太赫兹（THz）光谱在生物大分子研究中的应用

    不同状态的水，其吸收THz 射线的方式也不同，因此研究人员可以直接观测到蛋白质对其周围水分子的影响作用. Havenith等^[29]在蛋白质对溶剂的影响距离及影响方式的问题上进行了探讨，通过THz 光谱技术对溶剂化动力学、动态水化层厚度进行研究，发现蛋白质周围溶剂化层的相互重叠会导致λ蛋白的THz 吸收产生非单调趋势. 分子动力学模拟显示，蛋白质溶剂化层中的水与其距离10埃的自由水有着不同特征. 在水化层相互重叠等高蛋白浓度之处的计算所得数据和实验结果一致，都显示其吸收光谱有非单调性变化. 蛋白质对水分子网络运动的影响距离在20埃以上，这远大于理论长度. THz实验表明一个蛋白质可以影响1000个水分子. 该研究小组^[30]还利用THz 技术测量溶剂化蛋白质，以便研究生物大分子周围的动态水合层. 发现与浓度相关的THz 吸收系数会随着溶质诱导的溶剂化动力学改变，并受蛋白质浓度和动态水合层厚度的影响.

    THz 吸收可作为溶液中蛋白质浓度的函数，用以研究蛋白质和水之间动态耦合. 在高浓度的蛋白质-水溶液中，随着蛋白质浓度的增高，THz 吸收光谱以近似线性趋势降低^[26]. 在低浓度的蛋白质-水溶液中，蛋白质浓度与吸光率呈现非线性变化，蛋白质溶剂化层重叠和自由水消失表明了蛋白质稀溶液的转变^[29]. 计算机模拟也可以用于研究邻近生物分子表面的水分子动力学和结构. Whitmire等^[31]使用标准模式分析生物大分子力场模型，并计算谐函数近似值下的THz吸收光谱. Ebbinghaus等^[29]使用分子动力学模拟计算偶极波动.水分子的平移和旋转扩散可作为以蛋白质和水分子间距、氢键动力学的函数. Havenith研究小组通过模拟水分子的这一特性，得以深入研究生物分子和邻近水的动态耦合^[32].

5 结语

    THz 以其对生物分子的特征吸收、高灵敏度、宽带性等优点，为人们提供了一种研究生物分子结构及相关动力学、分子与环境相互作用的新方法. 目前关于THz 波段在生物学中的光谱和成像研究正处于一个飞速发展的时期，研究者们已在生物分子指纹图谱的获得、无标记生物探测以及水环境与分子的相互作用等方面做出了一些初步的研究成果. 但THz 作为一种新兴的光谱分析检测手段，与已经成熟的其他光谱技术相比仍难免存在着一系列问题，它在实验技术和理论分析技术方面仍有待完善. 在实验技术方面，由于THz 波的波长限制了THz 成像系统的空间分辨率，要在生物样品 (如生物细胞或生物组织) 上加一层控制材料是很困难的；目前大多数脉冲实验获取数据时间较长，对于生物样品可能会有样品的变性问题；一般光导天线辐射的THz 源有效频率较低,使得一些物质结构信息不能在谱图中得到充分的反映；另外,现有的THz 时域光谱系统及成像系统的设备还比较昂贵,信息处理过程也很复杂,有待进一步微型化和实用化. 在理论分析方面，有效的数据处理和图谱分析方法仍需要进一步探索，光谱数据也需要不断积累以利于研究THz光谱与分子结构之间的关系. THz 涉及到物理学、化学以及生物学等多学科的交叉与融合，随着研究的不断深入，该技术与多种学科之间的交叉势必会更加深入. 相信随着THz 技术的发展，它将在许多领域都展现出巨大的应用价值.

参 考 文 献
[1] Williams B S, Callebaut H, Kumar S, et al. 3.4-THz quantum cascade laser based on longitudinal-optical-phonon scattering for depopulation. Applied Physics Letters, 2003, 82(7): 1015~1017.
[2] 王晓红, 张亮亮, 胡颖, 等. THz辐射在DNA光谱研究中的应用. 光谱学与光谱分析, 2006, 26(3): 385~391.
Wang X H, Zhang L L, Hu Y, et al. Applications of terahertz pulse in the DNA molecule. Spectroscopy and Spectral Analysis, 2006, 26(3): 385.
[3] Kikuchi N, Tanno T, Watanabe M, et al. A membrane method for terahertz spectroscopy of amino acids. Aanalytical Sciences, 2009, 25(3): 457~459.
[4] Wang X M, Wang W N. Terahertz time-domain spectroscopy of sulfur-containing amino acids. Actachimica Sinica, 2008, 66(20): 2248~2252.
[5] Plusquellic D F, Kleiner I, Demaison J, et al. The microwave spectrum of a two-top peptide mimetic: The N-acetyl alanine methyl ester molecule. Journal of Chemical Physics, 2006, 125(10). 104312.
[6] Markelz A G, Roitberg A, Heilweil E J. Pulsed terahertz spectroscopy of DNA, bovine serum albumin and collagen between 0.1 and 2.0 THz. Chemical Physics Letters, 2000, 320 (1~2): 42~48.
[7] Markelz A G. Terahertz dielectric sensitivity to biomolecular structure and function. IEEE Journal of Selected Topics in Quantum Electronics, 2008, 14(1): 180~190.
[8] Balu R, Zhang H, Zukowski E, et al. Terahertz spectroscopy of bacteriorhodopsin and rhodopsin: Similarities and differences. Biophysical Journal, 2008, 94(8):3217~3226.
[9] Markelz A G, Knab J R, Chen J Y, et al.. Protein dynamical transition in terahertz dielectric response. Chemical Physics Letters, 2007, 442(4~6): 413~417.
[10] He Y F, Markelz A G. The Role of Structure in the Protein Dynamical Transition. 2008 33rd International Conference on Infrared, Millimeter and Terahertz Waves, 2008, 1~2: 746~748.
[11] He Y F, Ku P I, Knab J R, et al. Protein Dynamical Transition Does Not Require Protein Structure. Physical Review Letters, 2008, 101(17): 178103.
[12] Chen H, Chen G Y, Li S Q, et al. Reversible conformational changes of PsbO protein detected by terahertz Time-domain spectroscopy. Chinese Physics Letters, 2009, 26(8): 084204.
[13] Leitner D M, Gruebele M, Havenith M. Solvation dynamics of biomolecules: modeling and terahertz experiments. HFSP Journal, 2008, 2(6): 314~323.
[14] Kim S J, Born B, Havenith M, et al. Real-Time Detection of Protein–Water Dynamics upon Protein Folding by Terahertz Absorption Spectroscopy. Angewandte Chemie-international Edition, 2008, 47(34): 6486-6489.
[15] Shin H J, Oh S J, Kim S I, et al. Conformational characteristics of beta-glucan in laminarin probed by terahertz spectroscopy. Applied Physics Letters, 2009, 94(11): 111911.
[16] 杨丽敏, 赵夔, 孙红起, 等. 几种糖衍生物分子的THz光谱研究. 光谱学与光谱分析, 2008, 28 (5): 961~965.
Yang L M, Zhao K, Sun H Q, et al. THz absorption spectra of several carbohydrate derivatives. Spectroscopy and Spectral Analysis, 2008, 28 (5): 961~965.
[17] Fischer B, Hoffmann M, Helm H, et al. Chemical recognition in terahertz time-domain spectroscopy and imaging. Semicond. Sci. Technol. 2005, 20(7), S 246~S253.
[18] Wittlin A, Genzel L, Kremer F, et al. Far-infrared spectroscopy on oriented films of dry and hydrated DNA. Phys. Rev. A, 1986, 34: 493.
[19] Globus T, Khromova T, Gelmont B, et al. Terahertz characterization of dilute solutions of DNA - art. no. 609308. Biomedical Vibrational Spectroscopy III: Advances in Research and Industry, 2006, 6093: 9308.
[20] Fischer B M, Walther M, Jepsen P U. Far-infrared vibrational modes of DNA components studied by terahertz time-domain spectroscopy. Phys. Med. Biol., 2002, 47 (21): 3807.
[21] Li X W, Globus T, Gelmont B, et al. Terahertz Absorption of DNA Decamer Duplex. Journal of Physical Chemistry A, 2008, 112(47): 12090~12096.
[22] Weng B W, Xuan G C, Kolodzey J. Empirical mode decomposition as a tool for DNA sequence analysis from Terahertz spectroscopy measurements. 2006 IEEE International Workshop on Genomic Signal Processing and Statistics, 2006: 63~64.
[23] Brucherseifer M, Nagel M, Haring Bolivar P, et al. Label-free probing of the binding state of DNA by time-domain terahertz sensing. Applied Physics Letters, 2000, 77(24): 4 049~4 051
[24] Nagel M, Richter F, Bolivar P H, et al. A functionalized THz sensor for marker-free DNA analysis. Physics in Medicine and Biology, 2003, 48 (22): 3625~3636.
[25] Bründermann E, Born B, Funkner S, et al. Terahertz spectroscopic techniques for the study of proteins in aqueous solutions. Proceedings of SPIE, 2009, 7215: 72150.
[26] Xu J, Plaxco K W, Allen S J. Probing the collective vibrational dynamics of a protein in liquid water by terahertz absorption spectroscopy. Protein Science, 2006, 15: 1175~1181.
[27] Born B, Kim S J, Ebbinghaus S, et al. The terahertz dance of water with the proteins: the effect of protein flexibility on the dynamical hydration shell of ubiquitin. Faraday Discuss,2009, 141:161~173.
[28] Born B, Weingartner H, Brundermann E, et al. Solvation Dynamics of Model Peptides Probed by Terahertz Spectroscopy. Observation of the Onset of Collective Network Motions. Journal of the American Chemical Society, 2009, 131(10): 3752~3755.
[29] Ebbinghaus S, Kim S J, Heyden M, et al. An extended dynamical hydration shell around proteins. Proceedings of the National Academy of Sciences of the United.
[30] Havenith M, Leitner D, Gruebele M. The THz dance of the protein with the water.2008 33rd International Conference on Infrared, Millimeter and Terahertz Waves, 2008, 1~2: 238.
[31] Whitmire S E, Wolpert D, Markelz A G, et al. Protein flexibility and conformational state: A comparison of collective vibrational modes of wild-type and D96N bacteriorhodopsin. Biophysical Journal, 2003, 85(2): 1269~1277.
[32] Heyden M, Brundermann E, Heugen U, et al. Long-range influence of carbohydrates on the solvation dynamics of water-answers from terahertz absorption measurements and molecular modeling simulations. Journal of the American Chemical Society, 2008, 130(17): 5773~5779.