Photodynamics of Lys+-Trp protein motifs: hydrogen bonds ensure photostability.

Cation-pi interactions such as Lys(+)-Trp, are highly abundant structural motifs in proteins. Both, experimental and theoretical studies of small prototypical gas phase systems, H+Trp, H+Trp x (H2O)n and H+Gly-Trp, indicate such an arrangement as potential hot spot for photodamage and photoinstability. Here, we study the photodynamical properties of a Lys(+)-Trp pair in the protein human serum albumin (HSA) using nonadiabatic mixed time-dependent density functional theory/molecular mechanics simulations (TDDFT/MM). These simulations show that the findings for small protonated Trp complexes are largely transferable to a more complex protein environment. Under partially hydrated ("dry" conditions), when the -NH3+ head group is not fully solvated, photoexcitation of the tryptophan leads indeed to rapid photodissociation of the proximal charged amino group. In contrast, photostability is well maintained under fully solvated conditions when the lysine head group is fully hydrogen-bonded. In this case, photodynamics takes place in a pi-pi* state without interference of fast dissociative sigma*C-N) or sigma*N-H channels. These results highlight the crucial role of hydrogen bonds in ensuring the photostability of essential biological building blocks.

On the proton transfer mechanism in ammonia-bridged 7-hydroxyquinoline: a TDDFT molecular dynamics study.

We have investigated the mechanism of proton transfer in the lowest photoexcited state of 7-hydroxyquinoline.(NH3)3 using TDDFT based molecular dynamics. We observe a concerted mechanism according to which all protons are transferred simultaneously in a fast process (approximately 100 fs) that amounts to the net transport of one proton from the oxygen to the nitrogen of 7-hydroxyquinoline. In addition, the observed proton transfer pathway involves all three ammonia molecules and not only two as previously proposed. These differences arise from dynamical effects that occur at finite temperature. Our simulations provide a detailed time-resolved description of the proton transfer reaction mechanism for a prototypical molecular cluster and thus help to shed further light on the nature of this important and ubiquitous process.

New paradigm in molecular engineering of sensitizers for solar cell applications.

A novel thiocyanate-free cyclometalleted ruthenium sensitizer for solar cells is designed and developed. Upon anchoring to nanocrystalline TiO(2) films, it exhibits a remarkable incident monochromatic photon-to-current conversion efficiency of 83%. The solar cell employing a liquid-based electrolyte exhibits a short circuit photocurrent density of 17 mA/cm(2), an open circuit voltage of 800 mV, and a fill factor of 0.74, corresponding to an overall conversion efficiency of 10.1% at standard AM 1.5 sunlight. To understand the structural, electronic, and optical properties of the cyclometalleted ruthenium sensitizer, we have investigated using density functional theory (DFT) and time-dependent DFT (TDDFT). Our results show the HOMO is located mostly on ruthenium and cyclometalated ligand, while the LUMO is on 4-carboxylic acid-4'-carboxylate-2,2'-bipyridine. Molecular orbitals analysis confirmed the experimental assignment of redox potentials, and TDDFT calculations allowed assignment of the visible absorption bands. The present findings provide new design criteria for the next generation of ruthenium sensitizers and help foster widespread interest in the engineering of new sensitizers that interact effectively with the I(-)/I(3)(-) redox couple.

Computer simulation of uranyl uptake by the rough lipopolysaccharide membrane of Pseudomonas aeruginosa.

Heavy metal environmental contaminants cannot be destroyed but require containment, preferably in concentrated form, in a solid or immobile form for recycling or final disposal. Microorganisms are able to take up and deposit high levels of contaminant metals, including radioactive metals such as uranium and plutonium, into their cell wall. Consequently, these microbial systems are of great interest as the basis for potential environmental bioremediation technologies. The outer membranes of Gram-negative microbes are highly nonsymmetric and exhibit a significant electrostatic potential gradient across the membrane. This gradient has a significant effect on the uptake and transport of charged and dipolar compounds. However, the effectiveness of microbial systems for environmental remediation will depend strongly on specific properties that determine the uptake of targeted contaminants by a particular cell wall. To aid in the design of microbial remediation technologies, knowledge of the factors that determine the affinity of a particular bacterial outer membrane for the most common ionic species found in contaminated soils and groundwater is of great importance. Using our previously developed model for the lipopolysaccharide (LPS) membrane of Pseudomonas aeruginosa, this work presents the potentials of mean force as the estimate of the free energy profile for uptake of sodium, calcium, chloride, uranyl ions, and a water molecule by the bacterial LPS membrane. A compatible classical parameter set for uranyl has been developed and validated. Results show that the uptake of uranyl is energetically a favorable process relative to the other ions studied. At neutral pH, this nuclide is shown to be retained on the surface of the LPS membrane through chelation with the carboxyl and hydroxyl groups located in the outer core.

Microsolvation effects on the excited-state dynamics of protonated tryptophan.

To better understand the complex photophysics of the amino acid tryptophan, which is widely used as a probe of protein structure and dynamics, we have measured electronic spectra of protonated, gas-phase tryptophan solvated with a controlled number of water molecules and cooled to approximately 10 K. We observe that, even at this temperature, the bare molecule exhibits a broad electronic spectrum, implying ultrafast, nonradiative decay of the excited state. Surprisingly, the addition of two water molecules sufficiently lengthens the excited-state lifetime that we obtain a fully vibrationally resolved electronic spectrum. Quantum chemical calculations at the RI-CC2/aug-cc-pVDZ level, together with TDDFT/pw based first-principles MD simulations of the excited-state dynamics, clearly demonstrate how interactions with water destabilize the photodissociative states and increase the excited-state lifetime.