Papadopoulos P, Floudas G, Klok H-A, Schnell I, Pakula T
University of Ioannina, Department of Physics, 451 10 Ioannina, Greece.
Biomacromolecules. 2004 Jan-Feb;5(1):81-91. doi: 10.1021/bm034291q.
The structure and the associated dynamics have been investigated in a series of oligopeptides of gamma-benzyl-l-glutamate using DSC, WAXS, FTIR, NMR and dielectric spectroscopy, and rheology, respectively. The peptides with degrees of polymerization below 18 are mixtures of a lamellar assembly of beta sheets and of columnar hexagonal arrangement of alpha helices, whereas for longer chains, the intramolecular hydrogen bonds stabilize only the alpha-helical conformations. Multiple dielectrically active processes were found. Starting from low temperatures, the two Arrhenius processes (gamma and beta), with apparent activation energies of 20.6 and 50.2 kJ/mol, respectively, associate with the local relaxation of the side-chain methylene units (gamma process) and with more cooperative motions of the side chain dipoles sensitive to the 7/2 helical packing (beta process). The glass transition is manifested in the thermal properties with a step in the heat capacity and with an intense dielectric process bearing characteristics (molecular weight dependence, temperature dependence of relaxation times) known from amorphous polymers. Based on these findings, the alpha process is attributed to the relaxation of amorphous segments located between and at the end of helically ordered segments. Two slower processes were identified with opposite molecular weight dependence. The weak intermediate mode with an M2 molecular weight dependence of the characteristic relaxation times suggests amorphous-like chains, whereas the strong slower process originates from the loss of dipole orientational capacity caused by structural defects and reflects the migration of helical sequences along the chains. This identifies the helices as structures extending over rather short fragments of chains (i.e., of low persistence length). The viscoelastic response indicated that the structural defects arise from locally aggregated chains that inhibit the flow of oligopeptides.
分别使用差示扫描量热法（DSC）、广角X射线散射法（WAXS）、傅里叶变换红外光谱法（FTIR）、核磁共振法（NMR）和介电谱法以及流变学，对一系列γ-苄基-L-谷氨酸寡肽的结构及其相关动力学进行了研究。聚合度低于18的肽是β片层的层状组装和α螺旋的柱状六方排列的混合物，而对于较长的链，分子内氢键仅稳定α螺旋构象。发现了多个介电活性过程。从低温开始，两个阿累尼乌斯过程（γ和β），其表观活化能分别为20.6和50.2 kJ/mol，分别与侧链亚甲基单元的局部弛豫（γ过程）以及对7/2螺旋堆积敏感的侧链偶极子的更协同运动（β过程）相关。玻璃化转变在热性质中表现为热容的阶跃以及具有非晶态聚合物已知特征（分子量依赖性、弛豫时间的温度依赖性）的强烈介电过程。基于这些发现，α过程归因于位于螺旋有序段之间和末端的无定形链段的弛豫。确定了两个具有相反分子量依赖性的较慢过程。特征弛豫时间具有M2分子量依赖性的弱中间模式表明存在类似无定形的链，而较强的较慢过程源于结构缺陷导致的偶极取向能力丧失，并反映了螺旋序列沿链的迁移。这表明螺旋是在相当短的链片段上延伸的结构（即，持久长度低）。粘弹性响应表明结构缺陷源于局部聚集的链，这些链抑制了寡肽的流动。
