Tunable optical properties of silymarin flavonolignans

• Characterization of UV/Vis absorption properties of silymarin phenolic derivatives.
• TD-DFT calculations and molecular orbital analysis rationalize pH-dependence of the (dehydro)flavonolignans.
• Under pH variation around pKa, light absorption can induce a reversible flip-flop from UVB to UVA.

Exposure to UV radiation leads to several skin disorders. In particular, this can be attributed to an increase of the rate of DNA mutations, either due to direct absorption by proteins and DNA (mainly UVB), or due to ROS generation (mainly UVA). The plant extract silymarin is a promising sunscreen agent. It contains numerous flavonolignans and other polyphenol derivatives. In this joint experimental and theoretical study, we focus on silybin (SB), silychristin (SCH), silydianin (SD), and their 2,3-dehydroderivatives (DHSB, DHSCH, DHSD) and we describe their UV/Vis absorption properties, particularly the pH-dependence. Under acidic conditions, SB, SCH and SD exhibit a main and a minor absorption band in the UVB and UVA regions, respectively. The deprotonation in basic environment reverses this trend, making the UVA-absorption band predominant. The dehydrogenation of the C2-C3 bond (DHSB, DHSCH and DHSD) also shifts the main absorption band in the UVA region in acidic environment. In this case, basic environment causes further bathochromic shift, and the absorption band lies in the Vis region. A solvatochromic analysis revealed that, whereas the protonated forms are rather insensitive to other environmental factors, absorption properties of the deprotonated compounds strongly depend on intermolecular hydrogen bonding with solvent molecules. The time-dependent density functional theory (TD-DFT) calculations and molecular orbital analysis rationalized these effects.

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