New Evidence about the Spontaneous Symmetry Breaking: Action of

Article pubs.acs.org/JPCB

New Evidence about the Spontaneous Symmetry Breaking: Action of an Asymmetric Weak Heat Source Placido Mineo,*,†,‡ Valentina Villari,*,‡ Emilio Scamporrino,† and Norberto Micali‡ †

Dipartimento di Scienze Chimiche and I.N.S.T.M. UdR of Catania, Università di Catania, Viale Andrea Doria 6, I-95125 Catania, Italy ‡ CNR-IPCF Istituto per i Processi Chimico-Fisici, Viale F. Stagno d’Alcontres 37, I-98158 Messina, Italy

Downloaded by CENTRAL MICHIGAN UNIV on September 10, 2015 | http://pubs.acs.org Publication Date (Web): September 3, 2015 | doi: 10.1021/acs.jpcb.5b07199

S Supporting Information *

ABSTRACT: In the present study, we show how, in a stagnant water solution of uncharged aggregated achiral porphyrin-based molecules, a mirror-symmetry breaking (SB) can be induced and controlled by means of a weak asymmetric thermal gradient. In particular, it is shown that the optical activity of the aggregate porphyrin solution can be generated and reversed, in sign, only acting on the thermal ramp direction (heating or cooling). In order to avoid data misinterpretation, the aggregate structure modifications with the temperature change and the linear dichroism contribution to circular dichroism spectra were evaluated. A model simulation, using a finite element analysis approach describing the thermal flows, shows that small thermal gradients are able to give rise to asymmetric heat flow. The results reported here can be considered new evidence about the spontaneous symmetry breaking phenomenon induced by very weak forces having an important role in the natural chiral selective processes.

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motion, 29−36 and the simultaneous action of external fields.37−39 A simple way to study mirror-symmetry breaking can be considered the aggregation of nonchiral molecules.40 In this field, commonly used dyes are water-soluble porphyrin systems, which exhibit some peculiar properties: (i) a high molar absorption; (ii) a water solubility, generally due to the presence of suitable charged peripheral groups (carboxylate, sulfonate, pyridinium) or peripheral hydrophilic branches (PEG); and (iii) a capability to form self-assembled systems that can be tuned acting on the solution properties (as pH, ionic strength, and concentration). The self-aggregation of suitable porphyrin molecules can lead to the formation of H-type (face-to-face, Scheme 1a) and/or J-type (edge-to-edge, Scheme 1b) structures whose formation can be easily evidenced by examining the visible region of the extinction spectrum of the solution. However, these mesoscopic systems, generated from achiral building blocks, should not show chiral properties (and hence no circular dichroism, CD, signals) because the aggregation process is, generally, a random phenomenon leading to an achiral or racemic mixture. Nevertheless, it has been shown that the mirror-symmetry of such a system can be “broken” by means of an enantiomeric enrichment induced by chemical (through chiral templates) and/or physical (through asymmetric fields) perturbations. As an example, in an aggregated system, the enantiomeric composition can be perturbed by the addition of a chiral molecule (acting as “imprinting” during the

INTRODUCTION The spontaneous symmetry breaking (SSB) is one of the most strange and exciting natural phenomena.1,2 From the macroscopic to the microscopic world, examples of SSB are found (in some case without any plausible explanation) in several scientific fields; as an example, in astronomy, the abundance of spiral galaxies S-form (left-spiral) is 7% higher than those of Z-form (right-spiral), an excess indicated as cosmic parity violation;3 in nuclear physics, intrinsic asymmetry of β-particles emitted from radioactive nuclei;4−6 in biochemistry, L-amino acids and D-sugars are essentially the basis of the origin of life.7−15 A recent further evidence of a SSB phenomenon is the confirmation of the Higgs−Englert−Brout theory16−19 (inspired to the Nambu−Goldstone boson model20,21) about the origin of the subatomic particle mass (the Higgs boson), proved by means of the ATLAS and CMS experiments performed at CERN’s Large Hadron Collider. A common macroscopic evidence is the symmetry breaking in snail shells; in fact, the dextral shells (right-handed) are more abundant (about 90%) than left ones (left-handed), because, for an unclear reason, the snail’s glands produce right-handed material with a greater rate, favoring the growth of right shells.22 In general, the symmetry of a system can be “broken” in two ways: (i) an induced symmetry breaking by chemical and/or physical external stimulus able to perturb the natural random distribution of the species; (ii) a spontaneous breaking when the prevalent formation of a species form occurs without an apparent external solicitation. Examples of external stimuli, able to induce an enantioselection, can be considered some asymmetric photosynthesis and photolysis caused by the irradiation with circularly polarized light,1,2,23−25 the presence of a heterogeneous catalysis,26−28 the stirring and vortex © XXXX American Chemical Society

Received: July 24, 2015 Revised: August 25, 2015

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DOI: 10.1021/acs.jpcb.5b07199 J. Phys. Chem. B XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry B

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EXPERIMENTAL SECTION Instrumentation. 1H NMR spectra were obtained on a UNITYINOVA Varian instrument operating at 500 MHz, using VNMR for software acquisition and processing. Samples were dissolved in CD2Cl2, and the chemical shifts were expressed in ppm by comparison with the CH2Cl2 residue signal. The spectra were acquired at 300 K, with a spin lock time of 0.5 s. Positive MALDI-TOF mass spectra were acquired by a Voyager DE-STR (PerSeptive Biosystem) using a simultaneous delay extraction procedure (25 kV applied after 2600 ns with a potential gradient of 454 V/mm and a wire voltage of 25 V) and a detection in linear mode.49,50 The instrument was equipped with a nitrogen laser (emission at 337 nm for 3 ns) and a flash AD converter (time base 2 ns). trans-3-Indoleacrylic acid (IAA) was used as a matrix. Mass spectrometer calibration was performed as reported in previous cases.51−53 The m/z values reported in the spectra and in text refer to molecular ions considering the most abundant isotope of each element in the molecule. UV−visible spectra were recorded at 25 °C by a Shimadzu Model 1601 spectrophotometer, in quartz cells (1 cm optical path), using tetrahydrofuran (THF) or water as a solvent. The circular dichroism spectra were recorded by means of a J-815 spectropolarimeter (JASCO) equipped with a 150 W xenon lamp. The ellipticity, θ ∝ εL − εR, was obtained calibrating the instrument with a 0.06% (w/v) aqueous solution of ammonium d-10-camphorsulfonate and with a 0.08% (w/v) aqueous solution of tris(ethylendiamine)−Co complex (2(−)D[Coen3]Cl3·NaCl·6H2O). The measurements, performed at variable temperature in quartz cuvettes (1 cm optical path) and using water as solvent, were corrected from the contribution due to cuvette and water. The temperature of the quartz cuvette was regulated by means of a Jasco PTC-423S/15 Peltier-type temperature control system (with the systems cooled by an external water circulator). The temperature ramps, used in the CD experiments, were heating from 5 to 30 °C and cooling from 30 to 5 °C. Thanks to the peculiar geometry of the temperature control system, it was possible to exploit a very weak thermal force to give rise to a symmetry breaking phenomenon in supramolecular aggregates of uncharged porphyrins. Static light scattering (ELS) and dynamic light scattering (QELS) experiments were performed at both small and wide angles using a He−Ne laser and a homemade apparatus described in detail elsewhere.54 All measurements were performed at a concentration of 1.11 × 10−5 M in water. Particular care was devoted to the choice of cuvette (four-window Hellma cell) with low birefringence (see the Supporting Information). To avoid a gradient in refractive indices, a cuvette with a 10 mm path length has been used A computer simulation, by means of a finite element analysis procedure, was performed with COMSOL Multiphysics software (ver. 4.3). All the thermal parameters for the materials used (quartz, water, and air) were obtained from the COMSOL Multiphysics library. Thermographic images were obtained with a FLIR4 thermocamera (FLIR Systems Inc.). Temperatures on cuvette holder walls were measured with thermocouple digital thermometers (Delta OHM HD 2108). Synthesis of 5,10,15,20-Tetrakis(p-hydroxyphenyl)porphyrin. This porphyrin was prepared according to the

Scheme 1. Schematic Representation of H2TPPS (mesoTetrakis(4-sulfonatophenyl)porphine) H- (a) and J-Type (b) Aggregate Systems


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aggregation process) and/or by the action of an orienting tangential force. In particular, it has been possible to observe that the sign of the recorded CD signals can depend on the direction of the spin applied to the solution31,41−43 and/or on the specific nature of the enantiomeric species added during the aggregation process;44,45 in this last case, the porphyrin acts as a chirality amplifier.46,47 Surprisingly, in several other cases, also in the absence of an external perturbation, the appearance of a CD signal, in correspondence with the porphyrin H- and/or J-band, has been observed.44 This puzzling phenomenon has been often attributed to the casual presence of chiral contaminants, like cellular fragments or residues of lipids from the chromatographic porphyrin purification, thus justifying the poor reproducibility of the phenomenon for the use of a different batch of commercial porphyrins often obtained from not wellknown processes. Concerning this, we have recently demonstrated48 that, in a stagnant aqueous solution, a self-assembled achiral porphyrin can exhibit supramolecular chirality by subjecting the solution to a weak temperature gradient inducing the enantiomeric enrichment. In this contest, the appearance of the circular dichroism signals was considered the effect of the applied external asymmetric thermophoretic chiral force. However, an unresolved issue was the control of the CD signals, with the possibility of a sign inversion. In the present study, we show how this problem has been resolved; in fact, in a stagnant water solution, the symmetry breaking of aggregated achiral porphyrin-based molecules has been induced and controlled undergoing the solution to an asymmetric temperature gradient. In particular, it is shown that the optical activity (evidenced by the appearance of the CD signal) can be reversed by acting on the thermal ramp direction, starting from temperature values lower than room temperature (RT) and reaching temperatures higher than RT and vice versa. The dependence of SB from thermal variation was proved by means of (i) a thermal map of the top of the cuvette holder (during the thermal ramp); (ii) a direct measurement, with microthermocouple, of the temperature on the cuvette holder surfaces; and (iii) a model simulation (using a finite element analysis approach describing the thermal flows). This study strengthens our research about the mirrorsymmetry breaking phenomenon, putting in evidence that weak natural events can be responsible for unexpected chirality selections in animal and vegetal environments. B


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The Journal of Physical Chemistry B method described by Little et al.,55 starting from pyrrole and pacetoxybenzaldehyde in boiling propionic acid. Synthesis of ω-Methoxy-polyethyleneoxy Chloride. A commercial poly(ethylene glycol)methyl ether (PEGME) sample, constituted from a mixture of linear oligomers (Mn = 350 Da and a narrow mass distribution of about 1.01) having a methoxyl and a hydroxyl as end groups, was transformed into the corresponding chlorinated derivative (PEGMEC) by reaction with thionyl chloride.56 Synthesis of 5,10,15,20-Tetrakis[p(ωmethoxypolyethyleneoxy)phenyl] Porphyrin (PPEG4). PPEG4 was prepared by reaction between PEGMEC and 5,10,15,20-tetrakis(p-hydroxyphenyl) porphyrin.53 Briefly, in a 10 mL flask, 0.5 g of PEGMEC (about 1.43 mmol) was dissolved in 6 mL of a H2O/THF (1:1) mixture. Then, 0.12 g of 5,10,15,20-tetrakis(p-hydroxyphenyl)porphyrin (0.177 mmol) dissolved in 1.42 mL of a 0.5 mL aqueous solution of NaOH was added and the mixture refluxed for 24 h. Successively, 10 mL of NaOH and 10 mL of THF were added and the mixture refluxed for a further 24 h. The products in solution, analyzed by MALDI-TOF mass spectrometry, resulted in a mixture of porphyrin derivatives with different numbers (1−4) of methoxy-polyethyleneoxy (PEGME) branches. To obtain pure products, the solution was slightly acidified with CH3COOH and dried under a vacuum and the residue, dissolved in CH2Cl2, finally fractionated by column chromatography using silica gel as the stationary phase and a mixture of CH2Cl2/EtOH/TEA (97/2.5/0.5) as eluent. The separated pure fractions were characterized by means of MALDI-TOF and 1H NMR analysis (pertinent data are reported in the Result and Discussion section).

Figure 2. MALDI-TOF mass spectrum of the products of reaction between PEGMEC and 5,10,15,20-tetrakis(p-hydroxyphenyl)porphyrin. Inset: The MALDI-TOF spectrum of pure PPEG4.

n44 (with n = 20−41 and the first peak at m/z 1637), due to molecular ions of oligomers cationized with H+ or Na+, respectively (peaks correspond to the fourth family of signals at higher m/z values in Figure 2). The chemical structure of PPEG4 was also confirmed by 1H NMR whose spectrum, reported in Figure 3 (500 MHz,

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RESULTS AND DISCUSSION To study the effect of weak forces on the symmetry breaking phenomenon in nonchiral materials, a water-soluble uncharged porphyrin derivative, PPEG4 (see Figure 1), was prepared.

Figure 3. 1H NMR spectrum of PPEG4; see Figure 1 for relative signal assignments.

Figure 1. Chemical structure of PPEG4.

During the synthetic process, special care was taken in order to avoid any pollution by unwanted chiral contaminants. Thus, PPEG4, a star polymer with a porphyrin unit as the core and four polyethylene glycol (PEGME) branches (each with a polymerization degree of about 8) bound in its peripheral position, was obtained. Its MALDI-TOF mass spectrum, reported in Figure 2, shows an envelope of four groups of peaks indicating the presence of four families of porphyrin derivatives, different for the number (1−4) of PEGME branches. These were separated chromatographically, and pure PPEG4 was the first colored compound eluted from the column.56 Its mass spectrum, reported as the inset in Figure 2, shows two series of peaks at m/z values 735 + n44 (with n = 19−40 and the first peak at m/z 1571) and 757 +

CD2Cl2), shows a singlet at 8.90 ppm (8 H, C−H pyrrole protons, a), two doublets at 8.13 ppm (J = 8.6 Hz; 8 H, C−H phenyl protons in γ with respect to the phenolic oxygen, b) and 7.32 ppm (J = 8.6 Hz; 8 H, C−H phenyl protons in β with respect to the phenolic oxygen, c), and a singlet at −2.803 ppm (2 H, N−H pyrrole protons). Furthermore, for the PEG arms, the following appear: two triplets centered at 4.42 and 4.02 ppm (for a total of 16 H, CH2 groups in the α and β positions, respectively, with respect to the phenolic oxygen, d and e), an unresolved multiplet between 3.85 and 3.43 ppm (about 128 H, the other methylene groups of the PEG, PEO), and a few singlets between 3.33 and 3.30 ppm (12 H, the −OCH3 terminal groups of the branches, ω). C


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The UV−visible spectrum of PPEG4 in THF (dashed line in Figure 4) displays the typical absorption features of a free-base

Figure 4. Normalized UV−vis spectra of PPEG4 in THF (dotted line) and water (solid line) solution at the same concentration. Inset: The CD spectrum of PPEG4 in THF. Figure 5. (a) Average of eight scans of PPEG4 water solution; (b) eight single scans of PPEG4 water solution. In the figure, a′ and b′ are the traces of the correspondent absorption spectra [the high tension voltage (HT) is roughly proportional to absorbance].

porphyrin macrocycle with a Soret band at 421 nm (ε ≃ 430 000 cm−1 M−1) and four Q-bands in the range 500−680 nm. To ascertain about a possible optical activity of PPEG4 also in the absence of an aggregation phenomenon, the CD spectrum of PPEG4 in THF solution was performed and, as expected (see inset of Figure 4), it does not show any CD signal. As is well-known, in the spectrum of PPEG4 in aqueous solution (continuous line in Figure 4), the profile of the Soret band changes drastically, showing two bands at 402 nm (hypsochromic shift) and 440 nm (bathochromic shifts) in place of the band at 421 nm, typical of the exciton splitting for the formation of both H- and J-type self-aggregates.31,57−60 By dynamic light scattering experiments, a hydrodynamic radius (RH) of the aggregates of about 200 nm (see Supporting Information, Figure SI1) was measured. Their mesoscopic features, as indicated by the scattered intensity profile as a function of the exchange wave vector Q, point to a fractal structure with a gyration radius (Rg) of about 600 nm48 so that the value of the Rg/RH ratio is consistent with a tenuous structure of fractals. To examine the SSB phenomenon due to PPEG4 aggregates in a stagnant water solution, a CD measurement, consisting of the accumulation of eight scans, was performed at 20 °C, whose related trace is reported in Figure 5a. Two opposite bands appear, each in correspondence with the H- or J-absorption bands at about 400 and 440 nm, respectively (compare also with the UV signal in Figure 5a′). This spectrum was reproducible, so that any accidental stochastic fluctuation was excluded. A split-type Cotton effect should be expected48 so that the signal observed here suggested the possibility of an overlapping of different CD signals or the occurrence of a LD contribution. The eight scans were then individually checked, evidencing that the trace of each CD signal changed drastically during the experiment and the final spectrum was the sum of signals with different profiles. To better prove this occurrence, the cuvette was removed from the holder and inserted again; then, a new

series of spectra were acquired but in single scan mode. The obtained eight single scans are, for brevity, overlapped in Figure 5b. However, their traces confirm the drastic change of the CD bands at 400 and 440 nm, whereas the corresponding absorption spectra (Figure 5b′) remain almost unchanged, indicating that the local structures are substantially unvaried. To explain these results, two phenomena were considered: (i) the action of an uncontrolled temperature gradient in the stagnant solution and (ii) the effect of a weak flowing inside the solution induced by the insertion movement of the cuvette into the cell holder. About point i, recently,48 we have shown that PPEG4 aggregates, despite being achiral, become chiral under a weak thermal gradient, which acts as a perturbing force able to generate an enantiomeric enrichment. Nevertheless, for a further demonstration about the dependence of the aggregate’s chirality on the temperature gradient, new spectra of the stagnant PPEG4 aqueous solution were repetitively recorded during both a heating and successive cooling experiment in the temperature range 5−30 °C and vice versa (operating at a room temperature of 26 °C). Figure 6 shows the CD spectra achieved during the heating process; in particular, the significant change can be noticed of the signals from the initial weak CD values (spectrum at 5 °C) of about −2 mdeg, for H-type, and +5 mdeg for J-type aggregate species, to about +25 mdeg, for H-band, and −63 mdeg (at 433 nm) and +89 mdeg (at 437 nm) for the bisigned J-band in the spectrum at 30 °C. In particular, the maximum values were recorded in the spectra at 19 °C: about +42 mdeg, for the (asymmetric) H-band and −140 mdeg (at 433 nm) and +165 mdeg (at 437 nm) for the bisigned J-band. Analogously to all previous experiments, these changes appeared while the traces of the correspondent absorption spectra [Figure 6b, the high tension voltage (HT) is roughly proportional to absorbance] evidenced only a small blue-shift of about 0.4 nm for the H-band and a red-shift of about 1.2 nm D


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Figure 6. Spectroscopic features of PPEG4 aqueous stagnant solution during heating experiment (1 °C/min) from 5 to 30 °C performed by means of a Peltier-type thermostat: (a) circular dichroism spectra; (b) absorption spectrum (measured by the high tension, HT, of the detector); (c) 3D visualization of the CD spectra acquired during heating experiment.

Figure 7. Spectroscopic features of PPEG4 aqueous stagnant solution during cooling experiment (−1 °C/min) from 30 to 5 °C performed by means of a Peltier-type thermostat: (a) circular dichroism spectra; (b) absorption spectrum (measured by the high tension, HT, of the detector); (c) 3D visualization of the CD spectra acquired during cooling experiment.

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Figure 8. CD traces recorded at 25 °C during heating (red trace) and cooling (blue trace) processes. Inset: The measured CD signal corresponding to the H and J bands (for the latter being ΔCD = CD437 nm − CD433 nm).

for the J-band. Unlike what was observed for other porphyrinbased aggregated systems (for which a temperature increase causes the shift of the H- and J-bands toward the absorption wavelength of the monomer61,62), for PPEG4, the heating determined the strengthening of the energy interaction between the moieties of the aggregates. Therefore, according to the exciton coupling theory, the geometric arrangement of the dyes in the aggregates (i.e., distance between the porphyrin planes and their orientation) changes with temperature. Considering the limitations due to physical hindrance, the absorption shift could be due to a slipping and/or orientation change of adjacent porphyrins. An opposite temperature dependence of the CD signal was observed during the cooling process (Figure 7). In this case, the initial H- and J-type signals, at 30 °C, consisting, respectively, of a positive band with a maximum of about +12 mdeg and a negative band with a minimum of −32 mdeg (at 433 nm), became, respectively, about −13 mdeg and +30 mdeg at 5 °C with more intense values for the spectrum at 27 °C [about −48 mdeg for the H-band and +108 mdeg (at 433 nm) and −77 mdeg (at 437 nm) for the bisigned J-band]. Contrarily to the heating experiment, the cooling determined a small red-shift of about 0.9 nm for the H-band and a blue-shift of about 0.8 nm for the J-band, indicating a decrease of the interaction energy between the dyes in the aggregate. To better show the opposite trend of the CD signals during the two different thermal processes, the CD spectra at 25 °C recorded during heating (red trace) and cooling (blue trace) are reported in Figure 8. Furthermore, the values of the CD signals corresponding to the H- and J-bands, as a function of the temperature (see inset in Figure 8), show that both signal strength and sign depend on the temperature gradient, generated inside the cuvette, and on its ramp direction. The linear dichroism (LD) contribution48,63,64 to CD spectra was also investigated with an apparatus built up to measure linear dichroism and cuvette birefringence (see Supporting Information, Figure SI2 and eqs SI1 and SI2). The inset of Figure 9 shows that the LD amplitude increases under temperature increase; similar profiles, with unchanged sign, were also observed during cooling experiments (omitted for

Figure 9. Light scattered intensity profile at different temperature values (T = 20 °C, stars; T = 25 °C, circles; T = 30 °C, squares). Inset: LD curves recorded under increasing temperature.

brevity). From the measurement of LD and cuvette birefringence LB, such contribution was evaluated as less than 20% for J-band and about 60% for H-band with a significant signal distortion. To ascertain if the different CD traces were due to aggregate structure modifications at a mesoscopic scale, light-scattering measurements at 20, 25, and 30 °C were performed (Figure 9) but no indication about structural rearrangements resulted. The temperature gradient (and hence the mass flow generated), caused by the geometry of the cuvette’s holder (see Supporting Information, Figure SI3), is certainly responsible for the observed SSB phenomenon. Indeed, the heating unit is in contact only with one wall of the cuvette (indicated as T1 in the inset of Figure 10); the adjacent wall T2 is leaning to a metal support, whereas the other two walls, T3 and T4, held by springs, are freely exposed to the air. Thanks to such a geometry, during both heating and cooling experiments, points inside the solution are at different instantaneous temperatures, thus inducing the thermal gradient responsible for the SSB effect; the sign of the CD signal is a function of the thermal ramp direction. To show this, the temperature on the F


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Figure 11. CD spectra obtained after hand rotation (CW and CCW) of the cuvette containing an aqueous solution of PPEG4.

Figure 10. Finite element analysis computer simulation of a quartz made cuvette (having the dimensions of commercial ones) filled with 2.5 mL of water (similar to the experimental conditions), where the remaining empty volume was considered as air: (a) homogeneous heated cuvette (T1−6 = 300.0 K); (b) asymmetrical heating system (T1 = 300.0 K, T2 = 299.9 K, T3 = 299.8 K, T4 = 299.7 K, T5 = 300.1, T6 = 299.6 K); (c) asymmetrical heating system, reversed with respect to the long wall temperatures of situation b (T1 = 299.7 K, T2 = 299.8 K, T3 = 299.9 K, T4 = 300.0 K, T5 = 300.1, T6 = 299.6 K). In the inset, the walls’ names are indicated.

The comparison of these spectra evidences that the first rotation of the cuvette determined an evident change of the CD signal. The successive rotation in the opposite direction leads to a spectrum very similar to the initial one with only an increase of about −7 mdeg for H-type signals and of +10 mdeg for Jtype signals. In the literature, a similar behavior is reported for solutions of charged porphyrin species but only under the action of strong rotation rates (about 600 rpm).65 Differently, in the present case, the symmetry breaking of the aggregate species of the uncharged PPEG4 molecules occurs under much milder conditions, just for the action of a very weak flow or of a thermal gradient. To explain this different behavior, it can be considered that the strong Coulomb interactions occurring in a charged porphyrin system are absent in the PPEG4 case for which the porphyrin units interact only by weak pi−pi and/or hydrogen bonds.66 Finally, two solutions of PPEG4 were first stirred (into a cuvette holder with magnetic stir bar) clockwise (CW) and then counterclockwise (CCW)65 at a stirring rate of 7.8 Hz (spectra in Figure 12) and at a very lower rate of 0.15 Hz (spectra in the inset of Figure 12), respectively. The observed bisignate CD band is analogous to that observed in the stagnant solution under a small temperature gradient (Figure 8), and it is still quite huge even at very low stirring rate. Also in this case, the sign of the CD band depends on the rotation sense (CW or

walls inside the cuvette holder was measured with a thermocouple thermometer and the thermal images of the top of the holder’s walls were collected (see Supporting Information, Figures SI4 and SI5). In order to show that the small temperature difference at the holder’s walls is able to generate thermal flows, a model simulation, using a finite element analysis approach, in a common quartz cuvette containing 2.5 mL of water was performed. Several calculations, considering different sequences of wall temperatures (T1−T6 in Figure 10), were made. The most significant results were obtained comparing (Figure 10) the homogeneous model, with all walls having the same temperature (T1−6 = 300.0 K, Figure 10a), an asymmetrical model with the walls at very little different decreasing temperatures [T1 = 300.0 K, T2 = 299.9 K, T3 = 299.8 K, T4 = 299.7 K, T5 = 300.1, T6 = 299.6 K (Figure 10b)], and a similar case but with a reversed sequence of the wall temperatures [T1 = 299.7 K, T2 = 299.8 K, T3 = 299.9 K, T4 = 300.0 K, T5 = 300.1, T6 = 299.6 K (Figure 10c)]. The red arrows indicate the heat flow directions inside the cuvette calculated by the computer simulation. As expected, a chaotic heat flow results for model a, whereas a heat flow from warmer to colder zones results for models b and c, respectively, with an inverse sense. An asymmetrical heat flow was also found inverting the temperatures of the walls T3, T4, and/or those of the top (T5) and bottom (T6) walls (data omitted for brevity). In our opinion, the evidence of these asymmetrical heat flows (cases b and c), despite being very low, can explain the mirrorsymmetry breaking of the porphyrin aggregate systems. Finally, in order to check the effect of very weak fluid fluctuations inside the solution on the mirror-symmetry breaking of the achiral PPEG4 aggregated species, CD spectra (as a sum of eight scans) of a water solution in a quartz cuvette were acquired after some gentle and delicate handling of the cuvette. In particular, after recording a first CD spectrum (dashed line of Figure 11, first step), the cuvette was extracted from the cuvette holder, gently rotated 360° clockwise (CW), and reinserted to record a new CD spectrum (continuous line of Figure 11, second step); then, the cuvette was extracted again and gently rotated 360° counterclockwise (CCW) before recording a final CD spectrum (dotted line of Figure 11, third step).

Figure 12. Circular dichroism spectra of PPEG4 aqueous solution under stirring at 7.81 Hz both clockwise (CW) and counterclockwise (CCW) (the rotation sense refers to top-viewing). Inset: CD spectra under stirring at 0.15 Hz both clockwise (CW) and counterclockwise (CCW). G


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CCW) and, if the sample is maintained at a stable and homogeneous temperature, the CD signal vanishes when rotation is stopped.


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CONCLUSION The data reported here puts in clear evidence that in aggregates of intrinsically achiral porphyrins weak thermal flows resulting from very weak temperature gradients, as put in evidence from the model simulation approach, can be able to induce supramolecular chirality through a symmetry breaking phenomenon. Differently from previous cases of charged porphyrin molecules,65 it is likely that the absence of strong electrostatic interactions between the porphyrin units inside the aggregate of PPEG4 makes these more sensitive even to weak asymmetric forces. The results reported help to clarify the phenomenon of supramolecular chirality induction in the case of achiral uncharged aggregate species under the action of an asymmetrical weak physical perturbation.67 The evidence of this phenomenon can represent a first step toward the understanding of several natural processes which are still unexplained probably because very weak and uncontrolled perturbing forces are involved.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b07199. Size distribution of PPEG4 aggregates; scheme of the apparatus built up to measure linear dichroism and cuvette birefringence; equations describing CD and LD signals; top-view of the Peltier-type temperature control system; trend of the temperature variations at different walls, when submitted to heating; thermograms of the cuvette holder during heating ramp (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the University of Catania (Ricerca Scientifica di Ateneo 2012 and FIR 2014) and by the Ministero Istruzione Università e Ricerca (MIUR, Roma), project DIATEME - PON01_00074. Many thanks are due to Dr Nicoletta Russo for her help in spectroscopic acquisitions and data elaboration.

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REFERENCES

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