Macromolecules are characterized by distinctive arrangement of hydrogen bonds. Different patterns of hydrogen bonds give rise to distinct and stable structural motifs. An analysis of 4114 non-redundant protein chains reveals the existence of a three-residue, (i - 1) to (i + 1), structural motif, having two hydrogen-bonded five-membered pseudo rings (the first, an NH···OC involving the first residue, and the second being NH∙∙∙N involving the last two residues), separated by a peptide bond. There could be an additional hydrogen bond between the side-chain at (i-1) and the main-chain NH of (i + 1). The average backbone torsion angles of -76(±21)° and - 12(±17)° at i creates a tight turn in the polypeptide chain, akin to a γ-turn. Indeed, a search of three-residue fragments with restriction on the terminal Cα ···Cα distance and the existence of the two pseudo rings on either side revealed the presence 14 846 cases of a variant, termed NHN γ-turn, distinct from the NHO γ-turn (2032 cases) that has traditionally been characterized by the presence of NHO hydrogen bond linking the terminal main-chain atoms. As in the latter, the newly identified γ-turns are also of two types-classical and inverse, occurring in the ratio of 1:6. The propensities of residues to occur in these turns and their secondary structural features have been enumerated. An understanding of these turns would be useful for structure prediction and loop modeling, and may serve as models to represent some of the unfolded state or disordered region in proteins.

中文翻译：

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划定涉及NHNγ转角的新结构基序。

大分子的特征在于氢键的独特排列。氢键的不同图案产生不同且稳定的结构基序。对4114个非冗余蛋白链的分析显示，存在三个残基（i-1至至（i + 1））结构基序，具有两个氢键键合的五元伪环（第一个是Nthe H···OC涉及第一个残基，第二个是NH∙∙∙N涉及最后两个残基），由肽键分隔。在（i-1）的侧链和（i +1）的主链NH之间可能存在一个额外的氢键。i处的平均主链扭转角为-76（±21）°和-12（±17）°，在多肽链中产生紧密的转向，类似于γ转向。的确，对三个末端残基Cα····Cα距离受限制的残基进行搜索，并且在两侧均存在两个伪环，发现存在14 846例变种，称为NHNγ转角，与NHOγ不同转弯（2032例），其传统特征是存在连接末端主链原子的NHO氢键。与后者一样，新近识别出的γ匝也具有两种类型：经典和反向，以1：6的比率出现。已经列举了残基在这些回合中发生的可能性及其二级结构特征。对这些转折的理解对于结构预测和环建模将是有用的，并且可以用作代表蛋白质中某些未折叠状态或无序区域的模型。