C–H Stretch for Probing Kinetics of Self-Assembly into Macromolecular Chiral Structures at Interfaces by Chiral Sum Frequency Generation Spectroscopy

Abstract

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.

A narrow amide I vibrational band observed by sum frequency generation spectroscopy reveals highly ordered structures of a biofilm protein at the air/water interface

“…Chiral SFG has the advantages of being free of spectral background from achiral molecules, including water. 18, 19, 20 Due to the difference in selection rules, 20 the chiral SFG spectrum (Figure 1D) of BslA at the air/water interface is different from the achiral SFG spectrum (Figure 1C). It exhibits only one amide I band at 1639 cm −1 , which is at the high-frequency end of the antiparallel β-sheet B 2 vibrational mode and the low-frequency end of disordered structures.…”

Theoretical Sum Frequency Generation Spectroscopy of Peptides

“…Thus, it has become much-used for the study of interfacial proteins 18 − 38 and has enabled researchers to study these systems in real time and in situ . 19 , 24 , 27 , 31 − 33 , 39 − 44 In particular, many recent experiments have focused on the amide I (primarily CO-stretch) mode, which has also been extensively used for linear and 2D IR studies. 11 , 13 , 45 − 51 This mode is particularly attractive for proteins because it exhibits distinct spectral features for different secondary structures.…”

N‐H Stretching Modes Around 3300 Wavenumber From Peptide Backbones Observed by Chiral Sum Frequency Generation Vibrational Spectroscopy

“…Among these works, a surface‐specific vibrational technique, sum frequency generation (SFG) spectroscopy, has been used successfully to minimize the interference from water bulk, allowing detailed molecular studies of structures, kinetics, and interactions of proteins at interfaces . Nonetheless, most of these previous studies focused on the amide I and C‐H stretching regions . A few research groups have recently observed achiral SFG signals of N‐H stretching at ~3300 cm ‐1 from peptides and proteins at interfaces.…”

“…[5][6][7][8] Nonetheless, most of these previous studies focused on the amide I and C-H stretching regions. [9][10][11] A few research groups have recently observed achiral SFG signals of N-H stretching at~3300 cm -1 from peptides and proteins at interfaces. For example, Somorjai and coworkers tentatively discussed the assignment of thẽ 3300-cm -1 peak from the peptide LK 7 β (LKLKLKL) in the achiral SFG to be the N-H stretch from the backbone.…”