利用调制的定向敏感太赫兹光谱解析生物大分子振动

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"Modulated orientation-sensitive terahertz spectroscopy 技术在生物大分子红外振动研究中的应用" 在分子科学领域，特别是在生物大分子的研究中，理解蛋白质和其他大分子的内部振动至关重要。这些振动通常位于太赫兹（terahertz）频率范围内，这是一个介于微波和红外光谱之间的电磁频段。然而，由于这些振动频率紧密相邻，形成近乎连续的振动状态密度，这使得识别特定的吸收线并将其与特定的功能运动关联起来变得极其困难。此外，溶液的吸收以及局部的松弛动力学常常主导了这一频段的信号，进一步增加了分析的复杂性。 "Modulated orientation-sensitive terahertz spectroscopy"（调制的定向敏感太赫兹光谱法）是一种新兴的光谱技术，旨在解决上述挑战。这种技术通过调制太赫兹辐射的振幅、相位或频率，可以增强对特定振动模式的探测，并降低背景噪声的影响。它提高了对复杂生物分子体系中精细振动模式的分辨能力，从而有可能区分出那些在常规方法中难以辨认的信号。文章由Rohit Singh、Deepu Koshy George、Chejin Bae、K.A. Niessen和A.G. Markelz等作者共同撰写，分别来自德堡大学、弗吉尼亚理工学院、纽约州立大学水牛城分校的物理和电气工程部门。他们提出了一种策略，通过调制太赫兹光谱来去除溶剂和局部松弛运动的影响，以实现更精确的分子振动分析。该技术的应用不仅限于基础科学研究，还在药物发现、生物传感和生物标记等领域具有潜在价值。例如，它可以用来检测蛋白质结构的变化，这些变化可能与疾病的发生和发展有关，或者帮助设计更有效的药物靶向策略。此外，通过对生物分子的精细振动模式的深入理解，还可以推动新材料的设计和合成，尤其是在生物相容性和功能化材料的研发中。 "Modulated orientation-sensitive terahertz spectroscopy"为理解和利用生物大分子的太赫兹振动提供了一个强大的工具，有望开启新的研究途径，促进生物物理学、化学和医学等多个领域的进步。这项技术的创新之处在于其能够克服传统方法的局限性，使科学家能够更准确地解析复杂生物系统的动态特性。

Modulated orientation-sensitive terahertz spectroscopy

Rohit Singh,

Deepu Koshy George,

Chejin Bae,

K. A. Niessen,

and A. G. Markelz

Department of Physics and Astronomy, Depauw University, Greencastle, Indiana 46135, USA

Department of Physics, Virginia Tech, Virginia 24061, USA

Department of Electrical Engineering, University at Buffalo, Buffalo, New York 14260, USA

Department of Physics, University at Buffalo, Buffalo, New York 14260, USA

*Corresponding author: amarkelz@buffalo.edu

Received February 3, 2016; revised March 29, 2016; accepted March 31, 2016;

posted April 1, 2016 (Doc. ID 258858); published May 12, 2016

Intramolecular vibrations of large macromolecules reside in the terahertz range. In particular, protein vibrations

are closely spaced in frequency, resulting in a nearly continuous vibrational density of states. This density of

vibrations interferes with the identification of specific absorption lines and their subsequent association with spe-

cific functional motions. This challenge is compounded with the absorption being dominated by the solvent and

local relaxational motions. A strategy for removing the isotropic relaxational loss and isolating specific vibrations

is to use aligned samples and polarization-sensitive measurements. Here, we demonstrate a technique to rapidly

attain the anisotropic resonant absorbance using terahertz time domain spectroscopy and a spinning sample. The

technique, modulated orientation-sensitive terahertz spectroscopy (MOSTS), has a nonzero signal only for aniso-

tropic samples, as demonstrated by a comparison between a silicon wafer and a wire grid polarizer. For sucrose

and oxalic acid molecular crystals, the MOSTS response is in agreement with modeled results for the intermo-

lecular vibrations. Further, we demonstrate that, even in the presence of a large relaxational background, MOSTS

OCIS codes: (120.0120) Instrumentation, measurement, and metrology; (120.2130) Ellipsometry and polar-

imetry; (170.6280) Spectroscopy, fluorescence and luminescence; (300.6380) Spectroscopy, modulation;

(300.6390) Spectroscopy, molecular; (300.6495) Spectroscopy, terahertz.

http://dx.doi.org/10.1364/PRJ.4.0000A1

1. INTRODUCTION

The long-range intramolecular vibrations of proteins and

other large macromolecules reside in the terahertz (THz) fre-

quency range. Calculations have shown these motions may

provide the molecule with efficient access to functional struc-

tural change for biomolecules as well as possibly enable allo-

steric control through the dynamics [1–6]. To date there has

not been experimental proof of the relevance of these motions

to biology, due to the challenge of measuring these motions

and characterizing how they change with functional state and

mutation. X-ray and coherent neutron inelastic scattering

measurements are used to measure the vibrational density

of states (VDOS), but these measurements are challenging, re-

quiring ∼100 mg of protein and specialized facilities [7–10]. In

addition, the VDOS measurements cannot yet provide insight

into the role of these motions in function because specific mo-

tions associated with structural change cannot be isolated.

Optical techniques could potentially simplify the spectrum

by isolating those vibrations with optical coupling. However,

standard optical absorption spectra of proteins do not show

the hoped for resonant absorption structure and, instead, find

only a broad absorbance centered at 100–150 cm

−1

[11–13],

similar to the dielectric response of amorphous or glass-like

materials. This broad absorption could be first understood as

reflecting a majority of the intramolecular vibrations, which

are optically active; thus, the optical absorbance reflects

the broad VDOS. In addition to this, the amino acid side chains

and the solvent will also contribute a strong background from

relaxational dielectric loss [14,15].

It is tempting to simply remove all the water from a protein

sample to at least eliminate this background; however, the

molecular structure of proteins is highly dependent on the hy-

dration, and the removal of water results in distinctly different

material structure and dynamics. As the hydration decreases,

the protein’s backbone structure is no longer well defined

[16,17]. To study a uniform molecular sample with biologically

relevant dynamics, the protein sample must be hydrated to at

least 0.3 g water/g protein [16,17]. However, at this hydration

level, the absorption from the water alone is very large [18–21].

In addition, the amino acid side chain librational motions can

also contribute to a dielectric relaxation loss [22]. Thus, the

broad VDOS along with relaxational contributions from local

motions severely impede the isolating of specific intramolecu-

lar vibrations in the THz optical absorption of proteins.

Anisotropic absorption measurements of aligned samples

can overcome both of these challenges. The coupling of light

to a vibrational excitation is dependent on the relative direction

of the light polarization to the vibration’s transition dipole.

Figure 1(a) shows a displacement vector diagram of an intra-

molecular vibration calculated for cytochrome c using quasi-

harmonic mode analysis. The overall dipole transition direc-

tion is indicated on the figure by the red arrow. The solvent

molecules are randomly oriented over the surface of the

protein. The net absorption for any polarization direction is

given by

absθabs

isotropic



∕v

v − v



 γ



∂



∂q



· λθ



; (1)

Singh et al. Vol. 4, No. 3 / June 2016 / Photon. Res. A1

23 May 2016

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