The oxidation of phosphorus by sulfur at low temperatures (<100
°C) has been shown to produce complex mixtures
that include 12 of the 17 known binary phosphorus sulfides. Two of
these 17 sulfides have been observed in
these mixtures for the first time. The rate-determining and first
step in the reaction appears to be the formation
of the S8 diradical. This proposal is supported by the
observed rate at which the sulfides are formed and the
distribution of the sulfide product stoichiometries.
Photoinitiation of this oxidation at 0 °C produces a
similar
array of sulfides. The differences in the product distributions
between the thermal and photochemical processes
facilitate the understanding of the mother−daughter relationships
between the products. The effects of oxidation
by sulfur and reduction by phosphorus have been determined for several
of the known phosphorus sulfides. The
characterizations of phosphorus compounds in molten mixtures of
phosphorus and sulfur were performed by 31P
NMR and Raman spectroscopy directly on the reaction
mixtures.
The electrochemical redox behavior of Cu(II) has been investigated at glassy carbon electrode (GCE) in Britton-Robinson (BR) buffer as well as in a mixture of BR buffer and potassium chloride media. It is apparent that BR buffer can act as a supporting electrolyte and modify the electrochemical behavior of Cu(II) ion in aqueous medium. In BR buffer medium, Cu(II) undergoes one two-electrons redox process, while in a mixture of BR buffer and KCl media it follows two one-electron electrochemical reaction routes. Because of the formation of hydrogen bridge bond between the supporting electrolyte and GCE surface, the Cu(II) ion follows Cu(II)/Cu(0) electrochemical reaction path, while in presence of KCl such a hydrogen bridge bond is not formed and it undergoes successive Cu(II)/Cu(I) and Cu(I)/Cu(0) electrode reaction processes. Moreover, the linear variation of peak current with the square root of scan rate indicates that the electrochemical redox processes are diffusion controlled.
The oxidation of elemental phosphorus (P4) by elemental sulfur (S8) has been studied by a combination of 31P nmr and Raman spectroscopic techniques. At temperatures below the auto-ignition temperature, a molten mixture of the elements gives rise to a complex mixture of products which are dominated by phosphorus sulfides which have a high S/P4 ratio. Both the rate at which the products are formed and their distribution support the formation of a reactive P4S8 intermediate which then cascades through a series of reactions, incorporating the sulfur atoms into the phosphorus cage. Two new phosphorus sulfides (of composition P4S6 and P4S8) have been identified in these product mixtures.
INTRODUCTION
EXPERIMENTAL PROCEDUREAll reactions were performed either under vacuum or dry, oxygen free nitrogen using standard Schlenk techniques. The purity of the phosphorus (commercial vs. distilled) did not appear to affect the outcome of the chemistry.The Raman spectroscopy experiments were performed by observing reaction mixtures through a quartz flat sealed to the bottom of a mechanically stirred quartz reaction vessel. The reaction zone was a cylinder 2 cm long and 1 cm in diameter , heated with nichrome wire wound around the cylinder. The spectra were recorded using a fiber optic probe placed in contact with the quartz flat. The reaction was profiled by recording spectra at regular intervals over the course of 8-24 hours, depending on the mixture and the reaction temperature. 31P nmr spectroscopy was performed at 300 and 400 MHz (proton frequency) in standard 5 mm tubes. All reaction mixture nmr tubes were sealed under moderate vacuum (10-12 mmHg total pressure).
Abstract. The present manuscript provides a detailed physicochemical and thermodynamic characterization of 9-aminocamptothecin (9AC) which can be used as a tool to develop novel formulation strategies for optimum pharmacological activity. The pK a of 9AC was determined to be 2.43 at 37°C, while the basicity of quinoline nitrogen of 9AC was found to decrease with increasing temperature due to a positive enthalpy of deprotonation of 10.36 kJ mol −1. The equilibrium solubility as well as the intrinsic solubility of the drug was found to increase with increasing temperature and decreasing pH. The enthalpies of solution of unionized and ionized forms of 9AC obtained from isothermal and iso-pH equilibrium solubility measurements were found to be 36.01 and 24.72 kJ mol −1 , respectively.Equilibrium hydrolysis studies revealed the hydrolytic susceptibility of 9AC with only 14% of active lactone species remaining at physiological pH 7.4. The intrinsic partition coefficient log P of the free base, 9AC-lactone, was estimated to be 1.28 (a characteristic of molecules suitable for oral absorption). The estimated pK a and log P values of 9AC, combined with its increased solubility at lower pH, are features that can be utilized to develop novel drug delivery systems to optimize the antitumor activity of 9AC.
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