Rigid versus Flexible Protein Matrix: Light-Harvesting Complex II Exhibits a Temperature-Dependent Phonon Spectral Density

Abstract

Dynamics-function correlations are usually inferred when molecular mobility and protein function are simultaneously impaired at characteristic temperatures or hydration levels. In this sense, excitation energy transfer in the photosynthetic light-harvesting complex II (LHC II) is an untypical example because it remains fully functional even at cryogenic temperatures relying mainly on interactions of electronic states with protein vibrations. Here, we study the vibrational and conformational protein dynamics of monomeric and trimeric LHC II from spinach using inelastic neutron scattering (INS) in the temperature range of 20-305 K. INS spectra of trimeric LHC II reveal a distinct vibrational peak at ∼2.4 meV. At temperatures above ∼160 K, however, the inelastic peak shifts toward lower energies, which is attributed to vibrational anharmonicity. A more drastic shift is observed at about 240 K, which is interpreted in terms of a "softening" of the protein matrix along with the dynamical transition. Monomeric LHC II exhibits a higher degree of conformational mobility at physiological temperatures, which can be attributed to a higher number of solvent-exposed side chains at the protein surface. The effects of the changes in protein dynamics on the spectroscopic properties of LHC II are considered in comparative model calculations. The absorption line shapes of a pigment molecule embedded into LHC II are simulated for the cases of (i) a rigid protein matrix, (ii) a protein matrix with temperature-dependent spectral density of protein vibrations, and (iii) temperature-dependent electron-phonon coupling strength. Our findings indicate that vibrational and conformational protein dynamics affect the spectroscopic (absorption) properties of LHC II at physiological temperatures.

Solution Structure and Conformational Flexibility in the Active State of the Orange Carotenoid Protein. Part II: Quasielastic Neutron Scattering

“…Generally, the protein vibrations of both proteins are characterized by relatively high protein frequencies compared with other proteins typically showing vibrational peaks at ∼2.5 meV. 21,35 The relatively high vibrational frequencies observed can be explained by a relatively high degree of localization of the corresponding vibrations. Similarly high vibrational frequencies were recently reported for phycobiliproteins of A. marina.…”

“…18,19 In contrast, photosynthetic antenna complexes remain operative even at low temperatures, becausein terms of protein dynamicstheir function mainly requires harmonic protein vibrations. 20,21 Neutron spectroscopic methods like inelastic and quasielastic neutron scattering (INS and QENS) are powerful experimental tools for direct investigations of vibrational and conformational protein dynamics (for reviews, see refs 22−24). Because of the high incoherent scattering cross section of hydrogen atoms, which are almost homogeneously distributed in biomolecules, INS and QENS are widely used to study molecular motions in proteins.…”

“…Dynamics-function relationships are usually inferred when conformational protein motions and biological activity are simultaneously suppressed or cease below characteristic temperatures or hydration levels. For instance, specific electron transfer steps in photosynthetic reaction centers are impaired upon temperature decrease or dehydration and thus appear to require the presence of conformational protein dynamics. , In contrast, photosynthetic antenna complexes remain operative even at low temperatures, becausein terms of protein dynamicstheir function mainly requires harmonic protein vibrations. , …”

“…The latter transition occurs in the temperature range from 200 to 240 K depending on the particular protein and on its specific environment. Hydration water dynamics can also be studied by QENS using contrast variation. , INS spectra of proteins generally display a distinct inelastic peak centered at energies of 2–7 meV representing vibrational modes of the protein (see, e.g., refs , , and ). Furthermore, QENS data are complementary to molecular dynamics (MD) simulations of internal protein motions. ,− …”

Neutron scattering in photosynthesis research: recent advances and perspectives for testing crop plants

“…Further QENS studies of native and mutant (lacking the Chl a 612 pigment molecule) LHCII, coupled with absorption and fluorescence measurements, indicated correlation between the presence of conformational dynamics and the position of the excited electronic states in the antenna complex (Vrandecic et al 2015 ). The effect of the oligomeric state of the LHCII on the dynamics of this protein was also investigated up to physiologically relevant temperatures (Golub et al 2018 ).…”