John Stuehr

Laser Temperature‐Jump Apparatus for Relaxation Studies in Electrolytic Solutions

Temperature‐jump apparatus for relaxation studies in electrolytic solutions has been developed with laser heating and conductivity readout. The apparatus is suitable for quantitative studies of relaxation times in the range 10−1 to 3×10−6 sec. With a ruby laser, an appropriate solute must be added to aqueous solutions lacking a colored solute to adjust the optical density of the solution to an appropriate value. With a neodymium laser, the intrinsic absorption of water in aqueous solutions is adequate.

Ultrasonic Investigation of the Rate of Hydrolysis of Potassium Cyanide

The kinetics of the hydrolysis of the cyanide ion have been studied by ultrasonic absorption measurements over the frequency range 30 kc to 25 Mc. A single relaxation frequency has been observed which depends on concentration in the manner expected theoretically for the hydrolysis reaction. The relaxation frequency at 25°C is 450 kc for a 0.01M solution and 950 kc for 0.05M. The corresponding forward and reverse rate constants calculated for the reaction CN—+H2O⇌HCN+OH— are k1=5.2×104 sec—1 and k—1=3.7×109 M—1 sec—1 at 25°C. From the magnitude of the maximum excess acoustic absorption per wavelength (90×10—6 at 25°C for the 0.05M solution), the partial molal volume change ΔV° for the process is calculated to be — 12.4 cm3/mole. The temperature dependence of the relaxation frequency indicates an energy of activation for the process HCN+OH—→CN—+H2O of 6±2 kcal/mole.

Ion association and ultrasonic relaxation in hydrogen-bonding solvents: 1:1 Electrolytes in 2-propanol at 25°C

Ultrasonic absorption measurements have been carried out in aqueous solutions of Bu4NCl, Bu4NBr, Bu4NI, Me4NCl, and LiCl in 2-propanol over the frequency range 15–250 MHz. A single relaxation frequency was observed for each system. All the data were found to be consistent with a 2-step association mechanism in which the rate-determining step was the diffusion of the ions to form a solventseparated ion pair, followed by rapid desolvation to form the contact ion pair. Measured association rate constants were very nearly those predicted for a diffusion-controlled process.

A Technique for Reactant Introduction in Measurement of Rapid Reaction Rates

A simple technique is reported for rapid introduction of solid or liquid reactants into solution for continuous observation of ensuing reactions during ≥1 sec. The accuracy of the method is attested by congruent temperature dependence of the derived rate constants together with those from an independent procedure. The new technique allows measurement of solvolysis reactions considerably faster than those heretofore accessible.

Ultrasonic Absorption in Electrolytic Solutions

Ultrasonic absorption has been measured in various electrolytic solutions which have not been reported upon previously in the literature. Specific electrolytes include potassium cyanide, cadmium sulfate, chromic sulfate, potassium ferro‐ and ferricyanides, and stannous sulfate. Measurements have been made by the reverberation‐resonator method in both 12‐ and 50‐liter spherical resonators at frequencies from 40 to 230 kc. Cadmium sulfate and potsssium cyanide show abnormal absorption with relaxation frequencies above the range of the present measurements. In addition results will be reported for magnesium sulfate solutions in which the ionic interactions associated with the ionic atmosphere concept have been buffered out in good part by the use of large concentrations of acoustically nonactive, uni‐univalent electrolytes. The theoretical significances of these results will be discussed. [This research has been supported by the Office of Naval Research under Contract Nonr 1439(04).]

A rapid kinetic study of divalent metal interactions with flavin coenzymes

Abstract Research in these laboratories has focussed in recent years on the kinetics of divalent metal ion interactions with coenzymes. The principal kinetic tool has been temperature-jump relaxation spectroscopy. A large amount of kinetic information is now available for several nucleotides (e.g. AMP) and inorganic phosphates. The purpose of this paper is to report the first rapid kinetic study of the mechanism of divalent metal ion interactions with the coenzymes flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). The two compounds are structurally related to each other as well as to other coenzymes and phosphates that we have previously studied. FAD, for example, is structurally a combination of riboflavin phosphate and adenosine monophosphate (AMP). Ni(II) was chosen as the metal ion for these studies because of the large body of kinetic information that is already available for that ion. It can serve as a useful representative of divalent transition metal ion interactions with these and related coenzymes. The NiFMN system. Two relaxation effects were observed in the kinetic experiments: one (τ 1 ) on the order of 0.2 msec, the other (τ 2 ) at about 2 msec. The detailed concentration and pH dependencies of τ 1 and τ 2 are quite similar to those for the relaxation times found in the Ni-ribose phosphate and NiAMP systems respectively. The mechanism consistent with these observations is a dual-pathway, back-bound complex mechanism, shown schematically as I; in which NiL is the phosphate-bound complex, NiL′ the phosphate + base bound complex. The NiFAD System . FAD presents a number of different binding sites in the ionized phosphate bridge and the base nitrogens on both the adenine and isoalloxazine rings. The flexibility of this molecule facilitates both individual and simultaneous ring interactions with the phosphate-bound metal. The NiFAD system is unique in that four distinct relaxation times, τ–τ 2 , were found and characterized. The relaxation times ranged from 90 μsec to 20 msec and were found to be only slightly pH and concentration dependent. Based on the large body of prior data from our laboratory on simple nucleotide systems, we were able to associate specific relaxation times with reaction steps in scheme II and to determine the rate constants. The mechanism shown as II quantitatively accounts for the number and behavior of all the relaxations steps. In this scheme, F, P and A refer to the flavin, phosphate, and adenine moieties of the FAD molecule, respectively. The first step (1–2) involves bridging to the phosphate moiety only, followed by species involving interactions with the phosphate plus the flavin (5) or adenine moieties (3). The final complex (4) involves simultaneous interactions with all the components of the molecule.

Sound Absorption in Electrolytes

Further ultrasonic‐absorption measurements have been made at 104–108 cps in a number of electrolytes including iodides, chlorides, acetates, acetic acid, and mixtures of electrolytes. A number of these electrolytes exhibit interesting relaxation effects, which have been interpreted quantitatively in terms of specific ion‐ion and ion‐solvent interactions. Reexamination of several systems for which ultrasonic relaxation effects have been reported by other workers, however, has failed to confirm these relaxation effects. These surprising findings reflect the need for greater acuracy in absorption measurements, particularly at frequencies below 5 Mc/sec, and the need for caution among acousticians to prevent overinterpretation of their data under conditions where statistical fluctuations as well as consistent errors are substantial. In addition, several instances are cited where relaxation effects reported in the literature have been confirmed but the explanations are not compatible with accepted concepts of ...

Ultrasonic Absorption in Cyanide Solutions

Ultrasonic absorption measurements have been made in aqueous potassium cyanide solutions by the spherical resonator, statistical reverberation, and pulse‐reflector techniques at frequencies from 40 kc to 25 Mc. Only one relaxation frequency has been found in this range. For a KCN concentration of 0.05 mole/liter at 25°C, the relaxation frequency is 900 kc with an excess absorption per wave length (αλ) at this frequency equal to 9.0×10−5. The dependence of the absorption on concentration also has been measured. The experimental data support the conclusion that an hydrolysis reaction involving the reaction of the cyanide ion with water is responsible for the abnormal absorption. The relaxation data have been used to calculate the rate constants for this hydrolysis reaction. (This research has been supported by the Office of Naval Research.)