Coupling to Charge Transfer States is the Key to Modulate the Optical Bands for Efficient Light Harvesting in Purple Bacteria

The photosynthetic apparatus of purple bacteria uses exciton delocalization and static disorder to modulate the position and broadening of its absorption bands, leading to efficient light harvesting. Its main antenna complex, LH2, contains two rings of identical bacteriochlorophyll pigments, B800 and B850, absorbing at 800 and 850 nm, respectively. It has been an unsolved problem why static disorder of the strongly coupled B850 ring is several times larger than that of the B800 ring. Here we show that mixing between excitons and charge transfer states in the B850 ring is responsible for the effect. The linear absorption spectrum of the LH2 system is simulated by using a multiscale approach with an exciton Hamiltonian generalized to include the charge transfer states that involve adjacent pigment pairs, with static disorder modeled microscopically by molecular dynamics simulations. Our results show that sufficient inhomogeneous broadening of the B850 band, needed for efficient light harvesting, is only obtained by utilizing static disorder in the coupling between local excited and interpigment charge transfer states.

Shaping excitons in light-harvesting proteins through nanoplasmonics

Nanoplasmonics has been used to enhance molecular spectroscopic signals, with exquisite spatial resolution down to the sub-molecular scale. By means of a rigorous, state-of-the-art multiscale model based on a quantum chemical description, here we show that optimally tuned tip-shaped metal nanoparticles can selectively excite localized regions of typically coherent systems, eventually narrowing down to probing one single pigment. The well-known major light-harvesting complex LH2 of purple bacteria has been investigated because of its unique properties, as it presents both high and weak delocalization among subclusters of pigments. This finding opens the way to the direct spectroscopic investigation of quantum-based processes, such as the quantum diffusion of the excitation among the chromophores, and their external manipulation.

Coupling Real-Time Time-Dependent Density Functional Theory with Polarizable Force Field

Real-time time-dependent density functional theory (RT-TDDFT) is a powerful tool for obtaining spectroscopic observables and understanding complex, time-dependent properties. Currently, performing RT-TDDFT calculations on large, fully quantum mechanical systems is not computationally feasible. Previously, polarizable mixed quantum mechanical and molecular mechanical (QM/MMPol) models have been successful in providing accurate, yet efficient, approximations to a fully quantum mechanical system. Here we develop a coupling scheme between induced dipole based QM/MMPol and RT-TDDFT. Our approach is validated by comparing calculated spectra with both real-time and linear-response TDDFT calculations. The model developed within provides an accurate method for performing RT-TDDFT calculations on extended systems while accounting for mutual polarization between the quantum mechanical and molecular mechanical regions.

Theoretical description of protein field effects on electronic excitations of biological chromophores

Photoinitiated phenomena play a crucial role in many living organisms. Plants, algae, and bacteria absorb sunlight to perform photosynthesis, and convert water and carbon dioxide into molecular oxygen and carbohydrates, thus forming the basis for life on Earth. The vision of vertebrates is accomplished in the eye by a protein called rhodopsin, which upon photon absorption performs an ultrafast isomerisation of the retinal chromophore, triggering the signal cascade. Many other biological functions start with the photoexcitation of a protein-embedded pigment, followed by complex processes comprising, for example, electron or excitation energy transfer in photosynthetic complexes. The optical properties of chromophores in living systems are strongly dependent on the interaction with the surrounding environment (nearby protein residues, membrane, water), and the complexity of such interplay is, in most cases, at the origin of the functional diversity of the photoactive proteins. The specific interactions with the environment often lead to a significant shift of the chromophore excitation energies, compared with their absorption in solution or gas phase. The investigation of the optical response of chromophores is generally not straightforward, from both experimental and theoretical standpoints; this is due to the difficulty in understanding diverse behaviours and effects, occurring at different scales, with a single technique. In particular, the role played by ab initio calculations in assisting and guiding experiments, as well as in understanding the physics of photoactive proteins, is fundamental. At the same time, owing to the large size of the systems, more approximate strategies which take into account the environmental effects on the absorption spectra are also of paramount importance. Here we review the recent advances in the first-principle description of electronic and optical properties of biological chromophores embedded in a protein environment. We show their applications on paradigmatic systems, such as the light-harvesting complexes, rhodopsin and green fluorescent protein, emphasising the theoretical frameworks which are of common use in solid state physics, and emerging as promising tools for biomolecular systems.

An Ab Initio Description of the Excitonic Properties of LH2 and Their Temperature Dependence

The spectroscopic properties of light-harvesting (LH) antennae in photosyntehtic organisms represent a fingerprint that is unique for each specific pigment-protein complex. Because of that, spectroscopic observations are generally combined with structural data from X-ray crystallography to obtain an indirect representation of the excitonic properties of the system. Here, an alternative strategy is presented which goes beyond this empirical approach and introduces an ab initio computational description of both structural and electronic properties and their dependence on the temperature. The strategy is applied to the peripheral light-harvesting antenna complex (LH2) present in purple bacteria. By comparing this model with the one based on the crystal structure, a detailed, molecular level explanation of the absorption and circular dichroism (CD) spectra and their temperature dependence is achieved. The agreement obtained with the experiments at both low and room temperature lays the groundwork for an atomistic understanding of the excitation dynamics in the LH2 system.

Control of Coherences and Optical Responses of Pigment-Protein Complexes by Plasmonic Nanoantennae

The key for light-harvesting in pigment-protein complexes are molecular excitons, delocalized excited states comprising a superposition of excitations at different molecular sites. There is experimental evidence that the optical response due to such excitons can be largely affected by plasmonic nanoantennae. Here we employ a multiscale approach combining time-dependent density functional theory and polarizable classical models to study the optical behavior of the LH2 complex present in bacteria when interacting with a gold nanorod. The simulation not only reproduces the experiments but also explains their molecular origin. By tuning the chromophoric unit and selectively switching on/off the excitonic interactions, as well as by exploring different setups, we clearly show that the dramatic enhancement in the optical response, unexpectedly, is not accompanied by changes in the coherences. Instead polarization effects are dominant. These results can be used to design an optimal control of the light-harvesting process through plasmonic nanoantennae.

Photoprotection and triplet energy transfer in higher plants: the role of electronic and nuclear fluctuations

Photosynthetic organisms employ several photoprotection strategies to avoid damage due to the excess energy in high light conditions.

Electron attachment to molecules in a cluster environment

Low-energy dissociative electron attachment (DEA) to the CF(2)Cl(2) and CF(3)Cl molecules in a water cluster environment is investigated theoretically. Calculations are performed for the water trimer and water hexamer. It is shown that the DEA cross section is strongly enhanced when the attaching molecule is embedded in a water cluster, and that this cross section grows as the number of water molecules in the cluster increases. This growth is explained by a trapping effect that is due to multiple scattering by water molecules while the electron is trapped in the cluster environment. The trapping increases the resonance lifetime and the negative ion survival probability. This confirms qualitatively existing experiments on electron attachment to the CF(2)Cl(2) molecule placed on the surface of H(2)O ice. The DEA cross sections are shown to be very sensitive to the position of the attaching molecule within the cluster and the orientation of the electron beam relative to the cluster.

MRI of the small-bowel: how to differentiate primary neoplasms and mimickers

MRI of the gastrointestinal tract is gaining clinical acceptance and is increasingly used to evaluate patients with suspected small-bowel diseases. MRI may be performed with enterography or enteroclysis, both of which combine the advantages of cross-sectional imaging with those of conventional enteroclysis. In this paper, MRI features of primary small-bowel neoplasms, the most important signs for differential diagnosis and the diseases that can be considered as mimickers of small-bowel neoplasms, are discussed.

[Imaging of acute brain inflammatory disease]

The central nervous system inflammatory disease can be due to any kind of infective agent (bacterial viral, fungal and parasitic), but entails also multiple sclerosis, a primary demyelinating disease in which the causal agent is unknown. MR imaging is, in most often, the procedure of choice, due to her multiplanar and multiparametric imaging, and to her better contrast resolution. The post-contrast imaging with double dose of gadolinium and late sequences enable visualisation of smallest pathologic foci or slightest blood-brain barrier alterations, with a sensibility very higher than post-contrast CT scan. In addition, RM provide to many functional informations, by means of diffusion, perfusion and spectroscopy studies, Bold technique for cortical activation studies and Fiber Tracking technique, in order to demonstrate pathologic modification earlier than they are evident on morphologic imaging. Functional imaging is also employed to monitor response to treatment and damage reversibility.

[Vertebral fractures: radiological diagnosis, differential diagnosis and prognostic implications]

Vertebral fractures are a relevant problem for the heavy clinical implications and carrying disability. Vertebral fractures can be traumatic or pathologic, the latter can be benign or malignant, both mostly frequent in the elderly. An initial approach to this issue can use plain radiographs, but the correct extension and evaluation must involve CT and MR imaging. In particular MR is a useful tool for the prognostic evaluation of spine marrow injuries and the differential diagnosis of osteoporotic and metastatic fractures.

[Role of magnetic resonance in shoulder disease]

Next to the knee, the shoulder is the most common joint to be referred for MRI. Excellent soft tissue contrast and multiplanar acquisition provide optimal assessment of muscle, tendons, hyaline and fibrous cartilage, joint capsule, fat, bursae and bone marrow. In this article the most common indications for shoulder MRI are reviewed and discussed, but we focused primarily on the rotator cuff syndrome and shoulder instability. Correct diagnosis requires the use of appropriate pulse sequences and imaging planes, proper patient positioning, and a satisfactory surface coil. Moreover, technical improvements continuously augment the ability of MRI to study the shoulder; for example Magnetic Resonance arthrography is superior to the other imaging techniques in evaluation of glenohumeral joint. This interdependence between technical development in MRI and clinical advance in shoulder therapy ensures that MRI will continue to play an important role in the routine management of patients with shoulder disease.

[Can triphasic spiral CT provide data for the histologic characterization of liver masses?]

Spiral Computed Tomography (CT) has rapidly gained acceptance as the preferred CT technique for routine liver evaluation because it provides image acquisition at peak enhancement of the liver parenchyma during single breath hold. In this study, we evaluated a wide range of non cystic liver lesions with triphasic spiral CT technique, that allows imaging of the entire liver in arterial, portal and equilibrium phases. In particularly we focused on hemangioma, adenoma, focal nodular hyperplasia, hepatocarcinoma and metastases.