Understanding the Amide-II Vibrations in β‑Peptides

In this work, the vibrational characteristics of the amide-II modes in β-peptides in five helical conformations, namely, 8-, 10-, 12-, 14-, and 10/12-helices, have been examined. Remarkable conformational dependence of the amide-II spectral profile is obtained by ab initio computations as well as modeling analysis. Intramolecular hydrogen-bonding interaction and its influence on backbone structure and on the amide-II local-mode transition frequencies and intensities are examined. Through-space and through-bond contributions of the amide-II vibrational couplings are analyzed, and it was found that hydrogen-bonding interaction is not a determining factor for the coupling strength. The results reported here provide useful benchmarks for understanding experimental amide-II infrared spectra of β-peptides and suggest the potential application of this mode on monitoring the structures and dynamics of β-peptides

Vibrational Characterization of Two-Dimensional Graphdiyne Sheets

Graphdiyne is formed by two acetylene bonds conjugatively connecting two phenyl rings, which then extend to a sizable two-dimensional structure. In this work, a molecular size-dependent CC stretching vibrational transition intensity is predicted by density functional theory in graphdiyne; in particular, the strongest infrared-active transition is enhanced in intensity by a factor of as high as 2190 from phenyl-acetylenic dimer to graphdiynic 46-mer, showing ca. 300 times enhancement per acetylene bond on average, examined at the level of B3LYP/6-31G*. Such an enhancement of vibrational transition intensity is caused by intramolecular electronic delocalization that gives rise to an enlarged transition dipole moment as well as by intramolecular vibrational delocalization that gives rise to intensity borrowing in such two-dimensionally conjugated graphdiyne molecular sheets. The enhancement is found to be diminished in defected graphdiynes. The results suggest that the periodically appearing CC bond may be used as a vibrational spectroscopic marker for assessing the size of perfect graphdiyne oligomers, and the characteristic CC stretching absorption can potentially be used to differentiate perfect graphdiyne sheets from defected ones

Dissecting Amide‑I Vibration in β‑Peptide Helices

The vibrational properties of the amide-I modes of β-peptides in five helical conformations (8-helix, 10-helix, 12-helix, 14-helix, and 10/12-helix) from tetramer to heptamer were examined by ab initio calculations. The normal modes have been first decoupled into local modes, whose transition energies are found to be intrinsically sensitive to peptide structure and intramolecular hydrogen bonding interactions. By further removing the intramolecular hydrogen bonding interactions, pure local modes are obtained, whose transition energies still exhibit some conformational dependence in 8-helix and 10/12 hybrid helix, but not much in homogeneous 10-, 12-, and 14-helical conformations. This suggests that a set of nearly degenerated pure local-mode transitions can be specified when excitonic modeling the amide-I vibration in latter cases. The work provides important benchmark measurements for understanding the complexity of the amide-I absorption spectra of β-polypeptides

Vocalizations of female frogs contain nonlinear characteristics and individual signatures

<div><p>Anuran vocalization is sexually dimorphic, with males doing the bulk of vocalizing. Female vocalization is rare and has been observed in a handful of species, including the concave-eared torrent frog (<i>Odorrana tormota</i>). Females <i>O</i>. <i>tormota</i> have been reported to emit moderate-level calls to attract males. In contrast to males, female’s vocal signals show no evidence of nonlinear phenomena (NLP). However, with females emitting calls so infrequently that this conclusion must be considered tentative in light of the limited supporting data. The present study was undertaken to test the hypotheses that their vocalizations: 1. may not be purely linear, 2. may contain individual signatures, similar to their male counterparts. We recorded 671 calls from six captive gravid females and found that their vocalizations are as complex as male calls, with numerous calls exhibiting complex upward/downward frequency modulations, and 39% of female calls containing at least one component of the NLP, i.e., subharmonics, deterministic chaos, frequency jump, or biphonation. Furthermore, females in captivity tend to call in bouts throughout the day and night, and the call rate varies hourly with a maximum of >10 calls per minute matching the maximum call rate in males. Similar to males, female vocalizations carry individual signatures, and all sound parameters analyzed differ significantly between individuals. This represents the first report ever showing that vocalizations of female anurans: 1. contain NLP, 2. carry individual signatures. Presence of signatures in both the male and female vocalizations opens up the possibility for males (and females) to distinguish individual frogs in both sexes acoustically, and thus their sound communication ability may be more advanced than previously thought.</p></div

Temporal segmentation of calls.

<p>Shown here is the waveform (top trace) and spectrogram (bottom trace) of a call of female <i>Odorrana tormota</i>. The call was segmented into segments containing harmonic (Ha), subharmonics (Sh), deterministic chaos (Ch). This particular call does not show biphonation segments. Downward arrows indicate the times of occurrence of frequency jumps (FJ).</p

Two-Dimensional Infrared Study of ¹³C‑Natural Abundant Vibrational Transition Reveals Intramolecular Vibrational Redistribution Rather than Fluxional Exchange in Mn(CO)₅Br

In this work, molecular-symmetry enhanced ¹³CO natural abundant isotopic infrared transition was identified in Mn­(CO)₅Br dissolved in CCl₄ by FTIR spectroscopy. Diagonal and associated off-diagonal two-dimensional IR (2D IR) peaks of the ¹³CO-species were found to be spectrally separated from the all-¹²CO species, allowing a direct probe of the ¹³C natural abundant ensemble. Temperature-dependent FTIR experiment showed no evidence of ligand exchange in the metal carbonyl complex. Intramolecular vibrational redistribution dynamics among the CO stretching vibrational states were extracted using population-time dependent 2D IR diagonal and off-diagonal peaks for both radial mono-¹³CO and all-¹²CO isotopomers. This work demonstrates the potential use of natural abundant isotopic molecular species as a probe for revealing equilibrium and nonequilibrium structural dynamics in condensed-phase molecular systems

Lithium Sulfide (Li₂S)/Graphene Oxide Nanospheres with Conformal Carbon Coating as a High-Rate, Long-Life Cathode for Li/S Cells

In recent years, lithium/sulfur (Li/S) cells have attracted great attention as a candidate for the next generation of rechargeable batteries due to their high theoretical specific energy of 2600 W·h kg^–1, which is much higher than that of Li ion cells (400–600 W·h kg^–1). However, problems of the S cathode such as highly soluble intermediate species (polysulfides Li₂S_<i>n</i>, <i>n</i> = 4–8) and the insulating nature of S cause poor cycle life and low utilization of S, which prevents the practical use of Li/S cells. Here, a high-rate and long-life Li/S cell is proposed, which has a cathode material with a core–shell nanostructure comprising Li₂S nanospheres with an embedded graphene oxide (GO) sheet as a core material and a conformal carbon layer as a shell. The conformal carbon coating is easily obtained by a unique CVD coating process using a lab-designed rotating furnace without any repetitive steps. The Li₂S/GO@C cathode exhibits a high initial discharge capacity of 650 mA·h g^–1 of Li₂S (corresponding to the 942 mA·h g^–1 of S) and very low capacity decay rate of only 0.046% per cycle with a high Coulombic efficiency of up to 99.7% for 1500 cycles when cycled at the 2 C discharge rate

Spectral characteristics of calls of female <i>O</i>. <i>tormota</i>.

<p>Spectral characteristics of calls of female <i>O</i>. <i>tormota</i>.</p

Results of stepwise forward discriminant function analysis (DFA).

<p>Results of stepwise forward discriminant function analysis (DFA).</p

Results of univariate ANOVA tests that examined between-female variability.

<p>Results of univariate ANOVA tests that examined between-female variability.</p