Protonated water clusters, H + (H 2 O) n (n ) 5-8), from a supersonic expansion have been investigated by vibrational predissociation spectroscopy and ab initio calculations. The experimental spectra were obtained at an estimated cluster temperature of 170 ( 20 K. Recorded absorption bands at the frequency range of 2700-3900 cm -1 are attributed to the free-and hydrogen-bonded-OH stretches of the ion core and the surrounding solvent molecules. Ab initio calculations, performed at the B3LYP/6-31+G* level, indicate that geometries of the H + (H 2 O) 5-8 isomers are close in energy, with the excess proton either localized on a single water molecule, yieldingSystematic comparison of the experimental and computed spectra provides compelling evidence for both cases. The unique proton-transfer intermediate H 5 O 2 + (H 2 O) 4 was identified, for the first time, by its characteristic bonded-OH stretching absorptions at 3178 cm -1 . The existence of five-membered-ring isomers at n ) 7 is also evidenced by the distinct bonded-OH stretches at 3500-3600 cm -1 and by the free-OH stretch of threecoordinated H 2 O at 3679 cm -1 . Among these H + (H 2 O) 7 isomers is a newly discovered H 5 O 2 + -containing pentagonal ring, which is computed to be lowest in Gibbs free energy at 170 K. Its spectroscopic signature is the splitting of two equivalent bonded OH stretches into a doublet (3544 and 3555 cm -1 ) by vibrational coupling through ring closure. No profound spectral evidence, however, was found for the formation of four-membered rings although it is predicted to be favorable in terms of total interaction energy for H + (H 2 O) 7 . Six-membered ring and three-dimensional cagelike structures are also stable isomers but they are less strongly bound. The preference of five-membered-ring formation at n ) 7 appears to be the result of a delicate balance between entropy and enthalpy effects at the presently investigated cluster temperature. The correlation of this investigation with other studies of neutral water clusters and of the hydration of biological macromolecules is emphasized.

When a direct current (DC) electric field is applied across an ion-selective nanoporous membrane or a nanochannel with an overlapping Debye layer, a surprising microvortex instability occurs on the side of the membrane/channel through which counterions enter. Despite its micro and nano length scales, this instability exhibits all the hallmarks of other classical hydrodynamic instabilities—a subharmonic cascade, a wide-band fluctuation spectrum, and a coherent structure dominated by spatiotemporal dynamics. Moreover, the resulting convection enhances the ion flux into the ion-selective medium and gives rise to an overlimiting-current bifurcation in the current-voltage relationship. This hydrodynamically driven nonequilibrium ion flux does not seem to have any equivalent in cell membrane ion channels. Yet, by introducing asymmetric entrances to provide different polarized regions and/or viscous arrest of the vortex instability, one can fabricate a hydrodynamic nanofluidic diode. With other modifications, hysteretic, excitable, and oscillatory ion flux dynamics could also be elicited—all with strong hydrodynamic features.

The contact line in an evaporating drop can stay pinned to form a single ring or can shrink in a discontinuous stepwise manner and generate multiple rings. We demonstrate the latter with DNA solutions and attribute it to a pinning-depinning cycle that generates new contact lines. The new contact line recedes after depinning and is repinned at an internal precipitate ring that determines the location of the next contact line. Each precursor ring is formed when DNAs are trapped by an internal microstagnation flow and precipitation dynamics hence control this unsteady drop motion.

Isolation procedures have led to chlorophyll a (Chl a) contents of the photosystem I1 reaction center (RC) that range between about 4 and 6. Since this content for the bacterial RC is 4 (with two of those being associated with the special pair), the nature of the "extra" Chl a in R C preparations of photosystem I1 containing more than 4 Chl a molecules is currently of much interest. So too are the dynamics of primary charge separation in the R C which are triggered by excitation of the primary electron donor state, P680* (where P680 indicates that the lowest energy ground-state absorption band of the primary donor lies at 680 nm; the asterisk indicates lowest lying "r * (a) state). We report absorption and triplet-state bottleneck hole spectra (4.2 K) for RC preparations of photosystem I1 containing 4, 5 , and 6 Chl a molecules. The spectra reveal that the extra Chl a are due to 684-nm-absorbing Chl B, some contamination by the proximal antenna protein complex CP47, and, probably, also nonnative (disrupted) Chl a absorbing near 670 nm. The 684-nm Chl a were found to be easily disrupted by the ionic detergent Triton X-100 (much more so than P680). The results are inconsistent with the model that has the 684-nm band being thedimer (special pair) partner of P680. Nor can they be satisfactorily interpreted within the model that has the 684-nm band being P684 of a structurally very distinct subset of the RC ensemble. This "mixture" model has the ensemble comprised of P680 and P684 RC subsets. Importantly, the intensities of the 684-nm bands observed for the CP47 complex and the CP47-RC complex were also found to vary from preparation to preparation and be sensitive to Triton X-100. Two possibilities are considered: that the 684-nm Chl a are associated with the CP47-RC complex as a whole or that both CP47 and the R C possess 684-nm-absorbing Chl a or, equivalently, an intrinsic (fragile) 684-nm state. Irrespective of which of these two is correct, it is concluded that the number of Chl a in the hydrophobic interior of the RC of photosystem I1 is 4 and that the 684-nm Chl a are located in the exterior region of the R C protein complex. The lifetime of P680* of the 4 Chl a-RC preparation, which contains very little 684-nm Chl a (5% on a Chl a basis), was determined to be 1.9 ps at 4.2 K. This is identical to our previous determinations for higher Chl a content RC and CP47-RC samples. Thus, the 684-nm Chl a do not affect the lifetime of P680* at low temperatures, Le., do not serve as an efficient trap for P680*. A theoretical analysis of the burn wavelength dependence of the P680 hole spectra of the 4 Chl a preparation is given. In agreement with our previous work, the electron-phonon (protein) coupling is as strong (S = 2) as that observed for P870 and P960 of the bacterial R C Rhodobacter sphaeroides and Rhodopseudomonas viridis, respectively. However, the special pair marker mode (1 25/ 145 cm-l) progression of P870 and P960 is essentially silent in P680. This, together with the observation that the weakly absorbing, u...

Vibrational predissociation spectroscopy of protonated methanol clusters (tetramers and pentamers) reveals linear and cyclic structural isomers in a supersonic expansion. The cyclic pentamer, containing a five-membered ring, is identified by its characteristic free-OH stretch at 3647 cm -1 and hydrogen-bonded OH stretches at 3448 and 3461 cm -1 . Ab initio calculations indicate that the excess proton in these clusters can be either localized on one methanol unit in cyclic CH 3 OH 2 + (CH 3 OH) 3 and linear CH 3 OH 2 + (CH 3 OH) 4 or delocalized between two methanol molecules in linear C 2 H 9 O 2 + (CH 3 OH) 2 and cyclic C 2 H 9 O 2 + (CH 3 OH) 3 . Dynamic intracluster proton transfer can occur upon repeated ring opening and closing. The association of this process with the anomalously high proton mobility in liquid methanol is discussed.

Persistent nonphotochemical and population bottleneck hole-burning results obtained as a function of burn wavelength are reported for the CP47 proximal antenna protein complex of photosystem 11. Attention is focused on the lower energy chlorophyll a Qv states. Results are presented for the CP47 complex from two preparations. The Chl a content per CP47 complex was determined, spectroscopically, to be 14 f 2. On the basis of the analysis of the hole spectra and the 4.2 K static fluorescence spectrum, the lowest energy state of CP47 lies at 690 nm (fluorescence origin at 691 nm). The width of the weak 690-nm absorption band from inhomogeneous broadening is 100 cm-l. The linear electron-phonon coupling of the 690-nm state is weak with a Huang-Rhys factor (S) of about 0.2 and a mean phonon frequency (0,) of 20 cm-I, which explains why the Stokes shift ( 2 S~m ) is so small. The 690-nm state is found to be excitonically correlated with a, hitherto, unobserved state at 687 nm. However, the combined absorption intensity of the 690-and 687-nm states was determined to be equivalent to only 1 Chl a molecule. Results are presented which illustrate that these two states are fragile (Le., their associated chlorophyll a molecules are readily disrupted). Thus, it is possible that the correct number of Chl a molecules is 2, not 1. Indeed, the simplest interpretation consistent with the hole-burning data has the 687-and 690-nm states being associated with a Chl a dimer with the latter close to forbidden in absorption. The results indicate that the 687-nm state relaxes to the 690-nm state in about 70 fs. The zero-phonon hole widths for the higher integrity CP47 samples are used to determine the energy-transfer times for the higher energy absorbing states at 4.2 K. The absorption intensity of a previously identified state at 684 nm is found to vary from preparation to preparation. Diminution of the intensity of the 684-nm band is accompanied by increased absorption at -670 nm. This speaks to the fragility of the 684-nm absorbing Chl a. Consideration of the nature of the 684-nm-absorbing Chl a of CP47 is mainly reserved for the accompanying paper on the D1-D2-cyt b559 reaction center and CP47-Dl-D2-cyt bss9 complexes.

The hydrogen bonding structures of room-temperature ionic liquids 1,3-dimethylimidazolium methyl sulfate and 1-butyl-3-methylimidazolium hexafluorophosphate have been studied by infrared spectroscopy. High-pressure infrared spectral profiles and theoretical calculations allow us to make a vibrational assignment of these compounds. The imidazolium C-H bands of 1,3-dimethylimidazolium methyl sulfate display anomalous non-monotonic pressure-induced frequency shifts. This discontinuity in frequency shift is related to enhanced C-H...O hydrogen bonding. This behavior is in contrast with the trend of blue shifts in frequency for the methyl C-H stretching mode at ca. 2960 cm(-1). Our results indicated that the imidazolium C-H groups are more favorable sites for hydrogen bonding than the methyl C-H groups in the pure 1,3-dimethylimidazolium methyl sulfate. Nevertheless, both methyl C-H and imidazolium C-H groups are favorable sites for C-H...O hydrogen bonding in a dilute 1,3-dimethylimidazolium methyl sulfate/D(2)O mixture. Hydrogen bond-like C-H...F interactions were observed between PF(6)(-) and H atoms on the alkyl side chains and imidazolium ring for 1-butyl-3-methylimidazolium hexafluorophosphate.

A surface acoustic wave-based sample delivery and ionization method that requires minimal to no sample pretreatment and that can operate under ambient conditions is described. This miniaturized technology enables real-time, rapid, and high-throughput analysis of trace compounds in complex mixtures, especially high ionic strength and viscous samples that can be challenging for conventional ionization techniques such as electrospray ionization. This technique takes advantage of high order surface acoustic wave (SAW) vibrations that both manipulate small volumes of liquid mixtures containing trace analyte compounds and seamlessly transfers analytes from the liquid sample into gas phase ions for mass spectrometry (MS) analysis. Drugs in human whole blood and plasma and heavy metals in tap water have been successfully detected at nanomolar concentrations by coupling a SAW atomization and ionization device with an inexpensive, paper-based sample delivery system and mass spectrometer. The miniaturized SAW ionization unit requires only a modest operating power of 3 to 4 W and, therefore, provides a viable and efficient ionization platform for the real-time analysis of a wide range of compounds.