Transient Absorption of Chlorophylls and Carotenoids after Two-Photon Excitation of LHCII

Femtosecond transient absorption spectroscopy following two-photon excitation (2PE) is used to determine the contributions of carotenoids and chlorophylls to the 2PE signals in the main plant light-harvesting complex (LHCII). For 2PE, excitation at 1210 and 1300 nm was used, being within the known 2PE profile of LHCII. At both excitation wavelengths, the transient absorption spectra exhibit a shape characteristic of excited chlorophylls with only a minor contribution from carotenoids. We compare the 2PE data measured for LHCII with those obtained from 2PE of a lutein/chlorophyll a mixture in acetone. We estimate that although the 2PE cross section of a single carotenoid in acetone is similar to 1.7 times larger than that of a Chl a, due to the 1:3.5 carotenoid/Chl ratio in LHCII, only one-third of the absorbed 2PE photons excite carotenoids in LHCII in the 1200-1300 nm range.

Automated summary

Femtosecond transient absorption spectroscopy was used to investigate the contributions of carotenoids and chlorophylls to the two-photon excitation signals in the plant light-harvesting complex LHCII. Results showed that chlorophylls had a more significant role in the 2PE signals compared to carotenoids in LHCII. Additionally, due to the ratio of carotenoids to chlorophylls in LHCII, only a small fraction of absorbed 2PE photons would excite carotenoids.