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Does the Sum-Frequency Generation Signal of Aromatic C-H Vibrations Reflect Molecular Orientation?

ABSTRACT: Organic molecules with aromatic groups at the aqueous interfaces play a central role in atmospheric chemistry, green chemistry, and on-water synthesis. Insights into the organization of interfacial organic molecules can be obtained using surface-specific vibrational sum-frequency generation (SFG) spectroscopy. However, the origin of the aromatic C-H stretching mode peak is unknown, prohibiting us from connecting the SFG signal to the interfacial molecular structure. Here, we explore the origin of the aromatic C-H stretching response by heterodyne-detected SFG (HD-SFG) at the liquid/vapor interface of benzene derivatives and find that, irrespective of the molecular orientation, the sign of the aromatic C-H stretching signals is negative for all the studied solvents. Together with density functional theory (DFT) calculations, we reveal that the interfacial quadrupole contribution dominates, even for the symmetry-broken benzene derivatives, although the dipole contribution is non-negligible. We propose a simple evaluation of the molecular orientation based on the aromatic C-H peak area.

SUBMITTER: Matsumura F

PROVIDER: S-EPMC10278126 | biostudies-literature | 2023 Jun

REPOSITORIES: biostudies-literature

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Does the Sum-Frequency Generation Signal of Aromatic C-H Vibrations Reflect Molecular Orientation?

Matsumura Fumiki F Yu Chun-Chieh CC Yu Xiaoqing X Chiang Kuo-Yang KY Seki Takakazu T Bonn Mischa M Nagata Yuki Y

The journal of physical chemistry. B 20230607 23

Organic molecules with aromatic groups at the aqueous interfaces play a central role in atmospheric chemistry, green chemistry, and on-water synthesis. Insights into the organization of interfacial organic molecules can be obtained using surface-specific vibrational sum-frequency generation (SFG) spectroscopy. However, the origin of the aromatic C-H stretching mode peak is unknown, prohibiting us from connecting the SFG signal to the interfacial molecular structure. Here, we explore the origin o ...[more]

PMID: 37284731

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Project description:The ability to orient active proteins on surfaces is a critical aspect of many medical technologies. An important related challenge is characterizing protein orientation in these surface films. This study uses a combination of time-of-flight secondary ion mass spectrometry (ToF-SIMS), sum frequency generation (SFG) vibrational spectroscopy, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to characterize the orientation of surface-immobilized Protein G B1, a rigid 6 kDa domain that binds the Fc fragment of IgG. Two Protein G B1 variants with a single cysteine introduced at either end were immobilized via the cysteine thiol onto maleimide-oligo(ethylene glycol)-functionalized gold and bare gold substrates. X-ray photoelectron spectroscopy was used to measure the amount of immobilized protein, and ToF-SIMS was used to measure the amino acid composition of the exposed surface of the protein films and to confirm covalent attachment of protein thiol to the substrate maleimide groups. SFG and NEXAFS were used to characterize the ordering and orientation of peptide or side chain bonds. On both substrates and for both cysteine positions, ToF-SIMS data showed enrichment of mass peaks from amino acids located at the end of the protein opposite to the cysteine surface position as compared with nonspecifically immobilized protein, indicating end-on protein orientations. Orientation on the maleimide substrate was enhanced by increasing pH (7.0-9.5) and salt concentration (0-1.5 M NaCl). SFG spectral peaks characteristic of ordered ?-helix and ?-sheet elements were observed for both variants but not for cysteine-free wild type protein on the maleimide surface. The phase of the ?-helix and ?-sheet peaks indicated a predominantly upright orientation for both variants, consistent with an end-on protein binding configuration. Polarization dependence of the NEXAFS signal from the N 1s to ?* transition of ?-sheet peptide bonds also indicated protein ordering, with an estimated tilt angle of inner ?-strands of 40-50° for both variants (one variant more tilted than the other), consistent with SFG results. The combined results demonstrate the power of using complementary techniques to probe protein orientation on surfaces.

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Does the Sum-Frequency Generation Signal of Aromatic C-H Vibrations Reflect Molecular Orientation?

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Does the Sum-Frequency Generation Signal of Aromatic C-H Vibrations Reflect Molecular Orientation?

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