Tracking Energy Transfer across a Platinum Center.

Rigid, conjugated alkyne bridges serve as important components in various transition-metal complexes used for energy conversion, charge separation, sensing, and molecular electronics. Alkyne stretching modes have potential for modulating charge separation in donor-bridge-acceptor compounds. Understanding the rules of energy relaxation and energy transfer across the metal center in such compounds can help optimize their electron transfer switching properties. We used relaxation-assisted two-dimensional infrared spectroscopy to track energy transfer across metal centers in platinum complexes featuring a triazole-terminated alkyne ligand of two or six carbons, a perfluorophenyl ligand, and two tri(p-tolyl)phosphine ligands. Comprehensive analyses of waiting-time dynamics for numerous cross and diagonal peaks were performed, focusing on coherent oscillation, energy transfer, and cooling parameters. These observables augmented with density functional theory computations of vibrational frequencies and anharmonic force constants enabled identification of different functional groups of the compounds. Computations of vibrational relaxation pathways and mode couplings were performed, and two regimes of intramolecular energy redistribution are described. One involves energy transfer between ligands via high-frequency modes; the transfer is efficient only if the modes involved are delocalized over both ligands. The energy transport pathways between the ligands are identified. Another regime involves redistribution via low-frequency delocalized modes, which does not lead to interligand energy transport.

Structure/Function Analysis of Truncated Amino-Terminal ACE2 Peptide Analogs That Bind to SARS-CoV-2 Spike Glycoprotein.

The global burden of the SARS-CoV-2 pandemic is thought to result from a high viral transmission rate. Here, we consider mechanisms that influence host cell-virus binding between the SARS-CoV-2 spike glycoprotein (SPG) and the human angiotensin-converting enzyme 2 (ACE2) with a series of peptides designed to mimic key ACE2 hot spots through adopting a helical conformation analogous to the N-terminal α1 helix of ACE2, the region experimentally shown to bind to the SARS-CoV-2 receptor-binding domain (RBD). The approach examines putative structure/function relations by assessing SPG binding affinity with surface plasmon resonance (SPR). A cyclic peptide (c[KFNHEAEDLFEKLM]) was characterized in an α-helical conformation with micromolar affinity (KD = 500 µM) to the SPG. Thus, stabilizing the helical structure of the 14-mer through cyclization improves binding to SPG by an order of magnitude. In addition, end-group peptide analog modifications and residue substitutions mediate SPG binding, with net charge playing an apparent role. Therefore, we surveyed reported viral variants, and a correlation of increased positive charge with increased virulence lends support to the hypothesis that charge is relevant to enhanced viral fusion. Overall, the structure/function relationship informs the importance of conformation and charge for virus-binding analog design.

Detection of Human Neutrophil Elastase by Fluorescent Peptide Sensors Conjugated to TEMPO-Oxidized Nanofibrillated Cellulose.

Peptide-cellulose conjugates designed for use as optical protease sensors have gained interest for point-of-care (POC) detection. Elevated serine protease levels are often found in patients with chronic illnesses, necessitating optimal biosensor design for POC assessment. Nanocellulose provides a platform for protease sensors as a transducer surface, and the employment of nanocellulose in this capacity combines its biocompatibility and high specific surface area properties to confer sensitive detection of dilute biomarkers. However, a basic understanding of the spatiotemporal relationships of the transducer surface and sensor disposition is needed to improve protease sensor design and development. Here, we examine a tripeptide, fluorogenic elastase biosensor attached to TEMPO-oxidized nanofibrillated cellulose via a polyethylene glycol linker. The synthetic conjugate was found to be active in the presence of human neutrophil elastase at levels comparable to other cellulose-based biosensors. Computational models examined the relationship of the sensor molecule to the transducer surface. The results illustrate differences in two crystallite transducer surfaces ((110) vs. (1-10)) and reveal preferred orientations of the sensor. Finally, a determination of the relative (110) vs. (1-10) orientations of crystals extracted from cotton demonstrates a preference for the (1-10) conformer. This model study potentiates the HNE sensor results for enhanced sensor activity design.

Unidirectional coherent energy transport via conjugated oligo(p-phenylene) chains.

We discovered a way to funnel high-frequency vibrational quanta rapidly and unidirectionally over large distances using oligo(p-phenylene) chains. After mid-IR photon photoexcitation of a -COOH end group, the excess energy is injected efficiently into the chain, forming vibrational wavepackets that propagate freely along the chain. The transport delivers high-energy vibrational quanta with a range of transport speeds reaching 8.6 km/s, which exceeds the speed of sound in common metals (∼5 km/s) and polymers (∼2 km/s). Efficiencies of energy injection into the chain and transport along the chain are found to be very high and dependent on the extent of conjugation across the structure. By tuning the degree of conjugation via electronic doping of the chain, the transport speed and efficiency can be controlled. The study opens avenues for developing materials with controllable energy transport properties for heat management, schemes with efficient energy delivery to hard-to-reach regions, including transport against thermal gradients, and ways for initiating chemical reactions remotely.

Clinical Translational Potential in Skin Wound Regeneration for Adipose-Derived, Blood-Derived, and Cellulose Materials: Cells, Exosomes, and Hydrogels.

Acute and chronic skin wounds due to burns, pressure injuries, and trauma represent a substantial challenge to healthcare delivery with particular impacts on geriatric, paraplegic, and quadriplegic demographics worldwide. Nevertheless, the current standard of care relies extensively on preventive measures to mitigate pressure injury, surgical debridement, skin flap procedures, and negative pressure wound vacuum measures. This article highlights the potential of adipose-, blood-, and cellulose-derived products (cells, decellularized matrices and scaffolds, and exosome and secretome factors) as a means to address this unmet medical need. The current status of this research area is evaluated and discussed in the context of promising avenues for future discovery.

Proving and Probing the Presence of the Elusive C-H⋠⋠⋠O Hydrogen Bond in Liquid Solutions at Room Temperature.

Hydrogen bonds (H bonds) play a major role in defining the structure and properties of many substances, as well as phenomena and processes. Traditional H bonds are ubiquitous in nature, yet the demonstration of weak H bonds that occur between a highly polarized C-H group and an electron-rich oxygen atom, has proven elusive. Detailed here are linear and nonlinear IR spectroscopy experiments that reveal the presence of H bonds between the chloroform C-H group and an amide carbonyl oxygen atom in solution at room temperature. Evidence is provided for an amide solvation shell featuring two clearly distinguishable chloroform arrangements that undergo chemical exchange with a time scale of about 2 ps. Furthermore, the enthalpy of breaking the hydrogen bond is found to be 6-20 kJ mol-1 . Ab-initio computations support the findings of two distinct solvation shells formed by three chloroform molecules, where one thermally undergoes hydrogen-bond making and breaking.

Low-Temperature Vibrational Energy Transport via PEG Chains.

We used relaxation-assisted two-dimensional infrared spectroscopy to study the temperature dependence (10-295 K) of end-to-end energy transport across end-decorated PEG oligomers of various chain lengths. The excess energy was introduced by exciting the azido end-group stretching mode at 2100 cm-1 (tag); the transport was recorded by observing the asymmetric C═O stretching mode of the succinimide ester end group at 1740 cm-1. The overall transport involves diffusive steps at the end groups and a ballistic step through the PEG chain. We found that at lower temperatures the through-chain energy transport became faster, while the end-group diffusive transport time and the tag lifetime increase. The modeling of the transport using a quantum Liouville equation linked the observations to the reduction of decoherence rate and an increase of the mean-free-path for the vibrational wavepacket. The energy transport at the end groups slowed down at low temperatures due to the decreased number and efficiency of the anharmonic energy redistribution pathways.

Intense-field interaction regime with weak laser pulses and localized plasmonic enhancement: Reference-free demonstration by 3rd- and 5th-order infrared spectroscopies.

In bulk materials, intense field interaction is accompanied by undesired nonresonant processes. Plasmonic nanostructures localize enhanced fields exclusively in their vicinity. We report a 4-fold vibrational population inversion between all the excited and the ground states in the molecular monolayer on the surface of gold nanoantennas. Excited population assessment relies on a novel reference-sample-free evaluation of the field enhancement with 5th- and 3rd-order nonlinear infrared spectroscopies and on quantitative modeling of coherent excitation dynamics. This study opens opportunities for precise population control utilizing population inversion for vibrational transitions using weak fields.

Radiative Enhancement of Linear and Third-Order Vibrational Excitations by an Array of Infrared Plasmonic Antennas.

Infrared gold antennas localize enhanced near fields close to the metal surface, when excited at the frequency of their plasmon resonance, and amplify vibrational signals from the nearby molecules. We study the dependence of the signal enhancement on the thickness of a polymer film containing vibrational chromophores, deposited on the antenna array, using linear (FTIR) and third-order femtosecond vibrational spectroscopy (transient absorption and 2DIR). Our results show that for a film thickness beyond only a few nanometers the near-field interaction is not sufficient to account for the magnitude of the observed signal, which nevertheless has a clear Fano line shape, suggesting a radiative origin of the molecule-plasmon interaction. The mutual radiative damping of plasmonic and molecular transitions leads to the spectroscopic signal of a molecular vibrational excitation to be enhanced by up to a factor of 50 in the case of linear spectroscopy and over 2000 in the case of third-order spectroscopy. A qualitative explanation for the observed effect is given by the extended coupled oscillators model, which takes into account both near-field and radiative interactions between the plasmonic and molecular transitions.

Band-selective ballistic energy transport in alkane oligomers: toward controlling the transport speed.

Intramolecular transport of vibrational energy in two series of oligomers featuring alkane chains of various length was studied by relaxation-assisted two-dimensional infrared spectroscopy. The transport was initiated by exciting various end-group modes (tags) such as different modes of the azido (ν(N≡N) and ν(N═N)), carboxylic acid (ν(C═O)), and succinimide ester (νas(C═O)) with short mid-IR laser pulses. It is shown that the transport via alkane chains is ballistic and the transport speed is dependent on the type of the tag mode that initiates the transport. The transport speed of 8.0 Å/ps was observed when initiated by either ν(C═O) or νas(C═O). When initiated by ν(N≡N) and ν(N═N), the transport speed of 14.4 ± 2 and 11 ± 4 Å/ps was observed. Analysis of the vibrational relaxation channels of different tags, combined with the results for the group velocity evaluation, permits identification of the chain bands predominantly contributing to the transport for different cases of the transport initiation. For the transport initiated by ν(N≡N) the CH2 twisting and wagging chain bands were identified as the major energy transport channels. For the transport initiated by ν(C═O), the C-C stretching and CH2 rocking chain bands served as major energy transporters. The transport initiated by ν(N═N) results in direct formation of the wave packet within the CH2 twisting and wagging chain bands. These developments can aid in designing molecular systems featuring faster and more controllable energy transport in molecules.