J. Phys. Chem. 1996, 100, 8573-8579
8573
Qy-Excitation Resonance Raman Spectra of Chlorophyll a and Bacteriochlorophyll c/d Aggregates. Effects of Peripheral Substituents on the Low-Frequency Vibrational Characteristics James R. Diers,† Yinwen Zhu,‡ Robert E. Blankenship,‡ and David F. Bocian*,† Department of Chemistry, UniVersity of California, RiVerside, California 92521-0403, and Department of Chemistry and Biochemistry and Center for the Study of Early EVents in Photosynthesis, Arizona State UniVersity, Tempe, Arizona 85287-1604 ReceiVed: NoVember 30, 1995; In Final Form: February 20, 1996X
Low-frequency (80-700 cm-1) Qy-excitation resonance Raman (RR) spectra are reported for thin-solid-film aggregates of several chlorophyll (Chl) a and bacteriochlorophyll (BChl) c/d pigments. The pigments include Chl a, pyrochlorophyll a (PChl a), methylpyrochloropyllide a (MPChl a), methylbacteriochlorophyllide d (MBChl d), [E,M] BChl cS, [E,E] BChl cF, and [P,E] BChl cF. The BChl c/d’s are the principal constituents of the chlorosomal light-harvesting apparatus of green photosynthetic bacteria. Together, the various Chl a’s and BChl c/d’s represent a series in which the peripheral substituent groups on the chlorin macrocycle are varied in a systematic fashion. All of the Chl a and BChl c/d aggregates exhibit rich low-frequency vibrational patterns. In the case of the BChl c/d’s, certain modes in the very low-frequency region (100-200 cm-1) experience exceptionally strong Raman intensity enhancements. The frequencies of these modes are qualitatively similar to those of oscillations observed in femtosecond optical experiments on chlorosomes. The RR data indicate that the low-frequency vibrations are best characterized as intramolecular out-of-plane deformations of the chlorin macrocycle rather than intermolecular modes. The coupling of the out-of-plane modes in turn implies that the Qy electronic transition(s) of the aggregate have out-of-plane character. The RR spectra of the BChl c/d’s also reveal that the nature of the alkyl substituents at the 8 and 12 positions of the macrocycle plays an important role in determining the detailed features of the low-frequency vibrational patterns. The frequencies of the modes are particularly sensitive to larger substituent groups whose conformations may be more easily perturbed in the tightly packed aggregates. These factors also make aggregates of pigments containing larger substituents more susceptible to structural, electronic, and vibrational inhomogeneities. Collectively, the RR studies of the various pigments delineate the factors which influence the low-frequency vibrational characteristics of chlorosomal aggregates.
Introduction The (bacterio)chlorophyll pigments in photosynthetic apparati are typically immersed in a protein matrix whose architecture dictates the photophysical properties of these cofactors.1 Chlorosomes, which are the principal light-harvesting assemblies in green photosynthetic bacteria, are an exception.2-5 These structures are predominantly comprised of bacteriochlorophyll (BChl) c (or d or e) pigments which are arranged in extended rodlike assemblies containing very little protein.2-10 Owing to the low protein content of the chlorosomes, the structural and electronic properties of the supramolecular architecture are dictated by pigment-pigment rather than pigment-protein interactions.9,11-13 The interchromophore interactions in the chlorosomes give rise to intense, broad Qy absorption features which are significantly to the red (>1000 cm-1) of those observed for monomeric pigments.5,14 These interactions promote extremely rapid energy transfer among the pigments (e100 fs)13,15-24 This energy is ultimately funneled to a smaller BChl a containing protein which interfaces the light-harvesting assembly to the reaction center (RC).25 In solution, BChl c/d/e spontaneously assembles into aggregates which exhibit spectroscopic (absorption, fluorescence, linear and circular dichroism, and vibrational) properties that are strikingly similar to those of chlorosomes.5-7,9-12,26-32 †
University of California. Arizona State University. X Abstract published in AdVance ACS Abstracts, April 15, 1996. ‡
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Femtosecond optical experiments indicate that the self-assembled aggregates and chlorosomes also exhibit nearly analogous internal-energy-transfer kinetics and exciton-state evolution.24,32 The ultrafast absorption-difference profiles of both the natural and artificial assemblies are characterized by oscillations in low-frequency modes (
