Carotenoids phase distribution

The electrochromic shift of the carotenoids is usually calibrated with K-diffusion potential in the presence of valinomycin. One problem is that the shifts observed in respiring chromatophores (where the proton electrochemical potential is predominantly in the form of a membrane potential) are much larger than those induced by the calibrating diffusion potential, so that an extensive extrapolation is required. Thus, the carotenoids in illuminated chromatophores may indicate a membrane potential in excess of 300 mV, whereas the distribution of CNS, an electrically permeant anion, in the same system only indicates 140 mV [32]. The extent of this discrepancy, and the uncertainty as to whether the carotenoids see the bulk-phase potential, or only the local electrical field within the membrane, limits the confidence with which carotenoids may be used for quantitative as opposed to qualitative potential measurements. 

A comparison of LC methods for determination of cis-trans isomers of P-carotene was made on Vydac C18 201 TP and calcium hydroxide columns (186). The purity and relative distribution of P-carotene and its isomers in several commercially available products were evaluated by HPLC on several columns using a mobile phase of methanol/water (97 3) (187). Because carotenoids and chlorophylls are very sensitive to the nature of injection solvent, such as acetone, sample-solvent interaction may give rise to distorted and even false peaks (188). Because metal column frits may damage carotenoids. Teflon column frits should be used (189). Also artifacts may be produced on the column by reactions among the carotenoids, injection solvents, and mobile phase. Losses that occur during extraction and saponification can be reduced by use of suitable antioxidants (189). 

---