Effect of electrolytes and temperature on dications and radical cations of carotenoids: Electrochemical, optical absorption, and high-performance liquid chromatography studies
The effect of supporting electrolytes and temperature on the behavior of dications and radical cations of carotenoids is studied. Cyclic voltammograms (CVs) of canthaxanthin (I) at 23 and {minus}25 C show that Car{sup sm{underscore}bullet+} of I has similar stability during the time of the CV scan, when using tetrabutylammonium perchlorate (TBAPC), tetrabutylammonium tetrafluoroborate (TBATFB), or tetrabutylammonium hexafluorophosphate (TBAHFP) as supporting electrolyte. However, the stability of Car{sup 2+} decreases when using TBAPC or TBATFB; {beta}-carotene (II) shows similar behavior. The CV of I at {minus}25 C shows a strong cathodic wave (wave 6) near {minus}0.15 V (vs Ag) with an intensity about half that of the neutral oxidation wave when TBAPC or TBATFB is the supporting electrolyte. When TBAHFP is used, wave 6 (ca. {minus}0.05 V vs Ag) is ca. 8 times weaker than when TBAPC or TBATFB is used. This wave results from the reduction of a species that may be a decay product of Car{sup 2+} of I. Results show that these electrolytes commonly used in electrochemical studies may affect the studied systems to different extents. In simultaneous bulk electrolysis (BE) and optical absorption spectroscopic measurements, the absorption band of Car{sup 2+} of I in the presence of 0.1 M TBAHFP can be observed by lowering the BE temperature to {minus}20 C. In the presence of 0.1 M TBAPC or TBATFB, this band is not observed, even at {minus}50 C. Isomerization of neutral I (as shown by HPLC and its blue absorption band shift) is observed only when the Car{sup 2+} absorption band is absent during BE. This observation, along with an increase of the neutral absorption band after stopping BE, suggests that the equilibrium Car + Car{sup 2+} {r{underscore}equilibrium} 2Car{sup {sm{underscore}bullet}+} is shifted to the left because Car{sup 2+} decays more quickly than Car{sup {sm{underscore}bullet}+} in the presence of electrolyte and this is a major path for formation of cis neutral species from cis Car{sup {sm{underscore}bullet}+}. The optical absorption of Car{sup 2+} of I at 18 C in the presence of 0.1 M TBAHFP is obtained, which suggests that Car{sup 2+} of I is not as unstable at room temperature in the presence of TBAHFP as was thought before.
- Research Organization:
- Univ. of Alabama, Tuscaloosa, AL (US)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- FG02-86ER13465
- OSTI ID:
- 20013652
- Journal Information:
- Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical, Vol. 103, Issue 47; Other Information: PBD: 25 Nov 1999; ISSN 1089-5647
- Country of Publication:
- United States
- Language:
- English
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