Active Mechanical Threading by a Molecular Motor

Abstract

Molecular motors transform external energy input into directional motions and offer exquisite precision for nano-scale manipulations. To make full use of molecular motor capacities, their directional motions need to be transmitted and used for powering downstream molecular events. Here we present a macrocyclic molecular motor structure able to perform repetitive molecular threading of a flexible tetraethylene glycol chain through the macrocycle. This mechanical threading event is actively powered by the motor and leads to a direct translation of the unidirectional motor rotation into unidirectional translation motion (chain versus ring). The mechanism of the active mechanical threading is elucidated and the actual threading step is identified as a combined helix inversion and threading event. The established molecular machine function resembles the crucial step of macroscopic weaving or sewing processes and therefore offers a first entry point to a "molecular knitting" counterpart.

Keywords: Hemithioindigo; Indigoids; Molecular Machines; Molecular Motors; Photochemistry.

Figure 1

Mechanical and unidirectional threading of a TEG chain through a macrocycle powered by the operation of a molecular motor. a) Molecular structure of macrocyclic motors 1 and 2. b) Active translation of the unidirectional rotation into a directional linear threading motion. c) Stepwise mechanism of motor‐powered threading. The rotor part (light blue) serves as revolving door dragging the attached TEG chain through the macrocycle during motor operation.

Figure 2

Qualitative energy profile for the operation mechanism of macrocyclic motors 1 or 2. For isomers of type A and D two different structures (pre‐threaded i or already threaded ii) are possible. The actual threading step is possible for different isomer interconversion steps (dashed beveled arrows), either photochemically in a concerted fashion together with double bond isomerization (C to D_ii or A_i to B transition), as single thermally induced step (A_i to A_ii or D_i to D_ii ), or in a concerted, thermally induced fashion together with helix inversion (D_i to A_ii ).

Figure 3

Photochemical and thermal interconversions of macrocyclic motors 1 and 2 and spectral signatures used to identify the nature of the different isomers. a) ¹H NMR (400 MHz, CD₂Cl₂/CS₂=4/1 solution) spectra recorded during a full cycle of motor 1 rotation. Spectrum i: Starting point of pure A‐1. ii: After irradiation of A‐1 with 405 nm light at −105 °C isomer B‐1 is obtained in small amounts. iii: After thermal annealing isomer B‐1 converted exclusively to C‐1. iv: Starting point of pure C‐1. v: After irradiation of C‐1 with 450 nm light at −105 °C isomer D‐1 is obtained almost quantitatively. iv: After thermal annealing isomer D‐1 converted exclusively to A‐1. b) ¹H NMR (400 MHz, CD₂Cl₂ (i to iv) and C₂D₂Cl₄ (v to vii) solution) spectra recorded during operation of macrocycle 2. Spectrum i: Starting point of pure A‐2. ii: After irradiation with 405 nm light at 22 °C isomer C‐2 is obtained. iii: Starting point of pure C‐2. iv: After irradiation of C‐2 with 450 nm light at 22 °C isomer D‐2 is accumulated strongly. v to vii: Prolonged heating at 65 °C converts D‐2 first to C‐2 and then to A‐2 (see also Supporting Information). c) Segment of the NOESY spectrum evidencing Z configuration of A‐1. d) Segment of the NOESY spectrum evidencing E configuration of 1‐C. e) NOE spectra evidencing Z configuration of 1‐D.

Figure 4

a) Experimental ECD spectra for the two stable isomers A and C of HTI motor 1 possessing (S) configuration at the sulfoxide stereocenter. The helicity assignment is in good agreement with the theoretical description. Spectra were measured in CH₂Cl₂ solution at different indicated temperatures. Spectral changes upon irradiation of C‐2 at low temperatures show the hallmarks of a D‐isomeric structure with inverted helicity (pink versus blue spectrum, also compare to the corresponding spectra of C‐2 and D‐2 shown in subfigure (b). b) Comparison of experimental ECD spectra for the three stable isomers A, C, and D of HTI motor 2 possessing (S) configuration at the sulfoxide stereocenter in CH₂Cl₂ solution at 25 °C. Spectra of isomers C‐2 and D‐2 were originally measured for the (R) configured isomers and are mirrored to allow direct comparison with the spectra of 1 shown in sub‐Figure (a). c) Low energy minimum structures of isomers A to D of HTI motor 1 obtained from the theoretical analysis on the B3LYP‐D3BJ/6‐311G(d,p) IEFPCM(CH₂Cl₂) level of theory. From the minimum structures the Boltzmann‐averaged ECD spectra were calculated on the TD‐DFT B3LYP‐D3BJ/6‐311+G(d,p,) IEFPCM(CH₂Cl₂) level of theory. The global minimum structures are emphasized and the HTI motor component shown in darker colors.