Adsorbed Water Structure on Acrylate-Based Biocompatible Polymer Surface.

The role of water in the excellent biocompatibility of the acrylate-based polymers widely used for antibiofouling coating material has been realized previously. Here, we report femtosecond mid-infrared pump-probe spectroscopy of the OD stretch band of HOD molecule adsorbed on highly biocompatible poly(2-methoxyethyl) acrylate [PMEA] and poorly biocompatible poly(2-phenoxyethyl) acrylate [PPEA], both of which reveal that there are two water species with significantly different vibrational lifetime. PMEA interacts more strongly with water than PPEA through the H-bonding interaction between carbonyl (C═O) and water. The vibrational lifetime of the OD stretch in PPEA is notably longer by factors of 3 and 7 than those in PMEA and bulk water, respectively. The IR-pump visible-probe photothermal imaging further unravels substantial spatial overlap between polymer CO group and water for hydrated PMEA and a significant difference in surface morphology than those in PPEA, which exhibits the underlying relationships among polymer-water interaction, surface morphology, and biocompatibility.

Studying Water Hydrogen-Bonding Network near the Lipid Multibilayer with Multiple IR Probes.

A critical difference between living and nonliving is the existence of cell membranes, and hydration of membrane surface is a prerequisite for structural stability and various functions such as absorption/desorption of drugs, proteins, and ions. Therefore, a molecular level understanding of water structure and dynamics near the membrane is important to perceive the role of water in such a biologically relevant environment. In our recent paper [ J. Phys. Chem. Lett. 2016 , 7 , 741 ] on the IR pump-probe study of the OD stretch mode of HDO near lipid multibilayers, we have observed two different vibrational lifetime components of OD stretch mode in the phospholipid multibilayer systems. The faster component (0.6 ps) is associated with OD groups interacting with the phosphate moiety of the lipid, while the slower component (1.9 ps) is due to choline-associated water molecules that are close to bulklike water. Here, we additionally use hydrazoic acid (HN3) as another IR probe of which frequency is highly sensitive to its local H-bonding water density. Interestingly, we found that the vibrational lifetime of the asymmetric azido stretch mode of HN3 in the lipid multibilayer system is similar to that in neat water, whereas its orientational relaxation is a bit slower than that in bulk water. This indicates that due to the tight packing of lipid molecules, particularly the head parts, in the gel phase, HN3 molecules mostly stay near the choline group of lipid and interact with water molecules in the vicinity of choline groups. This suggests that membrane surface-adsorbed molecules such as hydrophilic drug molecules may interact with choline-associated water molecules, when the membrane is in the gel phase, instead of phosphate-associated water molecules.

Water Dynamics in Cytoplasm-Like Crowded Environment Correlates with the Conformational Transition of the Macromolecular Crowder.

Polyethylene glycol (PEG) is a unique polymer material with enormous applicability in many industrial and scientific fields. Here, its use as macromolecular crowder to mimic the cellular environment in vitro is the focus of the present study. We show that femtosecond mid-IR pump-probe spectroscopy using three different IR probes, HDO, HN3, and azido-derivatized crowder, provides complete and stereoscopic information on water structure and dynamics in the cytoplasm-like macromolecular crowding environment. Our experimental results suggest two distinct subpopulations of water molecules: those that interact with other water molecules and those that are part of a hydration shell of crowder on its surface. Interestingly, water dynamics even in highly crowded environment remains bulk-like in spite of significant perturbation to the tetrahedral H-bonding network of water molecules. That is possible because of the formation of water aggregates (pools) even in water-deficient PEGDME-water solutions. In such a crowded environment, the conformationally accessible phase space of the macromolecular crowder is reduced, similar to biopolymers in highly crowded cytoplasm. Nonetheless, the hydration water on the surface of crowders slows down considerably with increased crowding. Most importantly, we do not observe any coalescing of surface hydration water (of the crowder) with bulk-like water to generate collective hydration dynamics at any crowder concentration, contrary to recent reports. We anticipate that the present triple-IR-probe approach is of exceptional use in studying how conformational states of crowders correlate with structural and dynamical changes of water, which is critical in understanding their key roles in biological and industrial applications.

Ultrafast vibrational spectroscopy of cyanophenols.

Aromatic compounds with electron-donating or -accepting substituents exhibit interesting resonance effects on a variety of chemical reactivities and optical properties. To understand such effects and the possible relationship between vibrational energy dissipation pathways and resonance structures of aromatic compounds, we studied ortho-, meta-, and para-substituted cyanophenols and their anionic forms in methanol by using time- and frequency-resolved pump-probe and two-dimensional IR spectroscopy, where the nitrile group acts as an IR probe. From the measured transient spectra and singular-value decomposition analyses, we found that there is a combination band whose frequency is very close to that of the nitrile stretch mode. Due to the difference in the lifetimes of these two mode excited states, the transient pump-probe spectra commonly show notable blue-shifting behaviors in time. Comparing the vibrational lifetimes of neutral cyanophenols and cyanophenoxide anions in methanol and carrying out quantum mechanical/molecular mechanical molecular dynamics simulations to study hydrogen-bonding dynamics, we found that the vibrational energy of the nitrile stretch mode initially relaxes to intramolecular degrees of freedom instead of solvent modes. Also, the vibrational anharmonic frequency shifts, intrinsic lifetimes, and bandwidths of the nitrile stretch mode and the combination mode in these molecular systems are fully characterized, and their relationships with resonance structures are discussed. It is believed that the present work sheds light on the intrinsic vibrational relaxation process of the nitrile stretch mode in cyanophenols, even in the case when their IR spectra are congested by the spectrally overlapping combination bands, and the resonance effects of aromatic compounds on vibrational dynamics and relaxation processes.

Integrated and dispersed photon echo studies of nitrile stretching vibration of 4-cyanophenol in methanol.

By means of integrated and dispersed IR photon echo measurement methods, the vibrational dynamics of C-N stretch modes in 4-cyanophenol and 4-cyanophenoxide in methanol is investigated. The vibrational frequency-frequency correlation function (FFCF) is retrieved from the integrated photon echo signals by assuming that the FFCF is described by two exponential functions with about 400 fs and a few picosecond components. The excited state lifetimes of the C-N stretch modes of neutral and anionic 4-cyanophenols are 1.45 and 0.91 ps, respectively, and the overtone anharmonic frequency shifts are 25 and 28 cm(-1). At short waiting times, a notable underdamped oscillation, which is attributed to a low-frequency intramolecular vibration coupled to the CN stretch, in the integrated and dispersed vibrational echo as well as transient grating signals was observed. The spectral bandwidths of IR absorption and dispersed vibrational echo spectra of the 4-cyanophenoxide are significantly larger than those of its neutral form, indicating that the strong interaction between phenoxide and methanol causes large frequency fluctuation and rapid population relaxation. The resonance effects in a paradisubstituted aromatic compound would be of interest in understanding the conjugation effects and their influences on chemical reactivity of various aromatic compounds in organic solvents.

Femtosecond characterization of vibrational optical activity of chiral molecules.

Optical activity is the result of chiral molecules interacting differently with left versus right circularly polarized light. Because of this intrinsic link to molecular structure, the determination of optical activity through circular dichroism (CD) spectroscopy has long served as a routine method for obtaining structural information about chemical and biological systems in condensed phases. A recent development is time-resolved CD spectroscopy, which can in principle map the structural changes associated with biomolecular function and thus lead to mechanistic insights into fundamental biological processes. But implementing time-resolved CD measurements is experimentally challenging because CD is a notoriously weak effect (a factor of 10(-4)-10(-6) smaller than absorption). In fact, this problem has so far prevented time-resolved vibrational CD experiments. Here we show that vibrational CD spectroscopy with femtosecond time resolution can be realized when using heterodyned spectral interferometry to detect the phase and amplitude of the infrared optical activity free-induction-decay field in time (much like in a pulsed NMR experiment). We show that we can detect extremely weak signals in the presence of large achiral background contributions, by simultaneously measuring with a femtosecond laser pulse the vibrational CD and optical rotatory dispersion spectra of dissolved chiral limonene molecules. We have so far only targeted molecules in equilibrium, but it would be straightforward to extend the method for the observation of ultrafast structural changes such as those occurring during protein folding or asymmetric chemical reactions. That is, we should now be in a position to produce 'molecular motion pictures' of fundamental molecular processes from a chiral perspective.

Femtosecond spectral interferometry of optical activity: theory.

Optical activities such as circular dichroism (CD) and optical rotatory dispersion (ORD) are manifested by almost all natural products. However, the CD is an extremely weak effect so that time-resolved CD spectroscopy has been found to be experimentally difficult and even impossible for vibrational CD with current technology. Here, we show that the weak-signal and nonzero background problems can be overcome by heterodyned spectral interferometric detection of the phase and amplitude of optical activity free-induction-decay (OA FID) field. A detailed theoretical description and a cross-polarization scheme for selectively measuring the OA FID are presented and discussed. It is shown that the parallel and perpendicular electric fields when the solution sample contains chiral molecules are coupled to each other. Therefore, simultaneous spectral interferometric measurements of the parallel and perpendicular FID fields can provide the complex susceptibility, which is associated with the circular dichroism and optical rotatory dispersion as its imaginary and real parts, respectively. On the basis of the theoretical results, to examine its experimental possibility, we present numerical simulations for a model system. We anticipate the method discussed here to be a valuable tool for detecting electronic or vibrational optical activity in femtosecond time scale.

Vibrational dynamics of N-H, C-D, and C=O modes in formamide.

By means of heterodyned two-dimensional IR photon echo experiments on liquid formamide and isotopomers the vibrational frequency dynamics of the N-H stretch mode, the C-D mode, and the C=O mode were obtained. In each case the vibrational frequency correlation function is fitted to three exponentials representing ultrafast (few femtoseconds), intermediate (hundreds of femtoseconds), and slow (many picoseconds) correlation times. In the case of N-H there is a significant underdamped contribution to the correlation decay that was not seen in previous experiments and is attributed to hydrogen-bond librational modes. This underdamped motion is not seen in the C-D or C=O correlation functions. The motions probed by the C-D bond are generally faster than those seen by N-H and C=O, indicating that the environment of C-D interchanges more rapidly, consistent with a weaker C-D...O=C bond. The correlation decays of N-H and C=O are similar, consistent with both being involved in strong H bonding.

Multidimensional infrared spectroscopy of the N-H bond motions in formamide.

The heterodyned two-dimensional (2D) IR spectra and equilibrium dynamics of the N-H stretching motion of DCONHD in deuterated formamide, DCOND(2), were studied with 80 fs pulses at 3 microm. The time evolution of the heterodyned 2D IR spectra, pump-probe spectra, and photon echo peak shift demonstrate that interstate dynamics is occurring by relaxation of the original N-H excitation. The N-H vibrational frequency correlation function can be expressed as a sum of three exponentials with correlation times 0.24 ps, 0.8 ps, and 11 ps. The intermediate component is attributed to motions of the N-Hcdots, three dots, centeredO unit involving only slight angular variations of the N-H bond. The slow component is attributed to the structure breaking and making. The anisotropy decay confirmed that the significant angular N-H bond motion occurs on the 11 ps time scale. The fast component, which is the least well determined, might correspond to the modulation of the H-bond distance without angular motion. The correlation coefficient between the pumped and relaxed state distributions was +0.51, implying that the excited state phase memory is only slightly diminished by the relaxation of the N-H excitation. The relaxed modes are concluded to be local to the driven N-H mode.

Excitonic coupling strength and coherence length in the singlet and triplet excited states of meso-meso directly linked Zn(II)porphyrin arrays.

Excitation-energy transport (EET) phenomena in meso-meso directly linked Zn(II)porphyrin arrays in the singlet and triplet excited states were investigated with a view to electronic coupling strength and coherence length by steady-state and time-resolved spectroscopic measurements. To investigate energy transfer in the triplet states, we modified the Zn(II)porphyrin arrays with bromo substituents at both ends. The coupling strength of the Soret bands of the arrays was estimated to be about 2200 cm-1, and that of the Q bands is about 570 cm-1. The coherence length in the S1 state of the Zn(II)porphyrin arrays was determined to be 4-5 porphyrin units, which is comparable to that of the well-ordered two-dimensional circular structure B850 in the peripheral light-harvesting antenna (LH2) in photosynthetic purple bacteria. This indicates that the Zn(II)porphyrin arrays are well suited for mimicking natural light-harvesting antenna complexes. On the other hand, the rate of energy transfer in the triplet state is estimated to be on the order of 100 microseconds-1, and the very weak coupling between the triplet states (ca. 0.003 cm-1), indicates that the triplet excitation energy is essentially localized on a single porphyrin moiety.

Time-resolved spectroscopic study on photoinduced electron-transfer processes in Zn(II)porphyrin-Zn(II)chlorin-fullerene triad.

Femtosecond time-resolved transient absorption studies have been performed to investigate the photoinduced energy and electron-transfer processes in Zn(II)porphyrin-Zn(II)chlorin-fullerene triad in which energy and oxidation potential gradients are directed along the donor-acceptor-linked arrays. Fast energy transfer (approximately 450 fs) from photoexcited Zn(II)porphyrin to Zn(II)chlorin was observed upon selective photoexcitation of Zn(II)porphyrin unit in the triad. In a nonpolar solvent such as toluene, the energy transfer from the excited singlet state of Zn(II)chlorin to fullerene occurs and is followed by the formation of an intermediate state with a time constant of nanoseconds, which was attributed to the intramolecular exciplex between Zn(II)chlorin and fullerene. In benzonitrile, on the other hand, the photoexcitation of the triad results in the fast electron transfer (< 1 ps) from photoexcited Zn(II)chlorin to fullerene. The generated charge-separated species recombine with a time constant of approximately 12 ps. The relatively fast charge separation and charge recombination rates imply that the strong electronic coupling between Zn(II)chlorin and fullerene moieties is probably induced by the folded conformation between Zn(II)chlorin and fullerene moieties which enhances direct through-space interaction between the approximately contacted pi systems.