Up-converted fluorescence from photosynthetic light-harvesting complexes linearly dependent on excitation intensity

Abstract

Weak up-converted fluorescence related to bacteriochlorophyll a was recorded from various detergent-isolated and membrane-embedded light-harvesting pigment-protein complexes as well as from the functional membranes of photosynthetic purple bacteria under continuous-wave infrared laser excitation at 1064 nm, far outside the optically allowed singlet absorption bands of the chromophore. The fluorescence increases linearly with the excitation power, distinguishing it from the previously observed two-photon excited fluorescence upon femtosecond pulse excitation. Possible mechanisms of this excitation are discussed.

Vibronic Origin of the Qy Absorption Tail of Bacteriochlorophyll a Verified by Fluorescence Excitation Spectroscopy and Quantum Chemical Simulations

“…This emission, detectable even from whole bacterial cells, was linearly dependent on the excitation intensity and had the same spectral shape as the typical fluorescence accompanying the decay of the Q y excited state. Though a different explanation was favored in ref , later experiments with a tunable laser (data under preparation) indicated that the observed emission could be due to common fluorescence of excitonically coupled BChl a chromophores that were directly excited via their weak absorption tails.…”

“…Furthermore, a weak up-converted emission was recently observed at ambient temperature from the bacterial lightharvesting complexes containing multiple BChl a chromophores upon 1064 nm laser excitation, very far from the usual absorption range of these samples. 2 This emission, detectable even from whole bacterial cells, was linearly dependent on the excitation intensity and had the same spectral shape as the typical fluorescence accompanying the decay of the Q y excited state. Though a different explanation was favored in ref 2, later experiments with a tunable laser (data under preparation) indicated that the observed emission could be due to common fluorescence of excitonically coupled BChl a chromophores that were directly excited via their weak absorption tails.…”

“…However, there is mounting literature evidence, described in detail in a topical review, that certain low-energy states of chlorophyll-like molecules lead to functional operation of photosynthetic units, bypassing the established higher-energy mechanisms. Furthermore, a weak up-converted emission was recently observed at ambient temperature from the bacterial light-harvesting complexes containing multiple BChl a chromophores upon 1064 nm laser excitation, very far from the usual absorption range of these samples . This emission, detectable even from whole bacterial cells, was linearly dependent on the excitation intensity and had the same spectral shape as the typical fluorescence accompanying the decay of the Q y excited state.…”

“…It is absolutely clear from this figure that the BChl a absorption does not stop at 940 nm where the signal is over 6 orders of magnitude weaker than it is at 768.6 nm, the maximum of the Q y transition, but continues well beyond this mark. This agrees with the fact of observing the up-converted fluorescence of BChl a upon the 1064 nm laser excitation in ref . (see also Figure ).…”

Establishment of the Qy Absorption Spectrum of Chlorophyll a Extending to Near-Infrared

“…The rather large reach of absorption of photosynthetic pigments to the red has caught our attention before [ 26 ], when studying circular, BChl a -containing light-harvesting pigment–protein complexes from purple bacteria. Pigments in these complexes are closely packed, facilitating delocalization of the BChl a excited states, even at high temperatures [ 27 , 28 ], over a number of molecules.…”