Sul Soohwan, Karaiskaj Denis, Jiang Ying, Ge Nien-Hui
Department of Chemistry, University of California, Irvine, California 92697-2025, USA.
J Phys Chem B. 2006 Oct 12;110(40):19891-905. doi: 10.1021/jp062039h.
Femtosecond two-dimensional infrared (2D IR) spectroscopy has been applied to study the conformations of a model dipeptide, N-acetyl-L-prolinamide (AcProNH2) in deuterated chloroform (CDCl3). Spectral features in the amide-I and -II regions are obtained by rephasing (R), nonrephasing (NR), and reverse photon echo (RPE) pulse sequences with two polarization conditions. The 2D spectra obtained by the RPE and NR sequences with (0, 0, 0, 0) polarization reveal new spectral features associated with the multiple conformers of AcProNH2 that are difficult to discern using R sequence and linear-IR spectroscopy. The high resolving power of the RPE sequence comes from destructive interference between the positive and negative peaks of nearby vibrators, similar to the NR sequence. The RPE response functions that are useful for 2D spectral simulations are evaluated, including the effects of vibrational frequency correlations. The 2D spectra obtained with (45, -45, 90, 0) polarization exhibit clear cross-peak patterns in the off-diagonal region for the R and RPE sequences but in the diagonal region for the NR sequence. These patterns, free from strong diagonal contributions, are crucial for structure determination. DFT calculations, normal-mode analysis, Hessian matrix reconstruction, and vibrational exciton Hamiltonian diagonalization yield molecular parameters needed for quantitative simulations of 2D spectra: angles between transition dipoles, coupling constants, and off-diagonal anharmonicities of the amide-I and -II modes are obtained for solvated trans-C7 and cis structures and for gas-phase trans conformers in the region of phi = -120 degrees to 0 degrees and psi = -100 degrees to 180 degrees in the Ramachandran space. Systematic simulations based on a 4:1 population ratio of the solvated trans-C7 and cis structures reproduce well the 2D spectral features obtained at both polarization conditions. However, better agreement between the experimental and simulated cross-peak patterns can be reached if the dihedral angles of the major trans conformer are close to (phi, psi) = (-80 degrees , 100 degrees ). Our results suggest that the major conformer of AcProNH2 in CDCl3 deviates from the gas-phase global minimum, the trans-C7 form, to an extended intermediate between the C7 and polyproline-II structure. These results are discussed in relationship with earlier findings obtained by NMR, transient IR studies, and MD simulations.
飞秒二维红外（2D IR）光谱已被用于研究模型二肽N - 乙酰 - L - 脯氨酰胺（AcProNH2）在氘代氯仿（CDCl3）中的构象。通过具有两种偏振条件的重相位（R）、非重相位（NR）和反向光子回波（RPE）脉冲序列，获得了酰胺 - I和 - II区域的光谱特征。通过具有（0, 0, 0, 0）偏振的RPE和NR序列获得的二维光谱揭示了与AcProNH2的多个构象相关的新光谱特征，这些特征使用R序列和线性红外光谱难以辨别。RPE序列的高分辨能力来自附近振动器的正负峰之间的相消干涉，类似于NR序列。评估了对二维光谱模拟有用的RPE响应函数，包括振动频率相关性的影响。用（45, - 45, 90, 0）偏振获得的二维光谱在R和RPE序列的非对角区域显示出清晰的交叉峰模式，但在NR序列的对角区域显示出交叉峰模式。这些模式没有强烈的对角贡献，对于结构确定至关重要。密度泛函理论（DFT）计算、简正模式分析、海森矩阵重建和振动激子哈密顿量对角化产生了二维光谱定量模拟所需的分子参数：获得了溶剂化反式 - C7和顺式结构以及气相反式构象在拉马钱德兰空间中φ = - 120°至0°和ψ = - 100°至180°区域内的跃迁偶极矩之间的角度、耦合常数以及酰胺 - I和 - II模式的非对角非谐性。基于溶剂化反式 - C7和顺式结构4:1的布居比进行的系统模拟很好地再现了在两种偏振条件下获得的二维光谱特征。然而，如果主要反式构象的二面角接近（φ, ψ）=（- 80°, 100°），则实验和模拟的交叉峰模式之间可以达成更好的一致性。我们的结果表明，AcProNH2在CDCl3中的主要构象偏离气相全局最小值反式 - C7形式，向C7和多聚脯氨酸 - II结构之间的扩展中间体转变。结合早期通过核磁共振、瞬态红外研究和分子动力学模拟获得的结果对这些结果进行了讨论。
