Paschek Dietmar, Pühse Matthias, Perez-Goicochea Arnold, Gnanakaran S, García Angel E, Winter Roland, Geiger Alfons
Physikalische Chemie, Fakultät Chemie, TU Dortmund, Otto-Hahn-Str. 6, 44227 Dortmund, Germany.
Chemphyschem. 2008 Dec 22;9(18):2742-50. doi: 10.1002/cphc.200800540.
We determine the shift and line shape of the amide I band of a model AK peptide from molecular dynamics (MD) simulations of the peptide dissolved in methanol/water mixtures with varying composition. The IR spectra are determined from a transition dipole coupling exciton model. A simplified empirical model Hamiltonian is employed, which takes into account both the effect of hydrogen bonding and the intramolecular vibrational coupling. We consider a single isolated AK peptide in a mostly helical conformation, while the solvent is represented by 2600 methanol or water molecules, simulated for a pressure of 1 bar and a temperature of 300 K. Over the course of the simulations, minor reversible conformational changes at the termini are observed, which are found to only slightly affect the calculated spectral properties. Over the entire composition range, which varies from pure water to the pure methanol solvent, a monotonous shift towards higher frequency of the IR amide I band of about 8 wavenumbers is observed. This shift towards higher frequency is comparable to the shift found in preliminary experimental data also presented here on the amide I' band. The shift is found to be caused by two counter-compensating effects. An intramolecular red shift of about 1.2 wavenumbers occurs, due to stronger intramolecular hydrogen bonding in a methanol-rich environment. Dominating, however, is the intermolecular solvent-dependent shift towards higher frequency of about 10 wavenumbers, which is attributed to the less effective hydrogen-bond-donor capabilities of methanol compared to water. The importance of the solvent contribution to the IR shift, as well as the significantly different hydrogen formation capabilities of water and methanol, makes the amide I band sensitive to composition changes in the local environment close to the peptide/solvent interface. This allows, in principle, an experimental determination of the composition of the solvent in close proximity to the peptide surface. For the AK peptide case, we observe at low methanol concentrations a significantly enhanced methanol concentration at the peptide/solvent interface, supposedly promoted by the partially hydrophobic character of the AK peptide's solvent-accessible surface.
我们通过对溶解在不同组成的甲醇/水混合物中的模型AK肽进行分子动力学（MD）模拟，确定了该肽酰胺I带的位移和线形。红外光谱由跃迁偶极耦合激子模型确定。采用了一个简化的经验模型哈密顿量，该模型同时考虑了氢键效应和分子内振动耦合。我们考虑了一个处于主要螺旋构象的单个孤立AK肽，而溶剂由2600个甲醇或水分子表示，在1巴压力和300 K温度下进行模拟。在模拟过程中，观察到肽末端有轻微的可逆构象变化，发现这些变化仅对计算出的光谱特性有轻微影响。在从纯水到纯甲醇溶剂的整个组成范围内，观察到红外酰胺I带向高频方向单调位移约8波数。这种向高频的位移与这里也给出的关于酰胺I'带的初步实验数据中发现的位移相当。发现这种位移是由两种相互抵消的效应引起的。由于在富含甲醇的环境中分子内氢键更强，发生了约1.2波数的分子内红移。然而，占主导地位的是分子间溶剂依赖的向高频的位移约10波数，这归因于甲醇与水相比氢键供体能力较弱。溶剂对红外位移的贡献的重要性，以及水和甲醇形成氢键能力的显著差异，使得酰胺I带对肽/溶剂界面附近局部环境的组成变化敏感。原则上，这允许通过实验确定肽表面附近溶剂的组成。对于AK肽的情况，我们观察到在低甲醇浓度下，肽/溶剂界面处的甲醇浓度显著增加，推测这是由AK肽可溶剂接触表面的部分疏水特性促进的。
