Quantum Mechanical Calculations of Vibrational Sum-Frequency-Generation (SFG) Spectra of Cellulose: Dependence of the CH and OH Peak Intensity on the Polarity of Cellulose Chains within the SFG Coherence Domain

Abstract

Vibrational sum-frequency-generation (SFG) spectroscopy is capable of selectively detecting crystalline biopolymers interspersed in amorphous polymer matrices. However, the spectral interpretation is difficult due to the lack of knowledge on how spatial arrangements of crystalline segments influence SFG spectra features. Here we report time-dependent density functional theory (TD-DFT) calculations of cellulose crystallites in intimate contact with two different polarities: parallel versus antiparallel. TD-DFT calculations reveal that the CH/OH intensity ratio is very sensitive to the polarity of the crystallite packing. Theoretical calculations of hyperpolarizability tensors (β) clearly show the dependence of SFG intensities on the polarity of crystallite packing within the SFG coherence length, which provides the basis for interpretation of the empirically observed SFG features of native cellulose in biological systems.

Self-Phase-Stabilized Heterodyne Vibrational Sum Frequency Generation Microscopy

“…Because molecular selfassembly fully covers gold substrates, the difference in VSFG intensity reflects various domain conformations, but not surface coverage: e.g., domains with strong VSFG signals consist of head-to-tail conformations, because they are non-centrosymmetric, whereas domains with weak signals are composed of head-to-head configurations. 51 However, there is much more information to be learned than what the VSFG intensity image can reveal, because in the HD VSFG image, each pixel encodes a phase-resolved HD VSFG spectrum. To take advantage of the rich spectral information, we further characterize each domain by comparing all the spectral patterns of their imaginary spectra (corresponding to absorptive spectra), 30,33,34,36,47 including peak position, phase and line shapes.…”

Distinguishing Mesoscale Polar Order (Unidirectional vs Bidirectional) of Cellulose Microfibrils in Plant Cell Walls Using Sum Frequency Generation Spectroscopy

“…The transition dipole moments of the SFG-active OH stretch modes are in the (200) plane of the cellulose I unit cell with specific tilt angles from the chain axis. 30,[39][40] In contrast, the transition dipole moments of the SFG-active CH and CH 2 stretch modes are off the (200) plane. Thus, the OH dipoles can be enhanced when two crystallites are unidirectional (parallel) and cancelled when one crystallite is rotated by 180 o along the a-axis of the unit cell, making two crystallites bidirectional (antiparallel) to each other.…”

Sum Frequency Generation Vibrational Spectroscopy for Characterization of Buried Polymer Interfaces

“…156,157 So far, these novel techniques have not been widely applied to polymer interface studies. Additionally, molecular dynamics simulations, ab initio calculations, and time-dependent density functional theory can assist to predict and interpret the vibrational signatures obtained from SFG-VS. [158][159][160] Future work utilizing these advanced methodologies to interpret polymer interfaces would improve the understanding of molecular behaviors at buried polymer interfaces.…”

“…We need to mention that many biomolecules can be considered as natural polymers, such as cellulose, collagen polypeptide, deoxyribonucleic acid (DNA), and polymerized amyloid b-peptide. The abovementioned biopolymers have been extensively examined by SFG-VS, 160,[177][178][179][180][181] but were not discussed in this review. Excellent review articles related to the biological subjects of the SFG-VS can be found elsewhere.…”