Unveiling the excited state energy transfer pathways in peridinin-chlorophyll a-protein by ultrafast multi-pulse transient absorption spectroscopy.

Time-resolved multi-pulse methods were applied to investigate the excited state dynamics, the interstate couplings, and the excited state energy transfer pathways between the light-harvesting pigments in peridinin-chlorophyll a-protein (PCP). The utilized pump-dump-probe techniques are based on perturbation of the regular PCP energy transfer pathway. The PCP complexes were initially excited with an ultrashort pulse, resonant to the S0→S2 transition of the carotenoid peridinin. A portion of the peridinin-based emissive intramolecular charge transfer (ICT) state was then depopulated by applying an ultrashort NIR pulse that perturbed the interaction between S1 and ICT states and the energy flow from the carotenoids to the chlorophylls. The presented data indicate that the peridinin S1 and ICT states are spectrally distinct and coexist in an excited state equilibrium in the PCP complex. Moreover, numeric analysis of the experimental data asserts ICT→Chl-a as the main energy transfer pathway in the photoexcited PCP systems.

Investigation of the S1/ICT equilibrium in fucoxanthin by ultrafast pump-dump-probe and femtosecond stimulated Raman scattering spectroscopy.

Time-resolved multi-pulse spectroscopic methods-pump-dump-probe (PDP) and femtosecond stimulated Raman spectroscopy-were used to investigate the excited state photodynamics of the carbonyl group containing carotenoid fucoxanthin (FX). PDP experiments show that S1 and ICT states in FX are strongly coupled and that the interstate equilibrium is rapidly (<5 ps) reestablished after one of the interacting states is deliberately depopulated. Femtosecond stimulated Raman scattering experiments indicate that S1 and ICT are vibrationally distinct species. Identification of the FSRS modes on the S1 and ICT potential energy surfaces allows us to predict a possible coupling channel for the state interaction.

Optically controlled molecular switching of an indolobenzoxazine-type photochromic compound.

Photochromic forward (oxazine ring-opening) and backward (oxazine ring-closing) switching dynamics of an indolobenzoxazine compound were studied by femtosecond pump-repump-probe technique. A UV pulse was used to excite the ring-closed form of the photochromic compound, causing a C-O bond cleavage and the formation of a spectrally red-shifted isomer within a time scale of ca. 100 ps. A successive, temporally delayed near-IR pulse, resonant to the red-most absorption maximum of the ring-opened form, was used to reexcite the molecular system, causing a fast photoinduced oxazine ring closure, thereby "short-circuiting" the normally nanosecond lasting photocycle and returning ∼6% of the molecules to the main molecular ground state. Two possible models, involving the S1 excited state of the terminal photoproduct and its hot ground state, are introduced to explain the pre- and post-reexcitation spectral development and the photoinduced switching back mechanics.

Structure and properties of tethered bilayer lipid membranes with unsaturated anchor molecules.

The self-assembled monolayers (SAMs) of new lipidic anchor molecule HC18 [Z-20-(Z-octadec-9-enyloxy)-3,6,9,12,15,18,22-heptaoxatetracont-31-ene-1-thiol] and mixed HC18/ß-mercaptoethanol (ßME) SAMs were studied by spectroscopic ellipsometry, contact angle measurements, reflection-absorption infrared spectroscopy, and electrochemical impedance spectroscopy (EIS) and were evaluated in tethered bilayer lipid membranes (tBLMs). Our data indicate that HC18, containing a double bond in the alkyl segments, forms highly disordered SAMs up to anchor/ßME molar fraction ratios of 80/20 and result in tBLMs that exhibit higher lipid diffusion coefficients relative to those of previous anchor compounds with saturated alkyl chains, as determined by fluorescence correlation spectroscopy. EIS data shows the HC18 tBLMs, completed by rapid solvent exchange or vesicle fusion, form more easily than with saturated lipidic anchors, exhibit excellent electrical insulating properties indicating low defect densities, and readily incorporate the pore-forming toxin α-hemolysin. Neutron reflectivity measurements on HC18 tBLMs confirm the formation of complete tBLMs, even at low tether compositions and high ionic lipid compositions. Our data indicate that HC18 results in tBLMs with improved physical properties for the incorporation of integral membrane proteins (IMPs) and that 80% HC18 tBLMs appear to be optimal for practical applications such as biosensors where high electrical insulation and IMP/peptide reconstitution are imperative.

Surface-enhanced Raman spectroscopy for detection of toxic amyloid ß oligomers adsorbed on self-assembled monolayers.

Surface-enhanced Raman spectroscopy (SERS) was used to detect different spectral features of small (1-2 nm) and large (5-10 nm) synthetic amyloid Aß-42 oligomers, exhibiting high and no detectable neurotoxicities, respectively. Adsorption of peptides at self-assembled monolayers (SAM) terminated by methyl and pyridinium groups was employed to differentiate toxic and non-toxic oligomers. Three SAMs were analyzed: hydrophobic heptanethiol (HT) and octadecanethiol (ODT) as well as positively charged N-(6-mercapto)hexylpyridinium (MHP) chloride. SERS study revealed twofold adsorption effect, changes in the monolayer structure and appearance of new bands associated with the adsorbed peptides. A band at 1387 cm(-1), observed as a result of the SAM and Aß-42 interaction, is tentatively assigned to the peptide symmetric stretching vibration of carboxylate groups, and appears to be the most prominent spectral feature distinguishing toxic oligomers from the non-toxic Aß-42 forms. This band was identified in the spectra of Aß-42 adsorption on heptanethiol and MHP monolayers, while no clear perturbations were observed in the case of ODT monolayer.