Intramolecular charge-transfer state of carotenoids siphonaxanthin and siphonein: function of non-conjugated acyl-oxy group

• Published in: Photosynthesis research 2020-05, Vol.144 (2), p.127-135
• Main Authors: Staleva-Musto, Hristina, Kuznetsova, Valentyna, Bína, David, Litvín, Radek, Polívka, Tomáš
• Format: Article
• Language: English
• Subjects: Absorption spectroscopy; Acetonitrile; Acetonitriles - chemistry; Analysis; Biochemistry; Biomedical and Life Sciences; Carotenoids; Carotenoids - chemistry; Fucoxanthin; Hexanes - chemistry; Hydrogen Bonding; Life Sciences; Molecular Structure; n-Hexane; Nitriles; Original Article; Plant Genetics and Genomics; Plant Physiology; Plant Sciences; Polarity; Solvents; Solvents - chemistry; Spectrum analysis; X-Ray Absorption Spectroscopy - instrumentation; X-Ray Absorption Spectroscopy - methods; Xanthophylls - chemistry
• ISSN: 0166-8595; 1573-5079
• DOI: 10.1007/s11120-019-00694-x

We used ultrafast transient absorption spectroscopy to study excited-state dynamics of two keto-carotenoids, siphonaxanthin and siphonein. These two carotenoids differ in the presence of dodecanoyl-oxy group in siphonein, which is attached to the C19 carbon on the same side of the molecule as the conjugated keto group. We show that this dodecanoyl-oxy group, though not in conjugation, is still capable of modifying excited state properties. While spectroscopic properties of siphonein and siphonaxanthin are nearly identical in a non-polar solvent, they become markedly different in polar solvents. In a polar solvent, siphonein, having the dodecanoyl-oxy moiety, exhibits less pronounced vibrational bands in the absorption spectrum and has significantly enhanced characteristic features of an intramolecular charge-transfer (ICT) state in transient absorption spectra compared to siphonaxanthin. The presence of the dodecanoyl-oxy moiety also alters the lifetimes of the S 1 /ICT state. For siphonaxanthin, the lifetimes are 60, 20, and 14 ps in n-hexane, acetonitrile, and methanol, whereas for siphonein these lifetimes yield 60, 11, and 10 ps. Thus, we show that even a non-conjugated functional group can affect the charge-transfer character of the S 1 /ICT state. By comparison with fucoxanthin acyl-oxy derivatives, we show that position of the acyl-oxy group in respect to the conjugated keto group is the key feature determining whether the polarity-dependent behavior is enhanced or suppressed.