DeRider Michele L, Wilkens Steven J, Waddell Michael J, Bretscher Lynn E, Weinhold Frank, Raines Ronald T, Markley John L
Department of Chemistry, Graduate Program in Biophysics, University of Wisconsin-Madison, Madison, WI 53706, USA.
J Am Chem Soc. 2002 Mar 20;124(11):2497-505. doi: 10.1021/ja0166904.
Collagen-like peptides of the type (Pro-Pro-Gly)(10) fold into stable triple helices. An electron-withdrawing substituent at the H(gamma)(3) ring position of the second proline residue stabilizes these triple helices. The aim of this study was to reveal the structural and energetic origins of this effect. The approach was to obtain experimental NMR data on model systems and to use these results to validate computational chemical analyses of these systems. The most striking effects of an electron-withdrawing substituent are on the ring pucker of the substituted proline (Pro(i)) and on the trans/cis ratio of the Xaa(i-1)-Pro(i) peptide bond. NMR experiments demonstrated that N-acetylproline methyl ester (AcProOMe) exists in both the C(gamma)-endo and C(gamma)-exo conformations (with the endo conformation slightly preferred), N-acetyl-4(R)-fluoroproline methyl ester (Ac-4R-FlpOMe) exists almost exclusively in the C(gamma)-exo conformation, and N-acetyl-4(S)-fluoroproline methyl ester (Ac-4S-FlpOMe) exists almost exclusively in the C(gamma)-endo conformation. In dioxane, the K(trans/cis) values for AcProOMe, Ac-4R-FlpOMe, and Ac-4S-FlpOMe are 3.0, 4.0, and 1.2, respectively. Density functional theory (DFT) calculations with the (hybrid) B3LYP method were in good agreement with the experimental data. Computational analysis with the natural bond orbital (NBO) paradigm shows that the pucker preference of the substituted prolyl ring is due to the gauche effect. The backbone torsional angles, phi and psi, were shown to correlate with ring pucker, which in turn correlates with the known phi and psi angles in collagen-like peptides. The difference in K(trans/cis) between AcProOMe and Ac-4R-FlpOMe is due to an n-->pi interaction associated with the Bürg-Dunitz trajectory. The decrease in K(trans/cis) for Ac-4S-FlpOMe can be explained by destabilization of the trans isomer because of unfavorable electronic and steric interactions. Analysis of the results herein along with the structures of collagen-like peptides has led to a theory that links collagen stability to the interplay between the pyrrolidine ring pucker, phi and psi torsional angles, and peptide bond trans/cis ratio of substituted proline residues.
（脯氨酸-脯氨酸-甘氨酸）(10) 型类胶原蛋白肽折叠成稳定的三螺旋结构。第二个脯氨酸残基的H(γ)(3)环位置上的吸电子取代基可稳定这些三螺旋结构。本研究的目的是揭示这种效应的结构和能量起源。研究方法是获取模型系统的实验核磁共振（NMR）数据，并利用这些结果验证对这些系统的计算化学分析。吸电子取代基最显著的影响在于取代脯氨酸（Pro(i)）的环皱以及Xaa(i - 1)-Pro(i)肽键的反式/顺式比例。NMR实验表明，N - 乙酰脯氨酸甲酯（AcProOMe）以C(γ)-内型和C(γ)-外型两种构象存在（内型构象略占优势），N - 乙酰 - 4(R)-氟脯氨酸甲酯（Ac - 4R - FlpOMe）几乎完全以C(γ)-外型构象存在，而N - 乙酰 - 4(S)-氟脯氨酸甲酯（Ac - 4S - FlpOMe）几乎完全以C(γ)-内型构象存在。在二氧六环中，AcProOMe、Ac - 4R - FlpOMe和Ac - 4S - FlpOMe的K(反式/顺式)值分别为3.0、4.0和1.2。采用（杂化）B3LYP方法的密度泛函理论（DFT）计算结果与实验数据吻合良好。基于自然键轨道（NBO）范式的计算分析表明取代脯氨酰环的皱曲偏好是由于邻位交叉效应。主链扭转角φ和ψ与环皱相关，而环皱又与类胶原蛋白肽中已知的φ和ψ角相关。AcProOMe和Ac - 4R - FlpOMe之间K(反式/顺式)的差异是由于与 Bürg - Dunitz轨迹相关的n→π相互作用。Ac - 4S - FlpOMe的K(反式/顺式)降低可通过反式异构体因不利的电子和空间相互作用而失稳来解释。结合本文结果以及类胶原蛋白肽的结构分析，得出了一种理论，该理论将胶原蛋白的稳定性与吡咯烷环皱曲、φ和ψ扭转角以及取代脯氨酸残基的肽键反式/顺式比例之间的相互作用联系起来。
