cis-Resveratrol

cis-Resveratrol is the resveratrol stereoisomer with the two aryl groups on the same side of the central double bond, corresponding to the $Z$ configuration. It has the same neutral molecular formula, ${\mathrm{C}}_{14}{\mathrm{H}}_{12}{\mathrm{O}}_3$, and formula-based molar mass as trans-Resveratrol, but a different spatial arrangement.^[ref]

The isomers need different optical response factors. Brent C. Trela and Andrew L. Waterhouse measured an ethanol absorption maximum at $288\mathrm{nm}$ with molar absorption coefficient $12600\mathrm{L}\mathrm{mo}{\mathrm{l}}^{-1}\mathrm{c}{\mathrm{m}}^{-1}$ for the cis form; their stability tests also depended on light exposure and pH.^[ref]

At that maximum, an original $10\mu \mathrm{mol}{\mathrm{L}}^{-1}$ solution in a $1\mathrm{cm}$ path has ideal absorbanceThis is lower than the trans-isomer's corresponding example at its own maximum, but the two maxima occur at different wavelengths. Dividing their peak coefficients does not supply the cis/trans response ratio at one common detector wavelength.

For a mixture at a chosen wavelength $\lambda$, an additive Beer-Lambert law model isOne reading generally cannot identify both concentrations. Two wavelengths help only if their response rows are independent and the coefficients are appropriate to the solution. Multivariate calibration makes that requirement explicit; physical separation by chromatography supplies another route. Neither equal molecular formulas nor a single colour measurement establishes isomeric purity.