Tracing Superchirality in the Condensed Phase

Calculation of vibrational circular dichroism by means of ab initio molecular dynamics simulations

S. Jähnigen, D. Sebastiani, Institut für Chemie,Theoretische Chemie, MLU Halle-Wittenberg

In Short

• Spectroscopic detection of chiral configurations insupramolecular structures

• “Virtual experiment”: quantum chemical calcula-tions of spectral fingerprints

• Combine statistical sampling with quantum linearresponse calculations

• Obtain VCD spectra from Fourier-transformed cor-relation functions

Superchirality is the chirality that atoms andmolecules experience when put in a supramolec-ular embedding–that is, virtually all systems that donot come across in the gas phase. In the condensedphase non-covalent interactions play an importantrole for structure and functionality of matter, be itof biological or non-biological origin. As it happens,supramolecular structures may be chiral (just thinkof proteins that bind and recognise a messengermolecule) and impose their chirality on neighbour-ing domains. Hence, superchirality can be “felt” byinmates (Edwin A. Abbott, Flatland: A romance ofmany dimensions. 1884.)–but can it also be ad-dressed experimentally? Can it be measured?

Figure 1: Chiral supramolecular structures impose their chiralityon neighbouring domains. Vibrational circular dichroism (VCD) isable to grasp this superchirality experimentally.[1] UnderstandingVCD measurements requires accurate theoretical simulations ofrealistic models.(c) 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 2: Calculated anharmonic solid-state IR (top) and VCD(bottom) spectra of crystalline L-alanine and experimental data,measured as a nujol oil mull. The theoretical result is substantiallyinfluenced by the contribution of supramolecular correlations.Calculated anharmonic solid-state IR (top) and VCD (bottom)spectra of crystalline L-alanine. [1,2](c) 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Vibrational circular dichroism (VCD) representssuch a technique which is able to resolve superchi-rality where molecular vibrations act as antennaethat probe the molecule’s environment. It denotesthe chiral information that supplements conventional(i.e., achiral) vibrational spectroscopy based on theabsorption of infrared (IR) light. As VCD combinesthe structural specifity of IR spectroscopy with thechiral sensitivity of optical activity, it leads to a broadrange of applications such as the determination ofthe absolute configuration of chiral molecules andexhibits an outstanding ability to resolve inducedchirality in orginally achiral regions.

Recently, we revealed how coherent vibrationalmodes exploit the superchirality in L-alanine crys-tals, leading to non-local enhanced VCD features.There, the VCD-enhanced signal is ascribed to he-lical arrangement of oscillators in the crystal layers(Figure 2).[1] Delivering a quantitative atomic con-ception of supramolecular chirality induction, our abinitio scheme is applicable well beyond molecularcrystals, e.g., to address VCD in proteins and relatedcompounds. As enhanced-VCD phenomena havebeen described for amyloid fibrils that are relatedto neurodegenerative disorders such as Alzheimer’sdisease, realistic model are badly needed for VCDcalculations. Yet, this implies not only designing

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Figure 3: We design realistic models of supramolecular setupsand perform highly accurate ab initio molecular dynamics (AIMD)simulations.

a supramolecular setup in space, but also in time.Ab initio molecular dynamics (AIMD) build the keytechnology for a realistic phase space sampling, in-cluding entropic effects. Resembling a “virtual exper-iment”, the calculation of spetroscopic patterns restson a statistical ensemble of the molecular model.

This project aims for the theoretical modelling andunderstanding of vibrational circular dichroism (VCD)in the condensed phase. Our tool is ab initio molec-ular dynamics (AIMD) simulations that enables usto design realistic models of crystals, polymers, pro-teins, or solvated structures. Combining ab initiomolecular dynamics (AIMD) with Nuclear VelocityPerturbation Theory (NVPT), we reach a compre-hensive description of IR and VCD spectra basedon the Fourier-transform of the time-correlation func-tions of electronic and magnetic dipole momentsthat are calculated on the fly. Simulations with the–now topical–pretensions of being both, realistic andaccurate, require the means of high performancecomputation, for which the quantum chemistry andsolid state physics software packages that we use(CPMD,CP2K) have been optimised.

WWW

http://www.chemie.uni-halle.de

More Information

[1] S. Jähnigen, A. Scherrer, R. Vuilleumier, D. Se-bastiani, Angew. Chem. Int. Ed., 2018, 57,13344–13348. doi:10.1002/anie.201805671

[2] L. A. Nafie, Vibrational Optical Activity: Princi-ples and Applications, Wiley, Hoboken, 2011.

[3] A. Scherrer, R. Vuilleumier, D. Sebastiani,Journal of Chemical Physics, 2016, 145(8),084101. doi:10.1063/1.4960653

Funding

DFG Sonderforschungsbereich Transregio (SFB/TRR) 102, project A09

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