Ubiquinone has an absorbance band at 275 nm This band bleach

Ubiquinone has an absorbance band at 275 nm. This band bleaches when the quinone is reduced to either the semiquinone or the dihydroquinone. The semiquinone has an absorbance band at 450 nm, but neither the quinone nor the dihydroquinone absorbs at this wavelength. A suspension of purified bacterial reaction centers was supplemented with extra ubiquinone and reduced cyctochrome c and was then illuminated with a series of short flashes of light. The absorbance at 275 nm decreased on the odd-numbered flashes as shown on the first curve. The absorbance at 450 nm increased on the odd numbered flashes but returned to the original level on the even-numbered flashes as shown in the second curve.

Why does the absorbance at 450 nm increase on the first flash but decrease on the second flash? Make sure to explain how you arrived at your answer.

Solution

The absorbance increased on the first flash because of the presence of the thiol groups while it decreases later due to the presence of mercury(HG2+) which makes the flash rate fall continuously,