蛋白质不仅是构成生物体的物质基础，也是进行生命活动的功能基础。β-转角及β-折叠作为蛋白中重要的二级结构，在信息传递、免疫、代谢等过程发挥着重要的作用。由于β-转角及β-折叠在生物体中与蛋白作用复杂、稳定性差、易聚集等缺点，限制了其深入研究。因此，构建人工的β-转角及β-发夹体系用于阐述生理过程及疾病的发病机理引起了人们的广泛关注。本博士学位论文利用α, γ-氨基酸构建了一系列具有普适性强、易调控，甚至可在水溶液中稳定存在的新型人工β-转角及β-发夹结构，并对其折叠机理及影响其稳定的因素进行了深入研究。本论文取得的主要创新点如下：
1. 首先设计合成了一系列环区（loop）片段由两个α-氨基酸，两侧接有γ-氨基酸的四肽化合物。通过一系列的核磁实验及X-单晶衍射对其在有机溶剂中的构象进行了确定，实验证明上述四肽化合物在分子内氢键的作用下形成了稳定的β-转角结构，且此β-转角模型的环区片段可容纳不同序列的α-氨基酸形成“回折”结构，表明了其优异的普适性；通过与天然四肽分子对比发现，位于i和i + 3位置处的γ-氨基酸是其形成β-转角的必要条件；进一步研究表明上述β-转角模型可高效的诱导两条肽链序列形成反平行的β-折叠结构；实验证实中心环区二肽片段为两个丙氨酸时，形成的β-转角和β-发夹结构具有更高的稳定性。
   2. 设计合成了一系列环区（loop）片段由两个α-氨基酸，两侧接有γ-氨基酸和二硫键交联且溶于水的环肽化合物，通过一系列核磁方法分别对其在水溶液中的状态进行了确定，并对其稳定性进行了比较及研究。实验结果发现该系列化合物在水溶液中都是以β-转角的形式存在，具有较高的普适性；通过稳定性比较我们发现当i + 1号位置为D-构型氨基酸时，其稳定性会显著增强；通过对比发现位于i和i + 3位置的芳香γ-氨基酸是在水溶液中形成β-转角不可或缺的单元。
   3. 在第二部分β-转角研究的基础上，设计合成了一系列含有亲水肽链序列的β-发夹结构，通过一维及二维核磁对其在水溶液中的空间构象进行了确定，系统比较了各β-发夹结构在水溶液中的折叠能力与稳定性。实验发现本论文中所构建的人工β-转角可高效诱导肽链序列在水溶液中形成稳定的β-折叠结构；通过稳定性比较实验发现该β-发夹的折叠强度远高于含有D-Pro-Gly序列的β-发夹序列；通过对比发现在i与i + 3位置处γ-氨基酸的引入会进一步提高β-发夹的折叠强度。
综上，我们首次构建了具有普适性强、易调控，可在水溶液中稳定存在的人工β-转角和β-发夹结构，通过一维、二维核磁，理论计算，单晶结构等手段研究了转角和发夹结构的稳定性及其影响因素。以上研究工作对蛋白质二级结构在催化、抑菌、拮抗剂、抗体等领域的研究奠定了良好基础。

Proteins play an essential role in the basic of living organisms and various biological processes. As a major class of protein secondary structures, β-turns and β-sheets exhibit important biological functions in information transmission, immunity, metabolism, etc. To address the limitations of natural proteins, such as poor stability, ease of aggregation and complicated protein-protein interactions, research interests have led to the construction of many folding oligomers containing unnatural building blocks that adopt defined conformations including β-turns and β-hairpins. The analysis of the folding mechanism of β-turns and β-hairpins are of great importance for understanding many physiological processes and pathogenesis of diseases, as well as developing new diagnosis treatments. So far, artificial β-hairpin with good conformational stability in aqueous solution have been rarely reported.Hence, we developed a series of artificial β-turn and β-hairpin mimics containing α, γ-amino acid residues with wide universality, ease of manipulation, and high structural stability even in aqueous solution. Furthermore, the relative stability of corresponding β-turns and β-hairpins that differ in α-amino acid residues at the loop were studied. The main contents and results are shown as follows:
1. A series of hybrid tetrapeptides is reported. Each molecule consists a central dipeptide segment of α-amino acid residues flanked by two aromatic γ-amino acid residues on both termini. These tetrapeptides are found to fold into well-defined β-turn-like conformations by NMR and X-ray measurements. The loop of these β-turn motif accommodates different two-residue α-amino acid sequences, which demonstrates the universality of our β-turn mimics. Replacement of γ-amino acid residues with natural α-amino acid leads to ambigulous conformations of resulting molecules, demonstrates the critical importance of aromatic γ-amino acid residues in ensuring β-turn conformations. Furthermore, β-turn with different dipeptide loops could efficiently induce β-strands to form antiparallel β-sheet. The stabilities of the β-turn and β-hairpin conformations were found to vary with the α-amino acid sequences of the dipeptide loops. The loop with Ala-Ala sequence displays the highest stability in our study.
2. A series of water-soluble hybrid cyclopeptides which contain both α-amino acid and γ-amino acid residues are designed and synthesized. All the hybrid cyclopeptides were found to adopt well-defined conformations in aqueous solution revealed by NMR. The stabilities of the β-turn conformations were found to vary with the α-amino acid sequences of the dipeptide loops. Replacement of glycine or L-α-amino acid residue with a D-α-amino acid at i + 1 position resulted in the enhanced stability of the resultant cyclopeptide. The presence of the aromatic γ-amino acid residues is of critical importance in ensuring β-turn-like conformations.
3. Based on the previous work, a series of artificial β-hairpins composed of α, γ-amino acids were designed and synthesized. The conformations of β-hairpins in aqueous solution were defined by 1D and 2D NMR. The stabilities of different β-hairpins in aqueous buffers were also studied. The results show that the β-turn mimic containing α,γ-amino acids indeed drives oligopeptide chain into well-defined β-sheet conformations with high stability in aqueous solution. In addition, substituting the α-amino acid with γ-amino acid in the β-turn loops of hairpins enhanced the stability of the β-hairpin by comparing the downfield movements of Cα-H in 1H NMR. 
In summary, this study provides a novel strategy for construct of artificial β-turns and β-hairpins with wide universality, ease of manipulation, and high structural stability in aqueous solution high stability in aqueous solution. Such novel artificial β-turn and β-hairpin motifs possess potentials in catalysis, bacteriostasis, antagonist, and antibody.