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The detection of protamine, a polycation, by use of electrodes modified with self-assembled monolayers of thioctic acid is reported. These sensors can detect protamine concentrations as low as 0.5 μg/mL when [Ru(NH3)6]3+ is used as marker and 2.0 μg/mL when [Fe(CN)6]3- is used as marker. Binding of protamine to the thioctic acid monolayers controls the reduction rate of [Ru(NH3)6]3+ and [Fe(CN)6]3- at the electrode surface due to electrostatic attraction or repulsion between these markers and the monolayer, allowing the indirect detection of protamine. In physiological concentrations, the blood electrolytes sodium, potassium, calcium, and magnesium do not interfere. The sensors respond to protamine in diluted horse serum. They are selective for protamine over Polybrene, another polycation that neutralizes the anticoagulant activity of heparin. Protamine once bound to the electrode surface can be removed by washing with 0.1 M KCl of pH 5.1. An increase in the scan rate of cyclic voltammetry decreased the detection limit for protamine and increased the dynamic range. The sensor was used to detect the end point in heparin−protamine titrations.

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The compound PAQ, which consists of a tetra-arylporphine attached to methyl-p-benzoquinone via a single amide linkage, exhibits light-induced intramolecular electron transfer from the porphyrin excited-singlet state to the quinone at a rate which is strongly solvent-dependent. The rate constants are found to correlate well with the semiclassical Marcus theory of electron transfer, provided that the solvent effect on both the Gibbs energy change, AGO, for the electron-transfer reaction 'P*AQ -+ P'+AQ'-and the Marcus reorganisation energy, A, are considered. The AGO values are obtained from direct measurement of redox potentials in each solvent with various work-term corrections for Coulombic interaction in P'+AQ'-, and 1 is calculated from the optical and dielectric properties of each solvent. For fourteen solvents, reasonable agreement with Marcus theory is found using this approach on uncorrected AGO values and those corrected with a solventdependent work term ; a solvent-independent correction is not successful. For two solvent mixtures (acetonitrile-benzonitrile and acetonitrilechloroform), excellent agreement with Marcus theory is found using uncorrected AGO values and those corrected with a solvent-dependent work term. We have found that Weller's method for calculating A G O in various solvents from a single measurement in a reference solvent gives a poor correlation with Marcus theory, primarily because of a poor prediction of the solvent dependence of AGO.

surement conditions using the same plasticizer in all membranes. In addition, comparison is made with a 14-crown-4 ether neutral carrier as reported by Kitazawa et al. (5).Cation complexing properties of a large number of macrocyclic polyether-diester compounds have also been reported (6-19). These compounds generally form weaker complexes than the cyclic polyethers. They have been reported to show significant affinities for alkali, alkaline-earth, and primary alkylammonium cations (12,13,15,16,19). Gadzekpo et al. have reviewed the literature of lithium sensors (20). EXPERIMENTAL SECTION Reagents and Chemicals. High molecular weight poly(vinyl chloride) (PVC) was obtained from Fluka AG. Tris(2-ethylhexyl) phosphate was obtained by U. C. C. Flexoh. All solutions were