Nature of chemical bond in biology including hydrogen and hydration.

甲基CH_3基团构型在气态乙烷中，C-C键自由旋转，但交错构型最稳定，重叠构型最不稳定。我们对肌红蛋白和其他蛋白质的高分辨率中子衍射分析提供了这一信息。红氧还蛋白中的丙氨酸和异亮氨酸的氢原子被清楚地识别。提取并讨论了氨基酸残基中所有的CH_3基团构型。结果表明，绝大多数CH_3基团属于稳定的交错构象（角度接近60°），但也有少数基团的值接近于重叠构象（角度接近0°）。本文给出了肌红蛋白α-螺旋的氢键N-H…O=C的统计，其中H和O之间的长度为r，NHO和COH角分别为θ_1和θ_2。实际的N-H... O键角θ_1的非线性与H... O=C角θ_2偏离sp^2-取代羰基氧的孤对电子通常预期的120°值有关。蛋白质分子中原子的堆积所施加的空间约束，在这种情况下是α-螺旋，使得通常在小分子结构中发现的相当大的偏差成为必然。

Methyl CH_3 group configurationIn gaseous ethane, C-C bond rotates freely but a staggered conformation is most stable and an eclipsed conformation is most unstable. Our high-resolution neutron diffraction analyses of myoglobin and other proteins have provided this information. The hydrogen atoms of alanine and isoleucine in rubredoxin are clearly identified. All the CH_3 group configurations in amino acid residues are extracted and discussed. It is found that most of the CH_3 groups belong to the stable staggered conformations (angles close to 60°), but several % of them appear to have values close to eclipsed conformations (defined by angles close to 0°).Non-linear hydrogen bondsIn the neutron analysis of the α-helices of myoglobin, the linearity of the X-H…Y hydrogen bonds has been analyzed. The statistics of hydrogen bonds N-H…O=C of the α-helices of myoglobin have been obtained, where the length between H and O is r and the NHO and COH angles are θ_1, and θ_2, respectively. The non-linearity of the actual N-H…O bond angle θ_1 is correlated to the deviation of H…O=C angles θ_2 from the value of 120° normally expected for the lone pair electrons of an sp^2-hydridized carbonyl oxygen. It seems that the steric constraints imposed by the packing of atoms in a protein molecule, in this case an α-helix, necessitates considerable deviation normally found in small-molecule structures.