CONTENTS 1. Introduction 8471 2. Theoretical Background of Chiral SFG 8472 2.1. General Principles of SFG 8472 2.2. Effective Susceptibility of Chiral Surfaces 8473 2.3. Surface Susceptibility and Molecular Hyperpolarizability 8474 2.4. Chiral SFG Response: Bulk versus Interface 8476 3. Chiral SFG Experiments 8477 3.1. Polarization Settings for Chiral SFG Experiments 8477 3.2. Spectrometers for Chiral SFG Studies of Biomacromolecules 8478 3.3. Surface Platforms for Probing Biomacromolecules 8479 4. Structures of Biomacromolecules at Interfaces Probed by Chiral SFG 8480 4.1. Chiral Amide I Signals of Proteins or Peptides at Interfaces 8480 4.2. Chiral C−H Stretch Signals of DNA on Solid/ Water Interfaces 8480 4.3. Chiral N−H Stretch from Protein Backbone at Interfaces 8481 4.4. Chiral N−H Stretch and Amide I for Probing Secondary Structures at Interfaces 8482 4.5. Characterization of Various Vibrational Bands of Collagen 8484 4.6. Double Resonance for Detecting Chiral SFG Signal from Porphyrin J Aggregates 8484 5. Orientations of Biomacromolecules at Interfaces Probed by Chiral SFG 8485 5.1. Orientation of Antiparallel β-Sheet Structures at Interfaces 8485 5.2. Orientation of Parallel β-Sheet Structures at Interfaces 8486 6. Kinetics and Dynamics of Biomacromolecules at Interfaces Probed by Chiral SFG 8488 6.1. Kinetics of Protein Folding Probed by Chiral Amide I and N−H Stretch 8488 6.2. Kinetics of Proton Exchange in Protein Backbones Probed by Chiral N−H 8489 6.3. Kinetics of Protein Self-Assembly Probed by Chiral C−H Stretch of Protein Side Chains 8490 7. Calculations of Hyperpolarizability of Biomacromolecules 8491 7.1. Calculation of Hyperpolarizability of Biomacromolecules for Weak Vibrational Coupling 8491 7.2. Calculation of Hyperpolarizability of Biomacromolecules for Strong Vibrational Coupling 8492 7.3. Calculation of Hyperpolarizability by the ab Initio Quantum Chemistry Method 8492 7.4. Comparison of Calculation Methods for Hyperpolarizability of Biomacromolecules 8493 8. Summary and Outlook 8493 8.1. Summary 8493 8.2. Potential Applications 8493 8.3. Challenges and Outlooks 8494 Author Information 8495 Corresponding Author 8495 Notes 8495 Biographies 8495 References 8496 Note Added after ASAP Publication 8498

Studying hydrogen/deuterium (H/D) exchange in proteins can provide valuable insight on protein structure and dynamics. Several techniques are available for probing H/D exchange in the bulk solution, including NMR, mass spectroscopy, and Fourier transform infrared spectroscopy. However, probing H/D exchange at interfaces is challenging because it requires surface-selective methods. Here, we introduce the combination of in situ chiral sum frequency generation (cSFG) spectroscopy and ab initio simulations of cSFG spectra as a powerful methodology to probe the dynamics of H/D exchange at interfaces. This method is applied to characterize H/D exchange in the antiparallel β-sheet peptide LK7β. We report here for the first time that the rate of D-to-H exchange is about 1 order of magnitude faster than H-to-D exchange in the antiparallel structure at the air/water interface, which is consistent with the existing knowledge that O-H/D dissociation in water is the rate-limiting step, and breaking the O-D bond is slower than breaking the O-H bond. The reported analysis also provides fundamental understanding of several vibrational modes and their couplings in peptide backbones that have been difficult to characterize by conventional methods, including Fermi resonances of various combinations of peptide vibrational modes such as amide I and amide II, C-N stretch, and N-H/N-D bending. These results demonstrate cSFG as a sensitive technique for probing the kinetics of H/D exchange in proteins at interfaces, with high signal-to-noise N-H/N-D stretch bands that are free of background from the water O-H/O-D stretch.

We review the recent development of chiral sum frequency generation (SFG) spectroscopy and its applications to study chiral vibrational structures at interfaces. This review summarizes observations of chiral SFG signals from various molecular systems and describes the molecular origins of chiral SFG response. It focuses on the chiral vibrational structures of proteins and presents the chiral SFG spectra of proteins at interfaces in the C-H stretch, amide I, and N-H stretch regions. In particular, a combination of chiral amide I and N-H stretches of the peptide backbone provides highly characteristic vibrational signatures, unique to various secondary structures, which demonstrate the capacity of chiral SFG spectroscopy to distinguish protein secondary structures at interfaces. On the basis of these recent developments, we further discuss the advantages of chiral SFG spectroscopy and its potential application in various fields of science and technology. We conclude that chiral SFG spectroscopy can be a new approach to probe chiral vibrational structures of protein at interfaces, providing structural and dynamic information to study in situ and in real time protein structures and dynamics at interfaces.

Self-assembly of molecules into chiral macromolecular and supramolecular structures at interfaces is important in various fields, such as biomedicine, polymer sciences, material sciences, and supramolecular chemistry. However, probing the kinetics at interfaces remains challenging because it requires a real-time method that has selectivity to both interface and chirality. Here, we introduce an in situ approach of using the C-H stretch as a vibrational probe detected by chiral sum frequency generation spectroscopy (cSFG). We showed that the C-H stretch cSFG signals of an amphiphilic peptide (LK7β) can reveal the kinetics of its self-assembly into chiral β-sheet structures at the air-water interface. The cSFG experiments in conjunction with measurements of surface pressure allow us to propose a mechanism of the self-assembly process, which involves an immediate adsorption of disordered structures followed by a lag phase before the self-assembly into chiral antiparallel β-sheet structures. Our method of using the C-H stretch signals implies a general application of cSFG to study the self-assembly of bioactive, simple organic, and polymeric molecules into chiral macromolecular and supramolecular structures at interfaces, which will be useful in tackling problems, such as protein aggregation, rational design of functional materials, and fabrication of molecular devices.

Characterizations of protein structures at interfaces are important in solving an array of fundamental and engineering problems, including understanding transmembrane signal transduction and molecular transport processes and development of biomaterials to meet the needs of biomedical and energy research. However, in situ and real-time characterization of protein secondary structures is challenging because it requires physical methods that are selective to both interface and secondary structures. Here, we summarize recent experimental developments in our laboratory of chiral vibrational sum frequency generation spectroscopy (SFG) for analyzing protein structures at interfaces. We showed that chiral SFG provides vibrational optical signatures of the peptide N-H stretch and amide I modes that can distinguish various protein secondary structures. Using these signatures, we further applied chiral SFG to probe orientations and folding kinetics of proteins at interfaces. Our results show that chiral SFG is a background-free, label-free, in situ, and real-time vibrational method for studying proteins at interfaces. This recent progress demonstrates the potential of chiral SFG in solving problems related to proteins and other chiral biopolymers at interfaces.