Isomer-Specific Induced Circular Dichroism Spectroscopy of Jet

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Isomer-Specific Induced Circular Dichroism Spectroscopy of Jet-Cooled Phenol Complexes with (–)-Methyl L-Lactate Aram Hong, Cheol Joo Moon, Heeseon Jang, Ahreum Min, Myong Yong Choi, Jiyoung Heo, and Nam Joon Kim J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b03241 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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The Journal of Physical Chemistry Letters

Isomer-Specific Induced Circular Dichroism Spectroscopy of Jet-Cooled Phenol Complexes with (–)-Methyl L-Lactate Aram Hong, † Cheol Joo Moon, ‡ Heeseon Jang, † Ahreum Min, † Myong Yong Choi, ‡ Jiyoung Heo,*, § and Nam Joon Kim*, †

†Department

‡Department

of Chemistry, Chungbuk National University, Chungbuk 28644, Korea

of Chemistry (BK21+) and Research Institute of Natural Science, Gyeongsang National University, Jinju 52828, Korea

§Department

of Biomedical Technology, Sangmyung University, Chungnam 31066, Korea

Corresponding Author * E-mail: [email protected] (N.J. Kim) and [email protected] (J. Heo).

ACS Paragon Plus Environment

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Abstract

Induced circular dichroism (ICD) is the CD observed in the absorption of an achiral molecule bound to a transparent chiral molecule through noncovalent interactions. ICD spectroscopy has been used to probe the binding between molecules, such as protein-ligand interactions. However, most ICD spectra have been measured in solution, which only exhibit the averaged CD values of all conformational isomers in solution. Here, we obtained the first isomer-selective ICD spectra by applying resonant two-photon ionization CD spectroscopy to jet-cooled phenol complexes with (–)-methyl L-lactate (PhOH-(–)ML). The well-resolved CD bands in the spectra were assigned to two conformers, which contained different types of hydrogen-bonding interactions between PhOH and (–)ML. The ICD values of the two conformers have different signs and magnitudes, which were explained by differences both in the geometrical asymmetries of PhOH bound to (–)ML and in the electronic coupling strengths between PhOH and (–)ML.

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Circular dichroism (CD) consists of the differential absorption of left- and right-handed circularly polarized light (LCP and RCP) and, is generally observed for chiral molecules. However, CD is detected even in the absorption of an achiral molecule bound to a transparent chiral molecule through noncovalent interactions, which is denoted as induced circular dichroism (ICD)1. In contrast to CD of single chiral molecules, ICD arises from intermolecular interactions between achiral and chiral molecules. Therefore, since its discovery2,3, ICD spectroscopy has been extensively employed to investigate the binding properties of various host-guest complexes. For instance, the relative orientations of achiral guests with respect to a chiral host have been determined from ICD bands4,5. Additionally, the absolute configuration of a chiral molecule bound to a chromophoric achiral molecule was identified using ICD spectroscopy6,7. Recently, ICD spectroscopy has been used to probe the binding between drugs and target proteins8, 9. However, ICD spectroscopy has been primarily applied to complexes in solution, which imposes limits on its wide applications. First, solution-phase ICD spectra exhibit only broad spectral features with nearly unresolved vibronic bands, from which any high-resolution structural information can hardly be observed. Second, ICD spectra show only the averaged CD values of all different conformational isomers in solution. With these averaged spectra, extracting information about individual conformers is not possible. To avoid these limitations, it is necessary to apply ICD spectroscopy to isolated gasphase complexes, particularly, cooled by a supersonic expansion technique. The electronic spectra of molecules in a supersonic jet exhibit well-resolved vibronic bands, which can be assigned to the different conformers present in the jet by double resonance spectroscopic techniques10-12. Thus, ICD spectroscopy of jet-cooled complexes, which measures the ICD of each vibronic band in the electronic spectrum, can provide conformation-selective ICD values.


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These ICD values combined with quantum mechanical calculations are crucial to determine the structures, absolute configurations, and binding characteristics of individual conformers. The ICD of host-guest complexes arises from two different sources1. One source is the destruction of the geometrical symmetry of an achiral molecule through binding to a chiral molecule, which is often observed in the enantioselective synthesis using an asymmetric catalyst13. The other source is the coupling of the electric transition dipole moments (µ) between achiral and chiral molecules. To understand the influences of each of the two sources on the ICD values, it is also essential to measure conformation-selective ICD spectra. However, CD spectroscopy of jet-cooled molecules has not been widely studied due to the weak CD effects aggravated by low-density gas-phase molecules. The CD for ion yields of gaseous molecules in a molecular beam has been measured at selected wavelengths using resonance-enhanced multiphoton ionization (REMPI) spectroscopy14-17. Photoelectron CD spectra of jet-cooled molecules have been observed using one-photon ionization with vacuum UV synchrotron radiation or femtosecond REMPI18-21. Recently, we developed a CD spectroscopic technique applicable to jet-cooled chiral molecules, based on the work by Bonmarin and Helbing22, and obtained resonant two-photon ionization CD (R2PICD) and fluorescence-detected CD (FDCD) spectra23-25. Here, we obtained the first ICD spectra of the 1:1 complexes of phenol (PhOH) and (–)methyl L-lactate ((–)ML) produced in a supersonic jet using our CD spectroscopic technique. We chose PhOH and (–)ML due to their capability to form hydrogen-bonded complexes. The ICD of hydrogen-bonded complexes is of great interest because hydrogen bonding plays a key role in ligand-protein and protein-protein interactions26. Moreover, PhOH serves as a prototype of


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tyrosine residues in proteins, and (–)ML complexes have been investigated to understand chiral recognition processes27. The chirality transfer in ML complexes in solution and under matrix isolation conditions has also been studied using vibrational CD spectroscopy28,29. Additionally, (–)ML has no absorption in the near-UV region where PhOH has strong absorption, which simplifies the observation of ICD in host-guest complexes (Fig. S1). The ICD spectra of jet-cooled PhOH-(–)ML complexes exhibit well-resolved vibronic CD bands, which were assigned to two different conformational isomers. These conformers were identified to have two distinct types of hydrogen-bonding interactions between PhOH and (–)ML by UV-UV hole-burning (HB) and infrared (IR) ion-dip spectroscopy. We investigated how these different binding characteristics are reflected in the ICD spectra of the conformers.

Figure 1. (a) R2PI spectrum of PhOH-(–)ML near the origin band of the S0-S1 transition. UV-UV HB spectra were obtained by fixing a probe laser to the (b) α and (c) β bands. The tick labels in the upper axis represent the relative wavenumber from the origin band of bare PhOH.


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Fig. 1a displays the R2PI spectrum of PhOH-(–)ML produced in a supersonic jet near the origin band of the S0-S1 transition. The lowest-energy band, α, is redshifted by 415 cm-1 from the origin band of bare PhOH. The chromophore of PhOH-(–)ML in this wavelength region is the PhOH moiety, as the origin band of the S0-S1 transition of (–)ML occurs at a much higher frequency above 40000 cm-1 (Fig. S1). To determine the number of different conformational isomers of PhOH-(–)ML that contribute to the R2PI spectrum, UV-UV HB spectra were obtained by fixing a probe laser to the α and β bands (Figs. 1b and c, respectively). Most of the vibronic bands in the R2PI spectrum are observed in either of the two HB spectra, which exhibit distinct spectral features from each other. These observations indicate that PhOH-(–)ML exists as at least two conformers in the jet. The lowest-energy bands, α and β, were assigned as the origin bands of two conformers (labeled A and B, respectively). To determine the structures of these conformers, IR ion-dip spectra were obtained by fixing a UV laser at the α and β bands, and scanning the frequency of an IR laser over the spectral range (Fig. 2 and Table S1). Both IR spectra exhibit distinct spectral features from each other. The bands near 1750 and 3520 cm-1 correspond to the C=O and OH stretching vibrational modes of (–)ML, respectively, whereas the band between them corresponds to the OH stretching vibrational mode of PhOH. The bands with an asterisk are the overtones of the C=O stretching vibrational mode. The OH band of PhOH in conformer B is redshifted by 71 cm-1 from that of conformer A, implying that the OH group is involved in slightly stronger hydrogen-bonding interactions in conformer B. The IR ion-dip spectra were compared to the IR spectra of the low-energy conformers of PhOH-(–)ML predicted using calculations at the CAM-B3LYP-D3BJ/6-311++G(d,p) level (Table S2). The low-energy conformers are grouped into I and II (Fig. S2). In group I, the


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conformers contain a single hydrogen bond between PhOH and (–)ML, and an additional intramolecular hydrogen bond between the OH and C=O groups of (–)ML. The conformers in group II have two hydrogen bonds between PhOH and (–)ML, where the OH group of PhOH acts as both a hydrogen-bond acceptor and donor to the OH and C=O groups of (–)ML, respectively. The same hydrogen-bond patterns have been previously described in other complexes between methanol and ML30. From a comparison of the experimental and theoretical IR spectra, we assigned conformers A and B to groups I and II, respectively. However, further structural assignment was not possible because the IR spectra of the conformers in the same group are very similar to each other.

Figure 2. IR ion-dip spectra of PhOH-(–)ML obtained by fixing a UV laser to the (a) α and (b) β bands in Fig. 1a. Each spectrum is compared to the IR spectra of the low-lying conformers of PhOH-(–)ML predicted at the CAM-B3LYP-D3BJ/6-311++G(d,p) level. The optimized structures of conformers Ia and IIa at the same level of theory are shown on the right-hand side. The structures of the other low-energy conformers are in Fig. S2. Scale factors of 0.9698 and


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0.9484 were used for the IR spectra in the wavenumber regions of 1650~1800 cm-1 and 2800~3800 cm-1, respectively. The bands with an asterisk are the overtones of the C=O stretching vibrational mode. For further structural assignment of the conformers, the electronic spectra of the lowlying conformers were simulated considering the Franck-Condon and Herzberg-Teller contributions, and compared with the UV-UV HB spectra of conformers A and B (Fig. S3). Among the conformers in groups I and II, the electronic spectra of conformers Ia and IIa agree well with the HB spectra of conformers A and B, respectively. Therefore, we assigned conformers Ia and IIa, as conformers A and B, respectively.

Figure 3. (a) ICD spectra of PhOH-(–)ML (black) and PhOH-(+)ML (gray) obtained by subtracting the ion signals produced by the RCP pulses from those by the LCP pulses (IL-IR). (b) R2PI spectra of conformers A (purple) and B (green) in the same wavenumber regions with the ICD spectra.


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Fig. 3a displays the ICD spectra of PhOH-(–)ML and PhOH-(+)ML near the origin band of the S0-S1 transition. No CD effect is observed in the absorption of bare PhOH. However, upon forming a complex with (–)ML, CD effects appear near the absorption region of PhOH, where (– )ML is transparent, representing ICD. Both ICD spectra exhibit well-resolved CD bands, which are mirror images of each other. For PhOH-(–)ML, the CD signs of the origin bands of conformers A and B (α and β, respectively) are positive and negative, respectively. These CD signs correspond to those of conformers Ia and IIa estimated using the theoretical method developed by N. Lin et al.31 (Table S2). This result further supports the previous assignment of conformers A and B as conformers Ia and IIa, respectively. The vibronic progressions with an interval of 12 and 37 cm-1 are observed in the electronic spectra of conformers A and B, respectively (Fig. 3b). The vibrational modes of 12 and 37 cm-1 were assigned as the intermolecular bending vibrational modes between PhOH and (– )ML (Fig. S4). Interestingly, the vibronic progression with an interval of 12 cm-1 shows alternating CD signs between positive and negative (Fig. 3a). For instance, the CD sign of PhOH-(–)ML is positive for the transition from v”=0 in the S0 state to v’=0, 2, or 4 in the S1 state but becomes negative for the transition to v’=1 or 3. These alternating CD signs may result from the vibronic progression arising from two different modes, leading to the opposite CD signs from one another32. However, the presence of two different modes is unlikely because the energy gaps between the adjacent bands in the progression constantly decrease from 12 to 10 cm-1, as generally observed in a vibronic progression built on a single mode. Furthermore, a similar vibronic progression based on a 15 or 19 cm-1 mode was previously reported in the electronic spectra of ML complexes with aromatic compounds27, 33. Therefore, we suggest that the 12 cm-1 vibronic progression arises from a single


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mode. Theoretical predictions have also indicated that the alternation of CD signs or magnitudes may occur if a vibronic progression arises from a nontotally symmetric vibrational mode34, as is the case of the 12 cm-1 mode, an intermolecular bending vibration mode (Fig. S4). However, to theoretically reproduce the alternating CD signs of the vibronic progression, further developments of the theoretical method are required. The asymmetry factor is given by g=2(IL ‒ IR)/(IL + IR), where IL and IR are the ion signals produced by the LCP and RCP pulses, respectively14. For PhOH-(–)ML, the g values of the origin bands of conformers A and B in Fig. 3a were +3.3±0.1 % and -2.4±0.3 %, respectively. These distinct g values demonstrate that the ICD value is indeed sensitive to the binding characteristics between PhOH and (–)ML. To understand the large difference between the ICD values of the two conformers, we investiagted two factors producing ICD phenomena: (1) the destruction of the geometrical symmetry of PhOH and (2) the µ coupling or electronic coupling between PhOH and (–)ML. To quantitatively estimate the geometrical asymmetry, the rotatory strength R of PhOH was computed after removing (–)ML from the optimized structures of conformers Ia and IIa, which were assigned as conformers A and B, respectively. R corresponds to the imaginary part of a scalar product between µ and the magnetic transition dipole moment (M) and represents the experimental CD value. Table 1. Rotatory strengths R of PhOH with and without (–)ML in conformers Ia and IIa. Conformer

PhOH w/o (–)ML

PhOH-(–)ML

Ia

−3.17a

+7.14

IIa

−0.05

−1.86


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a

Unit in cgs (10-40 erg·esu·cm/Gauss).

Table 1 lists the R values of PhOH with and without (–)ML in conformers Ia and IIa. In conformer Ia, the R value of PhOH without (–)ML is nonzero but nearly half the value of the conformer, indicating that the geometrical symmetry of PhOH is indeed broken upon binding to (–)ML. More specifically, the OH group of PhOH is tilted away from the benzene ring plane by ~11o, forming a hydrogen bond with (–)ML (Fig. 4a). Interestingly, with this OH group tilted above or below the ring plane, PhOH cannot be superimposed by its mirror image like a chiral molecule.

Figure 4. Dihedral angles of the OH groups of PhOH in conformers (a) Ia and (b) IIa, and 2D representations of the matrix of ξv for the S0-S1 transition of conformers (c) Ia and (d) IIa. The tick label represents the atom number of the conformers as shown in (a). On the other hand, the OH group of PhOH in conformer IIa lies in the same plane as the benzene ring, producing a near zero value of R for PhOH without (–)ML (Fig. 4b). Therefore, the ICD of conformer IIa does not arise from the geometrical asymmetry of PhOH but arises from


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the electronic coupling between PhOH and (–)ML. This electronic coupling also plays a role in the ICD of conformer Ia because the R value is two times greater than and has the opposite sign to that of the conformer without (–)ML. To demonstrate the presence of the electronic coupling between PhOH and (–)ML and also evaluate its strengths of the two conformers, we employed the collective electronic oscillator (CEO) approach35,36. The CEO approach describes the electronic dynamics underlying the optical transition with a matrix of the electronic normal mode (ξv). In this matrix, the diagonal elements (ξv)XX represent the total induced charge on each atom X by optical transition, whereas the off-diagonal elements (ξv)XY indicate the flow of optically induced charges or electronic coherence between atoms X and Y. If |(ξv)XY| > |(ξv)YX| in the two-dimensional (2D) representation of a matrix of ξv , electron transfer occurs from X to Y during the optical transition (and vice versa). However, if |(ξv)XY|=|(ξv)YX|, no net charge transfer occurs between X and Y. Figs. 4c and d show 2D representations of ξv for the S0-S1 transitions of conformers Ia and IIa. The diagonal elements of conformer Ia indicate that charge is induced on C1~6 and O7 of PhOH and C16 and C18~19 of (–)ML by the optical transition. The large off-diagonal elements at X=1~7 and Y=16~19 indicate that electron transfer occurs from PhOH to (–)ML during the transition. These results imply that the S0-S1 transition is not localized on the PhOH chromophore but involves coherent electronic motion between PhOH and (–)ML, manifesting the electronic coupling between the two species. This coupling also exists in conformer IIa based on the large off-diagonal elements at X=1~7 and Y=16, and X=18~19 and Y=1~7. However, the coupling appears weaker in conformer IIa, as the off-diagonal elements are more than two times smaller than those in conformer Ia. Therefore, we suggest that the greater ICD magnitude of


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conformer Ia, compared to conformer IIa, results not only from the more pronounced geometrical asymmetry of PhOH but also from the stronger electronic coupling between PhOH and (–)ML. In summary, we measured the first ICD spectrum of a jet-cooled complex of an achiral (PhOH) and a chiral molecule ((–)ML), probed on the achiral chromophore, to our knowledge. The ICD spectrum exhibited well-resolved vibronic ICD bands, which were identified to originate from two different conformers. These conformers possessed two distinct types of hydrogen-bonding interactions between PhOH and (–)ML, yielding different ICD values. We found that these different ICD values result from the different geometrical asymmetries of PhOH imposed through binding with (–)ML and from different electronic coupling strengths between PhOH and (–)ML. These results demonstrate that isomer-specific ICD spectroscopy of jet-cooled host-guest complexes provides deep insight into the interactions between chiral and achiral molecules, which is crucial to understand molecular complexation or recognition processes in many biological reactions.

ASSOCIATED CONTENT Supporting Information Experimental and theoretical methods, observed IR band positions, relative energies and structures of the low-lying conformers, UV absorption spectra of PhOH and ML in methanol, simulated electronic spectra, pictorial representations of intermolecular bending vibrational modes.


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AUTHOR INFORMATION Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was financially supported by Samsung Science and Technology Foundation under Project Number SSTF-BA1602-06.

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Isomer-Specific Induced Circular Dichroism Spectroscopy of Jet

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