UV-Vis Spectrum of POM Pomegranate Juice
Below is a link to a UV-Vis spectrum of POM brand Pomegranate Juice. The graph shows two spectra- one is a simple dilution of POM-brand pomegranate juice. The other, lower extinction, spectrum was a simple dichloromethane (DCM) extraction of undiluted pomegranate juice as it comes out of the bottle. The extraction was done with a 1:1 v/v ratio of DCM to juice. Notably, the DCM extract contained no visible color. The layers emulsified and had to sit for ~10 minutes to separate. The DCM extract was dried over a bit of magnesium sulfate and filtered. The undiluted extract was submitted directly to analysis. The dashed curve is the spectrum of the extract.
What is interesting about the extract is that the absorption maxima do not align with the maxima of the whole juice. The DCM soluble fraction is quite different electronically from the balance of the components. Indeed, the extinction drops off to 0.026 by 350 nm and drops to near zero thereafter. It is important to note that the absorbance of the extract is based on a much more concentrated solution, so a direct comparison of absorbances with the highly diluted whole juice is not valid. Focus instead on the wavelength of the maxima.
I ran the spectrum of the whole juice as a 500 to 1 dilution in distilled water. No attempt was made to buffer the pH of the water or to filter the juice. I fully realize that there are experimental control issues to contend with here- i.e., pH dependence, turbidity, oxidative degradation due to air exposure, etc.
According to the literature, pomegranate juices contain varying amounts of polyphenolic, tannin-type species not just from the juice, but also material that is released from the rind in the pressing process. So further experiments should try to obtain juice that is pressed in a way to discourage the inclusion of materials from other plant tissues.
According to one source, the components of pomegranate juice can stabilize the level of PSA in men who have prostate cancer. Whether it works via the anti-oxidant properties or some other more specific interaction is unclear.
Just what is the point of running these spectra? My original interest related to the visible part of the spectrum. I wanted to know what the visible spectrum of this intensely colored juice looked like. What is evident is that for all of the extinction in the visible part of the spectrum (>350 nm), the UV band is much more intensely “colored”. That is, the extinction is much higher in the UV range (<350 nm). Why UV-Vis spectra? Because, silly, I don’t have an NMR. But I do have a UV-Vis spectrometer.
Well, that’s not quite true. I can run a proton NMR of the crude material, but given that pomegranate juice is a plant fluid, all I’m going to see is a forest of peaks. Actually, more to the point, others have isolated components from this fruit. My interest is in the reduction capacity of the pigments.
Extracting structural data from a UV-Vis spectrum is not really possible. UV-Vis spectroscopy is about electronic transitions and a wide variety of species overlap appreciably, so structural determinations of components in complex mixtures is out of the question. Furthermore, pomegranate juice is sensitive to oxidative degradation and is likely to be quite sensitive to pH (next on the agenda), so it’s thermal and O2 exposure history may be important (i.e., has it been Pasteurized, etc). So it’s back to the drawing board.