Chiral Phonons in Biocrystals

Chiral phonons are concerted mirror-symmetric movements of atomic groups connected by covalent and intermolecular bonds. Finding chiral phonons in biocrystals is fundamentally and technologically important because these lattice vibrations should be highly specific to their short- and long-range organizations. Based on theoretical and experimental data they might be expected but not identified or utilized. Here we show that terahertz chiroptical spectroscopy enables registration and attribution of chiral phonons in microcrystals of numerous amino acids and dipeptides. Theoretical analysis and computer simulations confirm that sharp mirror-symmetric bands observed for left and right enantiomers originate from collective vibrations of biomolecules interconnected by hydrogen bonds into helical chains. Structure-property relationships for strong phonons with rotatory components in biocrystals were also identified. Bladder stones and health supplements display strong spectral signatures of chiral phonons indicating their immediate importance for biomedicine.

Collective vibrations of a hydrodynamic active lattice

Recent experiments show that quasi-one-dimensional lattices of self-propelled droplets exhibit collective instabilities in the form of out-of-phase oscillations and solitary-like waves. This hydrodynamic lattice is driven by the external forcing of a vertically vibrating fluid bath, which invokes a field of subcritical Faraday waves on the bath surface, mediating the spatio-temporal droplet coupling. By modelling the droplet lattice as a memory-endowed system with spatially non-local coupling, we herein rationalize the form and onset of instability in this new class of dynamical oscillator. We identify the memory-driven instability of the lattice as a function of the number of droplets, and determine equispaced lattice configurations precluded by geometrical constraints. Each memory-driven instability is then classified as either a super- or subcritical Hopf bifurcation via a systematic weakly nonlinear analysis, rationalizing experimental observations. We further discover a previously unreported symmetry-breaking instability, manifest as an oscillatory–rotary motion of the lattice. Numerical simulations support our findings and prompt further investigations of this nonlinear dynamical system.

Intrinsic fluorescence in non-aromatic peptide structures is induced by collective vibrations, charge reorganisation and short hydrogen bonds, as shown in a new glutamine-related structure

Disentangling the origin of the optical activity of non-aromatic proteins is challenging due to their size and thus their high computational requisites. Here we show, in a much smaller model system, that the single amino acid glutamine undergoes a chemical transformation leading to an unreported glutamine-like structure which has a similar broad absorption spectrum reported previously for non-aromatic proteins. We further show computationally that the optical activity of the glutamine-like structure is directly coupled to short-hydrogen bonds, but also displays charge and vibrational fluctuations, the latter of which are also present in less optically active structures such as in L-glutamine. Since experimentally the glutamine-like structure is the brightest structure, we conclude that short-hydrogen bonds are the ones responsible for the large Stokes shift observed in optically active non-aromatic proteins.

Phonons in deformable microporous crystalline solids

Phonons are quantum elastic excitations of crystalline solids. Classically, they correspond to the collective vibrations of atoms in ordered periodic structures. They determine the thermodynamic properties of solids and their stability in the case of structural transformations. Here we review for the first time the existing examples of the phonon analysis of adsorption-induced transformations occurring in microporous crystalline materials. We discuss the role of phonons in determining the mechanism of the deformations. We point out that phonon-based methodology may be used as a predictive tool in characterization of flexible microporous structures; therefore, relevant numerical tools must be developed.