A simple artificial light-harvesting dyad as a model for excess energy dissipation in oxygenic photosynthesis.
Under excess illumination, plant photosystem II dissipates excess energy through the quenching of chlorophyll fluorescence, a process known as nonphotochemical quenching. Activation of nonphotochemical quenching has been linked to the conversion of a carotenoid with a conjugation length of nine double bonds (violaxanthin) into an 11-double-bond carotenoid (zeaxanthin). It has been suggested that the increase in the conjugation length turns the carotenoid from a nonquencher into a quencher of chlorophyll singlet excited states, but unequivocal evidence is lacking. Here, we present a transient absorption spectroscopic study on a model system made up of a zinc phthalocyanine (Pc) molecule covalently linked to carotenoids with 9, 10, or 11 conjugated carbon-carbon double bonds. We show that a carotenoid can act as an acceptor of Pc excitation energy, thereby shortening its singlet excited-state lifetime. The conjugation length of the carotenoid is critical to the quenching process. Remarkably, the addition of only one double bond can turn the carotenoid from a nonquencher into a very strong quencher. By studying the solvent polarity dependence of the quenching using target analysis of the time-resolved data, we show that the quenching proceeds through energy transfer from the excited Pc to the optically forbidden S(1) state of the carotenoid, coupled to an intramolecular charge-transfer state. The mechanism for excess energy dissipation in photosystem II is discussed in view of the insights obtained on this simple model system.
Berera
Rudi
R
Department of Biophysics, Division of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit, 1081 HV Amsterdam, The Netherlands.
Herrero
Christian
C
van Stokkum
Ivo H M
IH
Vengris
Mikas
M
Kodis
Gerdenis
G
Palacios
Rodrigo E
RE
van Amerongen
Herbert
H
van Grondelle
Rienk
R
Gust
Devens
D
Moore
Thomas A
TA
Moore
Ana L
AL
Kennis
John T M
JT
D008027:Light; D008954:Models, Biological; D010100:Oxygen / Q000378:metabolism*; D010788:Photosynthesis
D010100:Oxygen