I am testing various methods of extracting C-Phycocyanin from fresh Arthrospira platensis biomass. To quantify the phycocyanin concentration within my solvents after extraction, I use the equation:
Phycocyanin (mg/L) = (A620–0.474×A652)/5.34)
This equation is widely used in this context (Moraes et al., 2011; Patil et al., 2006; Zitelli et al., 2022; Vernès et al., 2015 + dozens more) and originates from a widely referenced paper in spirulina research (Bennet and Bogorad, 1973).
The original equation however, measures and absorbance value of 615, NOT 620, reading as:
Phycocyanin (mg/L) = (A615–0.474×A652)/5.34)
It appears at some point phycocyanin researchers switched to measuring 620 as opposed to 615 as standard, my guess due to a consensus with updating phycocyanins maximum absorbance peak. But all papers using the modified equation still reference the Bennet and Bogorad paper, with no acknowledgment or explanation to the adjustment. Some modern papers still use the unaltered equation when quantifying phycocyanin (Silveira et al., 2007).
If I measure the absorbance of my pyocyanin extracts at 615 and 620 I get considerably different values, inferring that comparison of extraction methods between papers that use different equations (Jaeschke et al., 2021) are likely inaccurate.
To anyone with experience quantifying phycocyanin spectrophotometrically, I assume you have wondered the same thing. If you can provide an explanation or insight, why is this widespread and unreferenced modification to the Bennet and Bogorad equation commonplace?
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