. Effects of monovalent anions of the Hofmeister series on DPPC lipid bilayers part I Swelling and in-plane equations of state. Biophys../., 93(5) 1580-1590, 2007. 

K. D. Collins. Ions from the Hofmeister series and osmolytes Effects on proteins in solution and in the crystallization process. Methods, 34(3) 300-311, 2004. 

The affinity is influenced by salt concentration in accordance with the Hofmeister series. It is increased in the presence of lyotropic anions such as sulphate and decreased in the presence of chaotropic anions at the other end of the series such as thiocyanate. Anions from the middle of the scale such as chloride have little effect on binding. 

In considering the differential or selective binding of one anion or another, the intrinsic properties of anions mean that we are not on a level playing field it is considerably easier to bind some anions than others. In general, in the absence of specific chemical recognition between anion and host, anion binding selectivity, particularly in solvent extraction experiments or in the detection of anions by membrane-based ion selective electrodes, follows the order of anion hydrophobicity. This order is termed the Hofmeister series, or lyotropic series and was first outlined in 1888 from experiments based on the... 

The majority of the devices mentioned thus far rely on the Hofmeister series for anion selectivity. However, for anions that deviate from this series, organometallic receptors can be utilised. The type of ligand or metal centre will influence the sensor selectivity due to the characteristics of the electron acceptance of the complex. An interesting development that is being explored here is the use of calixarenes. These have previously found use as cation-selective species, but with suitable substitution are now being incorporated within anion-selective devices. Compounds suitable as receptors for halides [61],benzoate [61] and acetate [62] have been developed. Reinhoudt and his co-workers have reported the production of a POj-selective CHEMFET based on a uranyl cation immobilised within a salophene ligand (Fig. 5), which shows selectivity over more lipophilic anions such as Br" and NOj [63]. 

Measurement of the inactivation rates of AMDH were performed under various conditions of salt types, salt concentrations, temperatures, and buffers (Mevarech and Neumann, 1977 Pundak et al, 1981 Zaccai et al., 1986b, 1989 Hecht andjaenicke, 1989a). It was found that (1) the inactivation process is of first order (which means that only one active form of the enzyme exists), (2) the rate constant for inactivation increases as the salt concentration decreases, (3) the temperature dependence of the rate constants of inactivation depends on type of salt, and (4) the dependence of the rate constants on salt type follows the Hofmeister series (von Hippel and Schleich, 1969), being lower for salting-out salts. The different models for the role of the salts in the stabilization of the AMDH will be discussed in Section IV,G. 

Several of the polymer membrane anion-selective electrodes described in the literature use quaternary ammonium salts as ion carriers (ionophores) (7). These electrodes respond according to the Hofmeister series (CIO4 > SCN > I > NO3 > Br - N3 > NC>2 > Cl > HCO3 acetate) (2, 5), which is the order of relative lipophilicity of the anions. Therefore, in strict terms, electrodes that respond according to this series could be considered "nonselective". 

A hydrophobic cobyrinate (Figure 2, structure 2) was used to prepare solvent polymeric membranes (10). The typical membrane composition was 1% (w/w) ionophore, 66% (w/w) plasticizer and 33% (w/w) polymer. Electrodes prepared with this ionophore, dioctyl sebacate (DOS) and poly(vinyl chloride) (PVC) presented, at pH 6.6, the selectivity pattern shown in Figure 3. The response of the electrodes was near-Nernstian for salicylate, thiocyanate, and nitrite. Their selectivity behavior clearly deviates from that of the Hofmeister series, with nitrite being the anion that presents the larger deviation. 

ISE Based on a Hydrophobic Cobalamin. Cobalamin 4 (Figure 2) is a hydrophobic vitamin B12 derivative that retains a proximal base (an imidazole ring), which provides the molecule with a resemblance to the original vitamin (Figure 1). The selectivity pattern of electrodes prepared with ionophore 4 at pH 5.5 is shown in Figure 4. These electrodes exhibit an anion selectivity pattern that differs from that of the Hofmeister series and from that of the previously reported cobyric acid derivatives (see section above). Indeed, electrodes based on 4 are selective for... 

The potentiometric behavior of electrodes based on these films was studied (Figure 8). These ISEs presented sub-Nemstian slopes for thiocyanate (from -40 to -53 mV/decade, depending on the buffer used), and had detection limits of 5xl0 7 M. The response time of the electrodes was typically less than 25 s. The selectivity pattern observed was thiocyanate > perchlorate > iodide > nitrite - salicylate bromide > chloride > bicarbonate > phosphate. This anion-selectivity behavior does not follow the Hofmeister series, with thiocyanate and nitrite being the ions that deviate the most from it. This indicates that there is a selective interaction of the immobilized porphyrin with the two anions. 

---