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键自由转动，但交错构象最稳定，遮挡构象最不稳定。我们对肌红蛋白和其他蛋白质的高分辨率中子衍射分析已经提供了这方面的信息。确定了Rubredoxin中丙氨酸和异亮氨酸的氢原子。提取并讨论了氨基酸残基上的所有CH3基团构型。发现大多数CH_3基团属于稳定的交错构象(夹角接近60°)，但有几%的CH_3基团具有接近遮挡构象(以接近0°的夹角定义)的值。非线性氢键在肌红蛋白的α螺旋的中子分析中，X-H…的线性对Y氢键进行了分析。氢键N-H…的统计得到了肌红蛋白的α螺旋的O=C，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N-H…的非线性O键角θ_1与H-…的偏差相关O=C使θ_2与sp^2氢化的羰基氧的孤立对电子通常预期的值成1 2 0°角。看来，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，需要在小分子结构中通常发现的相当大的偏差。

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.