No abstract

The physics of confined water has stimulated extensive research in recent years, in particular, regarding the role of hydrogen bonding as a significant factor in the observed dynamics. In this work, two-dimensional infrared spectroscopy was employed to investigate the response of the OH moiety of water in phospholipid membrane samples. The results show strong evidence for three distinct hydrogen bonding motifs (H2O with zero, one, or both OH moieties hydrogen bonded), whose relative proportions at the membrane interface are estimated.

Femtosecond infrared (IR) two-color pump-probe experiments were used to investigate the nonlinear response of the D2O stretching vibration in weakly hydrated dimyristoyl-phosphatidylcholine (DMPC) membrane fragments. The vibrational lifetime is comparable to or longer than that in bulk D2O and is frequency dependent, as it decreases with increasing probe frequency. Also, the lifetime increases when the water content of the sample is lowered. The measured lifetimes range between 903 and 390 fs. A long-lived spectral feature grows in following the excitation and is attributed to photoinduced D-bond breaking. The photoproduct spectrum differs from the steady state difference Fourier transform infrared (FTIR) spectrum, showing that the full thermalization of the excitation energy happens on a much longer time scale than the time interval considered (12 ps). Further evidence of the inhomogeneous character of the water residing in the polar region of the bilayer comes from the spectral anisotropy. The water molecules absorbing on the low frequency side of the absorption band show no decay at all of the anisotropy, while an ultrafast partial decay appears when the high frequency side of the spectrum is probed. The overall behavior differs remarkably from that observed with similar experiments in bulk water and in water segregated in inverse micelles. In weakly hydrated phospholipid membranes, water molecules are present mostly as isolated species, prevalently involved in strong, rigid, and persistent hydrogen bonds with the polar groups of the bilayer molecules. This specific character appears to have a direct effect on the structural stability and thermal properties of the membrane.

We present the results of two-pump and probe femtosecond experiments designed to follow the relaxation dynamics of the lowest excited state (S 1) populated by different modes. In the first mode, a direct (S0 3 S1) radiative excitation of the ground state is used. In the second mode, an indirect excitation is used where the S 1 state is populated by the use of two femtosecond laser pulses with different colors and delay times between them. The first pulse excites the S 0 3 S1 transition whereas the second pulse excites the S 1 3 Sn transition. The nonradiative relaxation from the Sn state populates the lowest excited state. Our results suggest that the S 1 state relaxes faster when populated nonradiatively from the Sn state than when pumped directly by the S0 3 S1 excitation. Additionally, the S n 3 S1 nonradiative relaxation time is found to change by varying the delay time between the two pump pulses. The observed dependence of the lowest excited state population as well as its dependence on the delay between the two pump pulses are found to fit a kinetic model in which the S n state populates a different surface (called S 1) than the one being directly excited (S 1). The possible involvement of the Ag type states, the J intermediate, and the conical intersection leading to the S 0 or to the isomerization product (K intermediate) are discussed in the framework of the proposed model. Halobacterium salinarum (Halobium salinarium) is a member of Halobacteria, which is part of the domain Archaea. In an anaerobic condition under light illumination, H. salinarum demonstrates a pH decrease in cell suspension and ATP synthesis (1, 2), the main features of photosynthesis. The photosynthetic activity in H. salinarum was attributed to bacteriorhodopsin (bR), a 26-kDa protein that is hexagonally packed within the purple membrane (1-4). Since the discovery of the purple membrane, its enigmatically efficient photosynthesis is in the center of the most active research. To our surprise, the archaic organism effectively utilizes more than 50% of the absorbed light energy (5, 6).Retinal, a light-absorbing polyene, is the bR chromophore and is linked to its Lys-216 by a protonated Schiff base. The chromophore undergoes light-induced isomerization and masters proton transfer across the membrane (7-11). The photoisomerization of retinal initiates a series of retinal protein structural reconformations of the bacteriorhodopsin that lead to the formation of different intermediates and finally return it to its initial state with all-trans-retinal. Overall, this photocycle consists of at least seven intermediates of bR with different visible absorption spectra and lifetimes bR 568 3 S 1 3 J 3 K 630 3 L 550 3 M 412 3 N 520 3 O 640 3 bR 568 (4).Retinal in solution shows relatively slow and nonselective photoisomerization around several double bonds (12-14) with low yield. The specific ultrafast, all-trans, 13-cis photoisomerization of the retinal in bR is the purple membrane photosynthesis key event which has a quantum yield of 55%. The p...

We have investigated size-dependent optical properties of poly-1,6-di(N-carbazolyl)-2,4-hexadiyne (poly-DCHD) nanocrystals in ensemble and in a single particle level. Our experimental results demonstrate that far-field microscopy coupled with an AFM setup is a reliable experimental approach for investigating optical properties of single organic nanoparticles. Single poly-DCHD nanocrystals show resonant Rayleigh scattering spectra with a lower resonance energy peak for nanocrystals having larger cross-section against its long axis. We also examined the extinction and the resonant Rayleigh scattering spectra in ensemble and the resonant Rayleigh and Raman scattering spectra and their anisotropy of single nanocrystals. The results indicate that the size-dependent resonance peak of the polymer-backbone electronic transition can be ascribed to structural confinement of nanocrystals, which reduces the effective conjugation length of π-electron in the polymer backbone for nanocrystals with a small cross-section.