Eisenberg Azaria Solomon, Juszczak Laura J
Department of Chemistry, Brooklyn College of the City University of New York, 2900 Bedford Avenue, Brooklyn, NY 11210, USA.
J Amino Acids. 2012;2012:735076. doi: 10.1155/2012/735076. Epub 2012 Jul 22.
Tryptophyl glycine (TrpGly) and glycyl tryptophan (GlyTrp) dipeptides at pH 5.5 and pH 9.3 show a pattern of fluorescence emission shifts with the TrpGly zwitterion emission solely blue shifted. This pattern is matched by shifts in the UV resonance Raman (UVRR) W10 band position and the W7 Fermi doublet band ratio. Ab initio calculations show that the 1340 cm(-1) band of the W7 doublet is composed of three modes, two of which determine the W7 band ratios for the dipeptides. Molecular dynamics simulations show that the dipeptides take on two conformations: one with the peptide backbone extended; one with the backbone curled over the indole. The dihedral angle critical to these conformations is χ(1) and takes on three discrete values. Only the TrpGly zwitterion spends an appreciable amount of time in the extended backbone conformation as this is stabilized by two hydrogen bonds with the terminal amine cation. According to a Stark effect model, a positive charge near the pyrrole keeps the (1)L(a) transition at high energy, limiting fluorescence emission red shift, as observed for the TrpGly zwitterion. The hydrogen bond stabilized backbone provides a rationale for the C(methylene)-C(α)-C(carbonyl) W10 symmetric stretch that is unique to the TrpGly zwitterion.
在pH 5.5和pH 9.3条件下，色氨酰甘氨酸（TrpGly）和甘氨酰色氨酸（GlyTrp）二肽呈现出荧光发射光谱的变化模式，其中TrpGly两性离子的发射仅发生蓝移。这种模式与紫外共振拉曼（UVRR）W10带位置的变化以及W7费米双峰带比率相匹配。从头算计算表明，W7双峰的1340 cm⁻¹带由三种模式组成，其中两种模式决定了二肽的W7带比率。分子动力学模拟表明，二肽呈现出两种构象：一种是肽主链伸展；另一种是主链卷曲在吲哚上。对这些构象至关重要的二面角是χ(1)，它具有三个离散值。只有TrpGly两性离子在伸展的主链构象中花费相当长的时间，因为这通过与末端胺阳离子的两个氢键而得以稳定。根据斯塔克效应模型，吡咯附近的正电荷使(1)L(a)跃迁处于高能状态，限制了荧光发射的红移，正如在TrpGly两性离子中所观察到的那样。氢键稳定的主链为TrpGly两性离子特有的C(亚甲基)-C(α)-C(羰基) W10对称伸缩振动提供了一种解释。
