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第四节 蛋白质的三维结构;一、研究蛋白质构象的方法;二、维持蛋白质三级结构的作用力;Fig. 1-24 Four types of noncovalent (“weak”) interactions among biomoleculus in aqueous solvent;1. 氢键(hydrogen bond) ;;2. 范德华力(van der Waals force); 3. 疏水相互作用(hydrophobic interaction); F = (Q1Q2)/(εR2)
Q1,Q2: 电荷电量 ε:介质的介电常数
R: 电荷质点间的距离 ; 5. 二硫键(disulfide bridge);三、多肽链折叠的空间限制;绕C ? –N 键旋转的角度称?(角), 绕C ? –C 键旋转的角度称 ?(角)。
多肽链的所有构象都可以用? 、 ? 两个构象角来描述。;Fig1-29. The peptide bond planes are joined by the tetrahedral bonds of the ?-carbon. The conformation shown corresponds to ? = +180°,? = +180°.;Fig. 1-30 By convention, φ and ψ are both defined as 0°when the two peptide bonds flanking an α carbon are in the same plane. In a protein, this conformation is prohibited by steric overlap between a carbonyl oxygen and an amino hydrogen atom. ;Fig. 1-31 Many of the possible conformations about a ?-carbon between two peptide planes.;3. 可允许的?角和? 角;Fig.1-32 A Ramachandran diagram showing the sterically reasonable values of the angles ? and ? .;四、蛋白质的二级结构(seconary structure);Although helices are not uncommon in manmade architecture, there are a common structure theme in biological macromolecules —protein, nucleic acids, and even polysaccharides.;(一) ?-螺旋(?- helix) ?= -57°, ?= -47°;Fig. 1-34 The ?- helix as viewed from one end, looking down the longitudinal axis. Note the positions of the R groups, represented by purple spheres. ;1. ?-螺旋的结构要点:;Fig. 1-35 The arrangement of N-H and C=O groups (each with an individual dipole moment) along the helix axis creates a large net dipole for the helix. Numbers indicate fractional charges on respective atoms. ;Fig.1-36 Four N-H groups at the N-terminal end of an α- helix and four C=O groups at the C-terminal end cannot participate in hydrogen bonding. ;Fig.1-37 The electric dipole of a peptide bond is transmitted along an α-helical segment through the intrachain hydrogen bonds, resulting in an overall helix dipole. In this illustration, the amino and carbonyl constituents of each
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