Chirality-induced spin polarization places symmetry constraints on biomolecular interactions

Abstract

Noncovalent interactions between molecules are key for many biological processes. Necessarily, when molecules interact, the electronic charge in each of them is redistributed. Here, we show experimentally that, in chiral molecules, charge redistribution is accompanied by spin polarization. We describe how this spin polarization adds an enantioselective term to the forces, so that homochiral interaction energies differ from heterochiral ones. The spin polarization was measured by using a modified Hall effect device. An electric field that is applied along the molecules causes charge redistribution, and for chiral molecules, a Hall voltage is measured that indicates the spin polarization. Based on this observation, we conjecture that the spin polarization enforces symmetry constraints on the biorecognition process between two chiral molecules, and we describe how these constraints can lead to selectivity in the interaction between enantiomers based on their handedness. Model quantum chemistry calculations that rigorously enforce these constraints show that the interaction energy for methyl groups on homochiral molecules differs significantly from that found for heterochiral molecules at van der Waals contact and shorter (i.e., ∼0.5 kcal/mol at 0.26 nm).spin | chirality | enantioselectivity | biorecognition | exchange interaction T he wealth of information on protein structure has led to a much better understanding of the relation between structure and function in biomolecular processes and biological machines (1); however, basic phenomena remain unexplained in terms of structure-function relationships. Biorecognition, which is based on noncovalent interactions between molecules, retains mysteries, and its calculation evades first principles theory (2, 3). This failure suggests that some essential features are not included in our current description of these interactions (4,5). In this study, we show that charge polarization, which occurs in any biorecognition event, is accompanied by spin polarization for chiral molecules, an effect that is missing in most treatments. The subsequent magnetic interaction energies are small and therefore, play no significant role in the interactions; however, the spin polarization constrains the symmetry of the wave function(s) involved with the intermolecular interaction, so that significant differences in energy emerge for interactions between molecules of the same chirality and those of opposite chirality. Thus, this phenomenon may impact quantitative modeling of biorecognition events and contribute to our understanding of enantiorecognition in nature (6).Nucleotides, amino acids, and sugars are chiral; namely, they do not possess mirror plane symmetry but have symmetry like a "hand" (cheir in Greek). Force field models for the interaction between biomolecules do not account for spin polarization or include terms with chiral symmetry. Noncovalent interactions between biomolecules are commonly described classically by way of force fields, which are constructed from their geometrie...

Magnetless Device for Conducting Three‐Dimensional Spin‐Specific Electrochemistry

“…[7,8] It has also been determined that, when chiral molecules are polarized by applying an electric field, the charge reorganization is accompanied by spin polarization of the electrons. [9] Theactual chirality of the enantiomer defines which spin direction is associated with which electric pole.…”

“…[10,11] In those studies,amagnetized substrate was coated with chiral molecules that transmit one preferred spin direction. [9] This term affects the repulsion part of the interacting potential, so that the repulsion is "softer" for the homochiral system than for aheterochiral system. Electrodes coated with chiral molecules were also used in the past for enantioselective electrochemistry studies,m ainly aimed at developing enantioselective electrochemical sensors.…”

“…[9] Figure 2p resents the results obtained when the GaN device is coated with as elf-assembled monolayer of achiral molecules (3-mercapto propanoic acid (MPA)) or with chiral a-helical oligopeptides of variable length l-SHCH 2 CH 2 CO-{Ala-Aib} n -COOH (alanine (Ala), a-aminoisobutyric acid (Aib), n = 5, 7, and 9, for AL-5, AL-7, and AL-9, respectively). We found that the enantioselectivity correlates with the spin polarization.…”

“…However, recently it has been suggested that enantioselectivity also involves an electronic term that relates to the electrons correlation energy;hence,itisaffected by the electrons spin. [9] This term affects the repulsion part of the interacting potential, so that the repulsion is "softer" for the homochiral system than for aheterochiral system.…”

“…Hence,w ef urther validate that chiral recognition occurs,n ot only with stereoselectivity but as pin-related interaction contributes very significantly to the recognition process. [9] Figure 2p resents the results obtained when the GaN device is coated with as elf-assembled monolayer of achiral molecules (3-mercapto propanoic acid (MPA)) or with chiral a-helical oligopeptides of variable length l-SHCH 2 CH 2 CO-{Ala-Aib} n -COOH (alanine (Ala), a-aminoisobutyric acid (Aib), n = 5, 7, and 9, for AL-5, AL-7, and AL-9, respectively). The a-helical oligopeptides are covalently bound to the surface (GaO x )w ith the carboxylic group (C-terminal) and to silver clusters (15-20 nm in diameter;S upporting Information, Figure S1) through the thiol group at the other end (N-terminal;f or ar epresentation, see Figure 1c;S upporting Information, Figures S2 and S3).…”