Two-Photon Circular Dichroism Spectrometer

双光子圆二向色性(TPCD)，即两个左圆偏振光子和两个右圆偏振光子的微分吸收，最早由Tinoo(1975)和Power(1975)从理论上讨论。这种技术在理论上类似于拉曼光学活性，但尚未在实验上得到证明，尽管相关的TP-Ord效应已经被证明(Gedanken&Amp；Tamir，1987)。我们建议在最近发展的商用钛宝石飞秒激光技术的基础上，利用非常灵敏的瞬时热光栅探测方法来构建TPCD光谱仪。我们主要感兴趣的领域是蛋白质的二级结构。鉴于这类蛋白质缺乏荧光，这种应用决定了一种直接吸收或基于热的方法来检测TPCD信号。作为一种补充传统圆二色谱(CD)的方法，TPCD在确定蛋白质二级结构和检测微妙构象变化方面具有几个潜在的优点。这些是：(1)提供一个新的线性独立的数据集，从而有可能唯一地确定二级结构；(2)从300到600 nm的TPCD谱覆盖了单光子能量到150到300 nm的电子激发态，从而将激发范围扩大到更高的能量，并涵盖了预期的更高能量的a-螺旋的激子成分；(3)有三个独立的TPCD谱(对于相同频率的双光子和随机取向的样品)证明了独立的信息。将要建造的光谱仪是基于高重复频率(1 KHz)的飞秒激光技术。双光子CD信号的检测将基于差分瞬变热栅法。这种方法最近已经被证明是一种非常灵敏的测量镉的方法，以前已经被证明是一种非常灵敏的方法来进行两个p霍顿吸收的测量。我们的建议是将这两种能力结合起来，提供新的双光子CD信息。基于使用该光栅法的先前双光子测量的详细信噪比计算和对预期的双光子微分截面的可靠估计表明，将获得良好的信噪比。

Two-photon circular dichroism (TPCD), the differential absorption of two left circularly polarized photons vs. two right circularly polarized photons, was first discussed theoretically by Tinoco (1975) and by Power (1975). This technique, which is theoretically similar to Raman optical activity, has not yet been demonstrated experimentally although the related TP-ORD effect has been demonstrated (Gedanken & Tamir, 1987). We propose to construct a TPCD spectrometer based on recently developed commercially available Ti:sapphire femtosecond laser technology using the very sensitive transient thermal grating detection method. Our primary area of interest is the secondary structure of proteins. Given the lack of fluorescence of such proteins, this application dictates a direct absorption or thermally based methodology for detection of the TPCD signal. TPCD offers several potential advantages as a method complimentary to conventional circular dichroism (CD) for the determination of protein secondary structure and detection of subtle conformational changes. These are (1 ) a new, linearly independent data set is provided potentially permitting a unique determination of secondary structure, (2) the TPCD spectrum extending from 300 to 600 nm covers electronic excited states corresponding in one-photon energy to 150 to 300 nm thus extending the range of the excitations to higher energy and covering the expected higher exciton components of the a-helix, (3) there are three independent TPCD spectra (for two-photons of the same frequency and randomly oriented samples) proving independent information. The spectrometer to be constructed is based on high-repetition rate (1 kHz) femtosecond laser technology. The detection of the two-photon CD signal will be based on the differential transient thermal grating method. This method has been recently shown to provide a very sensitive method for CD measurements and has previously been shown to be a very sensitive method for performing two-p hoton absorption measurements. Our proposal is to combine these two capabilities to provide new two-photon CD information. A detailed signal to noise calculation based on prior two-photon measurements using this grating method and a reliable estimate of the expected differential two-photon cross section shows that excellent signal to noise will be achieved.