Conformational energy and configurational statistics of poly-L-proline.
Conformational energy and configurational statistics of poly-L-proline.
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DOI：
10.1073/pnas.58.1.52
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发表时间：
1967-07
影响因子：
11.1
通讯作者：
P. Schimmel;P. Flory
中科院分区：
文献类型：
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作者：
P. Schimmel;P. Flory
在固态-I和溶液中分别研究了两种不同形式的聚-L-脯氨酸，分别命名为I和II。在I型中，酰亚胺基团为顺式构型;在II型中，酰亚胺基团为反式构型。“2通常存在于多肽中的氢键由于缺少酰胺基氢而被排除。因此，在固态中出现的螺旋结构必须由其他因素决定，主要是空间起源。内部丙氨酰和甘氨酰残基的构象能先前通过使用半经验势函数对内在键扭转势、非键合原子之间的相互作用和偶极-偶极相互作用进行了估计。6 ' 7分别计算了关于N-Ca和Ca C键的旋转角θ 0和θ 1的各种值的能量。配分函数、平均链尺寸和其他平均量通过为每个残基i的适当选择的（pi和i）值指定玻尔兹曼因子e-Ei/RT并对如此定义的所有“状态”w 0，4“求和来获得。6 -8在均方未扰动端到端距离的理论值和实验值之间建立了良好的一致性6 -9，不同氨基酸组成和序列的多肽链的平方偶极矩10。6-'0具有所有反式单元的聚-L-脯氨酸链的构型是本文的主要关注点。旋转角α p被认为是由吡咯烷环的刚性几何形状固定的;因此，残基的构象能仅取决于α。本文简要地报道了有关聚L-脯氨酸Ⅰ和Ⅰ; Ⅱ协同跃迁的进一步研究。构象能和链尺寸的计算。聚-L-脯氨酸II链的两个单元如图1所示。所有的键长和键角取自Sasisekharan的X-射线和光学衍射研究的聚-L-脯氨酸II.2的酰亚胺基团被分配到平面反式构型。连续的α-碳原子之间的距离则固定在lu = 3.80 A。因此，由x + 1个残基组成的聚-L-脯氨酸链可以被处理为连接x + 1个残基的连续α-碳原子的长度为llu的x个虚拟键的序列。聚-L脯氨酸II链中反式L-脯氨酰残基的构象能通过将由一个旋转角VI分开的原子之间的所有货车德瓦尔斯排斥（VR，k）和伦敦吸引（VLJ k）相互作用求和来计算。采用Brant等人7所获得的半经验势函数和参数。相邻酰亚胺基团之间的偶极相互作用和Cot C键的固有扭转势被合理地忽略了;这些对能量的贡献在这个受到严格空间障碍的链中并不重要（参见。讨论）。相应地，与绕Ca-C键的旋转相关的势被认为是：
Two distinctly different forms of poly-L-proline designated I and II, respectively, have been investigated in the solid state'-I and in solution.4' I In form I the imide group is in the cis configuration;3 in form II it is trans." 2 Hydrogen bonds of the kind usually occurring in polypeptides are precluded by the absence of an amido hydrogen. Hence, the helical structures occurring in the solid state' must be dictated by other factors, predominantly steric in origin. The conformational energies of internal alanyl and glycyl residues were estimated previously by use of semiempirical potential functions for intrinsic bond torsional potentials, for interactions between nonbonded atoms, and for dipole-dipole interactions.6' 7 Energies were calculated for various values of the angles of rotation so and 41 about the N-Ca and Ca C bonds, respectively. The partition function, average chain dimensions, and other average quantities were obtained by assigning a Boltzmann factor, e-Ei/RT, for suitably chosen values of (pi and /'i for each residue i and summing over all "states" wO,4' so defined, and over all residues.6-8 Good agreement was established between theoretical and experimental values of the mean-square unperturbed end-to-end distance6-9 and the mean-square dipole moment10 for polypeptide chains of varying amino acid composition and sequence.6-'0 The configuration of the poly-L-proline chain with all units trans is the main concern of the present paper. The rotation angle (p is taken to be fixed by the rigid geometry of the pyrrolidine ring; hence, the conformational energy of a residue depends on t,' alone. Further studies" dealing with poly-L-proline I and the cooperative I ;# II transition are reported in brief. Calculation of the Conformational Energy and Chain Dimensions.-Two units of a poly-L-proline II chain are displayed in Figure 1. All bond lengths and bond angles are taken from Sasisekharan's X-ray and optical diffraction investigation of poly-L-proline II.2 The imide group is assigned to the planar trans configuration. The distance between consecutive a-carbon atoms is then fixed at lu = 3.80 A. A poly-L-proline chain consisting of x + 1 residues may be treated, therefore, as a sequence of x virtual bonds of length llu joining the consecutive a-carbon atoms of x + 1 residues. Conformational energies of a trans L-prolyl residue in a poly-Lproline II chain were calculated by summing over all van der Waals repulsive (VR,,k) and London attractive (VLJk) interactions between atoms separated by one rotation angle VI. The semiempirical potential functions and parameters obtained by Brant et al.7 were used. Dipolar interactions between adjacent imide groups and the intrinsic torsional potential about the Cot C bond were legitimately neglected; these contributions to the energy are of minor importance in this chain which is subject to stringent steric hindrances (cf. Discussion). The potential associated with rotations about the Ca-C bond, accordingly, is taken to be