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Dynamic Control of Intramolecular Rotation by Tuning the Surrounding Two-Dimensional Matrix Field Taizo Mori, Hokyun Chin, Kazuhiro Kawashima, Huynh Thien Ngo, Nam-Joon Cho, Waka Nakanishi, Jonathan P Hill, and Katsuhiko Ariga ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b09320 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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Dynamic Control of Intramolecular Rotation by Tuning the Surrounding Two-Dimensional Matrix Field Taizo Mori,a,b* Hokyun Chin,c Kazuhiro Kawashima,d Huynh Thien Ngo,b Nam-Joon Cho,c, e Waka Nakanishi,b Jonathan P. Hill,b Katsuhiko Arigaa, b*

a

Graduate School of Frontier Science, The University of Tokyo, 5-1-5, Kashiwanoha,

Kashiwa 277-0827, Japan

b

World Premier International (WPI) Center for Materials Nanoarchitectonics (MANA),

National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan

c

School of Materials Science and Engineering, Nanyang Technological University,

Singapore, 637553, Singapore

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d

Global Research Center for Environment and Energy Based on Nanomaterials

Science (GREEN), National Institute for Materials Science, 1-1 Namiki, Tsukuba 3050044, Japan

e

School of Chemical and Biomedical Engineering, Nanyang Technological University,

Singapore, 637459, Singapore

KEYWORD: molecular machine, intramolecular rotation, BODYPY, fluorescence, TICT, air-water interface

Taizo Mori / [email protected], Katsuhiko Ariga / [email protected]


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ABSTRACT: The intramolecular rotation of 4-farnesyloxyphenyl-4,4-difluoro-4-bora3a,4a-diaza-s-indacene (BODIPY-ISO) was controlled by tuning its local physical environment within a mixed self-assembled monolayer at an air-water interface. Intramolecular rotation was investigated by considering the twisted intramolecular charge transfer (TICT) fluorescence of BODIPY-ISO, which increases in intensity with increasing viscosity

of

the

medium.

In situ fluorescence spectroscopy was performed on mixed monolayers of BODIPY-ISO with several different lipids at the air-water interface during in-plane compression of the monolayers. Depending on the identity of the lipid used, the fluorescence of the mixed monolayers could be enhanced by mechanical compression, indicating that the rotation of BODIPY-ISO can be controlled dynamically in mixtures with lipids dispersed at the airwater interface. Taken together, our findings provide insight into strategies for controlling the dynamic behavior of molecular machines involving mechanical stimuli at interfaces.

Molecular actions and movement involve a variety of functions including chemical reactions and molecular recognition.1–3 In biological systems, molecular motions play an


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important role in molecular recognition, where concerted reactions can be controlled sequentially.4,5 Recently, molecular machines have been synthesized and their functions controlled by tuning their mechanical motion.6–9 The mechanical action of molecular machines can be more accurately controlled within a two-dimensional field (e.g., in a biological membrane) than in a three-dimensional space such as a solvent or crystal.10 Individual molecular movements and/or reactions have previously been manipulated by using the tip of a scanning tunneling microscope (STM) on a metal surface.11,12 Molecular machines have also been controlled at soft interfaces such as an air-water interface or on a lipid bilayer. It has even been recently reported that molecular machines can be used to open cell membranes13 while, at an air-water interface, molecular machines can be controlled through the application of small (1 kcal/mol or piconewton (pN)) mechanical forces,14 (for inducing variation in the conformation and function of a protein).15 The airwater interface is therefore a suitable field to manipulate and analyze molecular motions.16,17 We have designed and synthesized various molecular machines in order to investigate the possibility of controlling their conformation and function at the air-water interface.14,18– 4 ACS Paragon Plus Environment

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24

Molecular bending and flip-flop motions affect molecular recognition processes and can

be operated at the air-water interface by applying a dynamic mechanical stimulus.14,18–23 In contrast, molecular displacement or translocation involving intramolecular rotation could not be controlled even in an ordered monolayer.24 We have investigated the intramolecular rotation of molecules upon compression at an air-water interface by measuring the twisted intramolecular charge transfer (TICT) fluorescence25 of julolidine derivatives.24,26,27 The fluorescence intensity of TICT molecules increases with increasing viscosity of the solvent in which it is contained, and is promoted by inhibition of intramolecular rotation. Fluorescence intensities of julolidine derivatives contained in monolayers at the air-water interface were not suppressed during in-plane compression of the monolayers. This suggests that rotation of the julolidine derivatives is not inhibited in densely packed ordered monolayers, similarly to the case for molecular rotors and motors which freely rotate at a metal surface according to STM analysis.11, 12 It seems that controllable molecular machines at the air-water interface are subject to compression from the water phase while, in contrast, julolidine derivatives rotate in the air phase (Scheme 1). Thus, it appears that the rotation of the molecules might be affected by 5 ACS Paragon Plus Environment

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varying the ‘flexibility’ within the air phase. Appreciating that the surface viscosity of lipids varies with increasing surface pressure,28–32 we hypothesized that molecular machines might be controlled when contained or doped within a mixed monolayer, which would act as a medium for the application of mechanical force to the dopant molecules at the airwater interface.


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Scheme 1. Schematic illustration for operation of molecular machines: molecular pliers (bending), molecular manipulator (flip-flop), and molecular rotor (rotation), at the air-water interface.

In this work, the intramolecular rotation of the TICT-type intensely fluorescent molecule 4-farnesyloxyphenyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY-ISO)33,34 was controlled dynamically based on the presence of tunable lipid matrices at the air-water interface (Scheme 2). The mobility of the TICT-type molecule was controlled by tuning its local environment. The fluorescence emission due to BODIPY-ISO increases in intensity with increasing viscosity of the prevailing medium, similarly to julolidine derivatives. Intramolecular rotation of BODIPY-ISO is arrested in the locally excited (LE) state and commences rotation in the TICT state. Therefore, the rotation of the BODIPY-ISO can be controlled dynamically in mixed lipid monolayers by molecular compression at the airwater interface. This study provides information concerning the control of molecular


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machines and motions involving mechanical stimuli not only at air-water interfaces but also at liquid-solid interfaces.


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Scheme 2. (a) Chemical structure and energy diagram of the molecular rotor, BODIPYISO. In its locally exited (LE) state, BODIPY-ISO does not rotate and exhibits fluorescence emission. In its twisted intramolecular charge transfer (TICT) state, BODIPY-ISO undergoes intramolecular rotation and no fluorescence emission occurs. (b) Static control of BODIPY-ISO rotation in solvent and a lipid bilayer, respectively. (c) Dynamic control of BODIPY-ISO rotation within a lipid matrix by compression at the airwater interface.

RESULT and DISCUSSION Optical properties of BODIPY-ISO in solution

BODIPY-ISO was synthesized according to a protocol described in the literature.35 The rotation rate of BODIPY-ISO estimated from 1H NMR measurements in CDCl3 should be similar to those reported for other BODIPY derivatives.35-40 BODIPY-ISO exhibits monomer-type fluorescence emission with a maximum at 520 nm in dilute solutions and two additional emission bands at 615 and ca. 715 nm in concentrated solutions (Figure S3). In the solid state, BODIPY-ISO exhibits only a fluorescence emission maximum at


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ca. 715 nm. Emission bands at 615 and 715 nm are respectively derived from excimer emission of J-aggregated BODIPY-ISO and its micro/nano crystalline form.36–40 Thus, both of these emission bands originate from aggregated states of BODIPY-ISO. Fluorescence emission of the BODIPY-ISO chromophore is shifted to longer wavelength by its aggregation and the intramolecular rotation is also inhibited in the aggregated states. Fluorescence spectra of BODIPY-ISO in solvents of different viscosities are showed in Figure 1. BODIPY-ISO exhibits the general characteristic of a TICT molecule that the fluorescence intensity of monomer emission increases with increasing viscosity of the solvent. A linear relationship between log(fluorescence intensity) and log(viscosity) was confirmed for BODIPY-ISO. Fluorescence intensity tends to be saturated in a high viscosity medium indicating that the rotation rate of molecules is significantly depressed and/or that the number of the rotation inhibited molecules increases in the high viscosity solvent.


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Figure 1. (a) Fluorescence spectra of BODIPY-ISO in different solvents, methanol (MeOH) ethylene glycol, and mixtures of ethylene glycol (E) /glycerol (G) (v/v) = 8/2, 5/5, 4/6, 3/7, 2/8, and glycerol at viscosity of 0.542, 0.608, 1.99, 16.1, 39.5, 140, 209, 309,


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452, 945 Pa·s, respectively. (b) Linear relationship between log(intensity) and log(viscosity).


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Optical properties of BODIPY-ISO monolayer

BODIPY-ISO in chloroform was spread on a water surface and the fluorescence spectrum of the monolayer was monitored during mechanical compression (Figure 2). Intensities of the fluorescence spectra obtained were normalized according to occupied molecular area at the corresponding surface pressure taking into account the effect of the increasing lateral density of the monolayer at higher surface pressures. The BODIPY-ISO monolayer collapses at surface pressures of 12 mN m-1 at a molecular area of 0.26 nm2, which is smaller than that expected area (0.55 nm2) based on its molecular structure. Aggregated state emission appears at 615 and ca. 715 nm prior to monolayer formation, and the intensities of these peaks increases with increasing surface pressure with a concurrent decrease in intensity of monomer emission at 520 nm. Aggregated state emission at 615 nm due to close  stacking at 0.36 – 0.42 nm appeared below the molecule area of 1.5 nm2, for which an intermolecular distance of 0.76 nm was estimated.41,42 Thus, it seems that BODIPY-ISO is aggregated at the air-water interface because intermolecular distances of 0.32 nm were estimated at molecular area of 0.26


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nm2. Fluorescence spectra observed from a BODIPY-ISO solution in toluene dispersed at an air-water interface showed similar results (Figure S4). The intensity of monomer emission increased during drying. When dry, the film exhibited predominantly aggregation state emission with no monomer emission. These results suggest that BODIPY-ISO is aggregated and that intramolecular rotation is inhibited at the air-water interface for pure BODIPY-ISO.

Figure 2. Fluorescence intensities vs. molecular area (circles connected with lines) at 520, 615, and 715 nm observed from a monolayer of BODIPY-ISO. The corresponding

–A isotherm is also shown as a black line. Fluorescence spectra observed from the


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monolayer of BODIPY-ISO at different molecular areas excited at 400 nm. Molecular area decreases passing from the blue line to the green line.

Physical properties of monolayers

BODIPY-ISO was also dispersed in several different lipids (Scheme 3) as matrices for the formation of stable mixed monolayers at the air-water interface. Matrix lipids containing long alkyl chains, such as stearic acid (SA) or 1-hexadecanol (palmityl alcohol, PA), form stable and rigid monolayers as does cholesteryl acetate (CA). In addition, monounsaturated oleic acid (OA) and methyl oleate (MO) and 1,2-dipalmitoyl-sn-glycero3-phosphocholine (DPPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) form lipid bilayers and were also used as lipid matrices. –A isotherms and fluorescence spectra of the mixed monolayers on the water surface were measured for each matrix containing BODYPY-ISO at a molar mixing ratio of 1/10. The –A isotherms reveal that the mixtures between BODIPY-ISO and each matrix form stable monolayers, as shown in Figure S5 and S6. The limiting molecular areas of BODIPY-ISO in SA, PA and CA were


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estimated to be 0.3 and 0.4, 0.5 nm2 per molecule, respectively. These values in SA and PA are smaller than that estimated from the cross-sectional area of the BODIPY-ISO molecule (0.55 nm2). In contrast, the limiting area for CA was close to this value or that for the other matrices used here were larger than this value. This suggests that the rigid lipids SA, PA showed better miscibility with BODIPY-ISO than the other matrices. Phase transitions were estimated by considering the compressibility (Cs = -1/AdA/d) isotherm. Minima in plots of Cs vs. A correspond to phase transitions after monolayer formation.43 Thus, the minimum at smaller molecular area indicates collapse of a monolayer. The Cs-A isotherms of the mixed monolayers of BODIPY-ISO with PA, DOPC or DPPC contain two local minima (Figure S7–S10). However, while pure DOPC exhibits no phase transition during monolayer formation, its mixture with BODIPY-ISO exhibited a phase transition from expanded to condensed phases. Thus, pure DOPC exhibits a single expanded liquid phase in its monolayer, while in contrast the mixture of BODIPY-ISO and DOPC exhibits two phases of expanded and condensed liquid phases. The other mixed monolayers showed no phase transitions prior to monolayer collapse.


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Scheme 3. Chemical structures and schematic illustrations of lipids

Table 1. Physical properties of monolayers. Limiting molecular area of BODIPY-ISO for lipid (ALim). Compression modulus of pristine lipids (Cs-1L) and lipid mixed monolayers with BODIPY-ISO (Cs-1mix).

Lipid

ALim / nm2

Cs-1L/ mN m-1

Cs-1mix / mN m-1

SA

0.3

100

50

PA

0.4

180, 800

80, 500

CA

0.5

250

50, 250


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OA

0.9

70

60

MO

1.2

30

34

DPPC

1.1

25, 160

30, 80

DOPC

2.7

100

55, 55

The compression modulus (Cs−1) was plotted as a function of surface pressure to compare the rigidity of the monolayer with/without matrices (Table 1). The compression modulus of pure BODIPY-ISO is around 20 mN m-1, which corresponds to a liquid expand state. Both pure CA and the mixture with BODIPY-ISO exhibit a large maximum compression modulus of around 250 mN m-1 corresponding to a liquid condensed state. Although the compression modulus of pure PA corresponds to the solid state, the mixed monolayer (500 mN m-1) was softer than the pure monolayer (800 mN m-1). The maximum compression moduli of mixed monolayers of BODYPY-ISO with SA and phospholipids (DPPC, DOPC) lie between 50 and 100 mN m-1 and correspond to states between liquid condensed and expanded state. These values are lower than those for pure lipids (over 100 mN m-1), which correspond to liquid-condensed states. Monolayers of OA and MO 18 ACS Paragon Plus Environment

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with/without BODIPY-ISO exhibited compression moduli around 70 and 30 mN m-1, respectively. To summarize, the rigidities of monolayers of CA, OA, MO do not vary by adding BODIPY-ISO, but monolayers of SA, PA, DPPC, DOPC become softer when mixed with BODIPY-ISO. According to the smaller limiting molecular areas of BODIPYISO in lipid matrices relative to its estimated molecular area, BODIPY-ISO should be better miscible in SA and PA than the other lipids studied here. However, the miscibility of BODIPY-ISO in DPPC and DOPC might be improved after phase transition from liquid expanded to condensed phase.

Optical properties of mixed monolayers with lipids at the air-water interface

The intramolecular rotation of the molecular rotors upon compression of the mixed monolayers was investigated by using in situ surface fluorescence spectroscopy. Fluorescence intensities of the mixed monolayers with SA and PA, both of which have long alkyl chains, were relatively low and remained almost constant during monolayer compression (Figure S11). This suggests that intramolecular rotation is not inhibited even


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under compression within SA or PA monolayers. Thus, the fluorescence intensity of BODIPY-ISO is independent of the viscosities of SA and PA. As previously discussed, the limiting molecular areas of BODIPY-ISO in SA and PA were smaller than that estimated from the cross-sectional area of BODIPY-ISO molecule. In addition, the maximum compression moduli of mixed monolayers of BODYPY-ISO with SA and PA were lower than those for pure lipids. These results suggest that monolayers of SA and PA become less rigid or are softened after addition of the BODIPY-ISO. It also seems that molecular motion of even densely packed sterically bulky parts of the molecule such as the boron-dipyrromethene moiety could be inhibited under molecular compression, although that of the farnesyl chain, which is linked through a single bond and has an associated free volume, would not be inhibited even within a rigid monolayer. As a result, the BODIPY part of the chromophore that affects fluorescence can effectively rotate even in the densely packed environment of the lipid monolayer.


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Figure 3. Variation of fluorescence intensity with area per molecule (circles connected lines) observed from mixed monolayers of BODIPY-ISO and lipids, (a) CA and (b) OA, (c) DPPC. -A isotherms are also shown as black lines. BODIPY-ISO/each matrix at molar ratios of 1/10. Fluorescence spectra observed from the monolayer of BODIPY-ISO


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at different molecular area excited at 400 nm. Molecular area becomes smaller passing from the blue line to the green line.

In contrast, the fluorescence intensities of BODIPY-ISO contained in monolayers with other lipids became larger upon compression of the individual monolayers (Figure 3 and S11–S13), although the profiles for each mixture exhibit some differences due to differences in the physical properties of the lipids. The fluorescence intensity of the mixture with CA rapidly increased with increasing surface pressure below the molecular area of 0.5 nm2 (Figure 3a). As surface intensity correlates substantially with compression modulus, the intramolecular rotation must be inhibited when BODIPY-ISO is contained within the rigid CA matrix under monolayer compression. In the case of OA and MO, the intensities of the monomer emission at 520 nm are larger above molecular areas of 0.7 nm2 prior to monolayer formation (Figure 3b and S12). Below molecular areas of 0.7 nm2, monomer emission decreases with the emergence of excimer emission of J-aggregated BODIPY-ISO at ca. 610 nm. It seems that the intramolecular rotation of BODIPY-ISO is


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inhibited in the ‘gas’ state of soft matrices such as OA and MO, and that the homodimeric molecules responsible for excimer emission are formed by the mobility of BODIPY-ISO within the lipid monolayers during compression. The intensity of monomer emission from the mixed monolayer with DPPC was enhanced with increasing surface pressure at molecular areas above 0.6 nm2 in the liquidexpanded state and then reduces following the phase transition (Figure 3c). In addition, the surface fluorescence of the mixed monolayer with DPPC was independent of the viscosity of the lipid. As a result, the intramolecular rotation of BODIPY-ISO in DPPC is inhibited in the expanded liquid state but commences rotation between the liquid expanded and condensed states. Neat DPPC exhibits two phase transitions at 25 and 160 mN m-1. The BODIPY-ISO mixture in DPPC showed two different compression moduli: one at around 25 mN m-1 (which is similar to that of pure DPPC and the MO monolayer with/without BODIPY-ISO at ~30 mN m-1) and one at 80 mN m-1, substantially lower than the value for the second transition of neat DPPC or that of SA at around 100 mN-1, and less than that of the mixture with BODIPY-ISO by around 50 mN-1. Thus, it can be confirmed that mechanical compression affects the molecular rotation of BODIPY-ISO 23 ACS Paragon Plus Environment

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contained in the similarly rigid environment of lipids such as MO, but this does not apply to the soft molecular rotor dispersed in a harder lipid monolayer such as SA. The intensity of the monomer fluorescence emission of the mixed monolayer with DOPC is enhanced with increasing surface pressures at molecular areas below 0.8 nm2 (after its phase transition; see Figure S13). Aggregated state emission was observed at molecular areas below 1.3 nm2 with increasing surface pressure after monolayer formation followed by a reduction in aggregated state emission intensity below molecular areas of 0.8 nm2. The intensities of the monomer emission also increased after the phase transition. Thus, BODIPY-ISO is aggregated upon monolayer formation in DOPC then aggregates are dissolved after the phase transition from expanded to compressed states.23 In addition, intramolecular rotation of dissolved BODIPY-ISO remains inhibited in the compressed liquid phase for OA and MO. Based on surface fluorescence measurements on the dynamic interface, it seems that excimer formation of BODIPY-ISO under the densely packed conditions during compression leads to a decrease in the intensity of monomer emission. Fluorescence intensities of BODIPY-ISO in OA, MO and DPPC increased and decreased prior to and 24 ACS Paragon Plus Environment

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following monolayer formation, respectively. However, lipid viscosities at the air-water interface usually increase with increasing surface pressure, but fluorescence emission intensity of BODIPY contained in the lipid monolayers was independent of their viscosities. Surface fluorescence of BODIPY-ISO on a static water surface was measured by dropping mixed solutions with lipids, such as OA and DPPC, DOPC, in order to clarify the effect of molecular aggregation and avoid any possible dynamic effects of lipid viscosity variation (Figure S14). Regarding BODIPY-ISO concentration, the fluorescence emission intensity of pure BODIPY-ISO in solution at the air-water interface (Figure S4) was greater than that of the mixed monolayer (where BODIPY-ISO was mixed in each matrix at a molar ratio of 1/10) by one or two orders of magnitude. This indicates that the rotation rate of BODIPY-ISO in lipids at the air-water interface is greater than that of BODIPY-ISO in solution. Monomer emission from the mixtures with lipids at the static interface behaved similarly to that found at the dynamic interface by decreasing the molecular area. Aggregated state emission at 715 nm was not observed in the mixtures with OA or DPPC, and was reduced in intensity for DOPC. These features contrast with those found at the dynamic interface 25 ACS Paragon Plus Environment

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and support our proposal that inhibition of molecular rotation is caused by variation of molecular interactions between BODIPY-ISO and the lipid for OA and DPPC, and that molecular compression induces aggregation of BODIPY-ISO. In addition, it seems that rotation of BODIPY-ISO in the lipids at the air-water interface is independent of the viscosities of OA and DPPC. In contrast, aggregation of BODIPY-ISO in DOPC was disrupted by increasing surface pressure of the lipids23 and molecular rotation of the dissolved BODIPY-ISO was still inhibited as the viscosity of DOPC increased during monolayer compression BODIPY derivatives have been used as probes of natural lipid bilayers and the phospholipid DOPC forms a stable bilayer at room temperature. The fluorescence spectra of the DOPC bilayer containing BODIPY-ISO were measured to monitor intramolecular rotation in the bilayer. While monomer emission could be observed, aggregated state emission was not. This suggests that BODIPY-ISO is well dispersed in DOPC and that intramolecular rotation is inhibited in lipid bilayers due to their high viscosity similarly to the case for compressed lipid monolayers. In addition, similar rotation rates were found


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in both monolayer and bilayer because the order of the intensity of monomer emission was found to be almost equal for both cases (Figure S13, S14b, S15).

CONCLUSION

Intramolecular rotation of the TICT-type fluorescence molecule was controlled in several matrices at the air-water interface under mechanical compression. Rotation behaviors varied depending on the monolayer matrix used, as is summarized in Scheme 4. Intramolecular rotation was not inhibited in long-alkyl-chain-containing stearic acid and 1-hexadecanol monolayers even in compressed states. This is because the farnesyl chains linked to the BODIPY moiety have sufficient space available to rotate in the condensed phase and because of softening of the lipid matrix cause by addition of the molecular rotor. That is, mechanical force due to compression was not fully transferred to the molecular rotors when contained in those compressed lipid monolayers. In contrast, rotation of BODIPY moiety in cholesteryl acetate, which has a more rigid structure, could be controlled by varying the mechanical force at the air-water interface.


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In the case of softer matrices involving monounsaturated alkyl chains such as oleic acid and methyl oleate, intramolecular rotation was inhibited in the expanded monolayer while molecules formed excimers and other aggregates in the condensed monolayer. In the case of lipids with monounsaturated fat, the applied mechanical force is efficiently transferred to the rotor because of the similar rigidity of lipid and BODIPY-ISO in the compressed state. On the other hand, intramolecular rotation of BODIPY-ISO in the mixed monolayer with DPPC is inhibited in the expanded state because the rigidity of the molecular rotor is close to that of DPPC, similarly to the case for oleic acid. The mechanical state of the mixed monolayer with DPPC is similar to that with stearic acid in a condensed state so that the molecular rotor was not inhibited and could rotate again. Molecular rotation is independent of lipid viscosity and could be controlled by altering the molecular interaction between molecular rotor and lipids during monolayer compression. In contrast, BODIPY-ISO is aggregated upon monolayer formation in DOPC with aggregates being dissolved during compression. Molecular rotation of the dissolved BODIPY-ISO remains inhibited due to increases in viscosity with increasing surface pressure. The DOPC bilayer mixture behaved similarly. 28 ACS Paragon Plus Environment

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It can be seen from these results that the molecular machine can be effectively controlled when contained in the sterically hindered environment of a lipid monolayer at the air-water interface. This is supported by evidence from our other trials of molecular machine control at the air-water interface.14,18-24 For instance, a molecular manipulator containing a central cyclophane ring appended through flexible spacers with four rigid, bulky substituents(as ‘walls’), could be operated mechanically at the air-water interface.18 In addition, molecular recognitions of a cholesteryl-armed cyclic amine could be tuned by mechanical compression. Both molecular machines have rigid, bulky cholesterol moieties, which are capable of transferring the mechanical stimulus from a monolayer.19,20 In other work, the dihedral angle of a rigid binaphthyl moiety of an amphiphilic molecule contained in a stable monolayer could be controlled leading us to coin the term ‘molecular pliers’.14,23 In contrast, the intramolecular rotation of julolidine derivatives was not inhibited at the air-water interface, because the rigid rotating moieties are smaller than the julolidine part of the molecule.24 The conformation of a pyrene-labelled xylose could be varied by mechanical compression when mixed with rigid lipids (but not in soft lipids) at an air-water


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interface.22 Thus, motility of molecular machines depends strongly on their immediate environments. Our studies provide information concerning control over the motion of the molecular machines at two-dimensional interfaces. Molecular machines with rigid, bulky rotator should be easy to control at the air-water interface; alternatively, the molecular machines could be controlled within a matrix having similar physical properties to the machine molecules themselves. In addition, our observation regarding flexible dynamic control can be extended to other interfaces such as those between liquids and solids.


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Scheme 4. Schematic view of molecular rotation at dynamic interfaces under various conditions of compression and lipid identity.


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METHOD/EXPERIMENTAL SECTION General. Analytical thin-layer chromatography (TLC) was performed on a glass plate coated with silica gel (230-400 mesh, 0.25 mm thickness) containing a fluorescent indicator (silica gel 60F254, Merck). Flash silica gel column chromatography was performed on silica gel 60N (spherical and neutral gel, 40-50 µm, Kanto). Proton (1H) and carbon (13C) nuclear magnetic resonance (NMR) spectra were recorded in CDCl3 on a JEOL JNR-AL300 (300 MHz) spectrometer. Mass spectra were obtained using an Applied Biosystems Voyager DE STR SI-3 instrument (MALDI-TOF MS). Fluorescence emission spectra were obtained using JASCO FP-6500 fluorimeter. Materials. Solvents and materials were purchased from Aldrich, Tokyo Chemical Industry Co., Wako Chemical Co. and GL Science, and were used without further purification. Deionized (DI) water used for the LB subphase was distilled and deionized using a PURELAB Option R7, Flex (ELGA). Its specific resistance was greater than 18 MΩ cm. Spectroscopic grade chloroform (DOJINDO LABORATORIES) was used as the spreading

solvent.

For

synthesis

and

characterization

Supplementary Information. 32 ACS Paragon Plus Environment

of

BODIPY-ISO,

see

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Monolayer measurements. Preparation of LB films was performed by the similar method reported.2,3 Measurement of surface pressure–area, π-A isotherms and thin–film preparation were performed on a USI-3-777C3 Langmuir-Blodgett system (USI Co., Fukuoka). The trough contains 1 L subphase and has a working area of 544 × 150 = 8.16 × 104 mm2. Samples for study were dissolved in chloroform and spread on the pure water in the trough. After 15 minutes (for evaporation of the chloroform), the film was compressed at the rate of 0.2 mm sec–1 to the predetermined surface pressures at 20.0 ± 0.2 °C. Fluorescence spectroscopy of Langmuir monolayers at the air-water interface. Fluorescence spectra were measured using a photodiode array-equipped spectrometer (Otsuka Electronics, model MCPD-9800) with an excitation wavelength of 400-450 nm. Incident light was directed through an optical fiber at an angle of 60o against the water surface, and emission was detected by an optical fiber positioned in parallel. An inclined mirror was set in the trough to deflect the excitation light beam away from the detector optical fiber. To account for the effect of the increase of density of molecules upon compression, fluorescent intensities were multiplied by area per molecule. 33 ACS Paragon Plus Environment

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Fluorescence spectroscopy of BODIPY-ISO at static air-water interface. A stock solution of BODIPY-ISO was dispersed on the water surface in a glass dish of 3.5 × 103 mm2 area (67 mm dish). A lipid solution was then gradually dropped onto the monolayer of BODIPY-ISO on the water surface. The spreading amounts at molecular area of 0.5 (OA) and 1.0 (DPPC, DOPC) nm2 correspond to the molecular area of the mixed monolayer (molecular ratio of BODIPY-ISO/each lipid was 1/10), respectively. Fluorescence from the surface was detected with the same MCPD system used for in situ measurements at the air-water interface. SALB

for

bilayer

formation.

Zwitterionic

lipids,

1,2-dioleoyl-sn-glycero-3-

phosphocholine (DOPC, transition temperature: -16.5 °C) and 1,2-dipalmitoyl-sn-glycero3-phosphocholine (DPPC, transition temperature: 41.3 °C), are widely used for formation of lipid bilayers as models of cell membranes because of the ease of separation of their phases at room temperature. Some conventional techniques (e.g., vesicle fusion) exist for formation of lipid bilayers on materials. However, lipid bilayers consisting of phospholipids of gel phases such as DPPC cannot be formed by these techniques at room temperature. 34 ACS Paragon Plus Environment

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A coverslip (rectangle type) was rinsed with DI water, ethanol, and DI water sequentially before drying in a stream of N2 gas. The surface of the coverslip was treated with UV for 15 min immediately before use. After treatment, the coverslip was assembled with a microfluidic chamber (Ibidi, GmbH), which includes an inlet and outlet for solvent exchange. DI water was injected to the microfluidic chamber at a flow rate of 200 μL/min using a syringe pump (HARVARD APPARATUS). DI water was gradually replaced with isopropanol at a flow rate of 200 μL/min using the pump. DOPC lipid and BODIPY-ISO dissolved in DI water-isopropanol solvent were injected into the microfluidic chamber for formation of lipid bilayer on the coverslip at a flow rate of 100 μL/min using the syringe pump for 15 min. Lipid bilayer was formed by changing the solvent to DI water in the microfluidic chamber at a flow rate of 50 μL/min using the pump for 30 min.

ASSOCIATED CONTENT


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Supporting Information. Information for synthesis of BODIPY-ISO, NMR spectra, additional fluorescence spectra, –A isotherms, Cs–A isotherms. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author TM: [email protected]. KA: [email protected]

Author Contributions

W. N proposed the project and N. H. synthesized the BODIPY-ISO. T.M. and H. C., K. K. measured surface fluorescence and analysed them. N.-J. C. and K. A. directed the project. T.M., J. H. and wrote the manuscript with feedback from all the authors.

Funding Sources This work was partially supported JSPS KAKENHI (16H07436, JP16H06518 (Coordination Asymmetry), 18H05419 (Molecular Engine)), and CREST (JPMJCR1665). Additioanl support was provided by the Creative Materials Discovery


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Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future Planning (NRF-2016M3D1A1024098).


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