A study conducted using NMR spectroscopy, DFT calculation, and data mining of high resolution protein crystal structures revealed that C-H…π type interactions are in fact quite common in proteins. Simple H atom sharing between two electronegative atoms represents the most common type of H bond, however, it is now accepted that there are additional types of weaker H bonds e.g., C-H groups as donors, N-H…S, and C-H…π interactions with aromatic amino acid residues as acceptors. As H bonds are difficult to directly observe, they are generally inferred when the donor-acceptor distance is less than ~3.5 Å, and the angles at the donor/acceptor are >90°. The D…A distances are typically between 1.7 and 2.4 Å, and the angle at the H atom can be anywhere from 130° to 170°, compared to ~150° at the acceptor. There is quite a large variation in H bond properties as observed in protein structures, especially in bond angles when comparing secondary structure elements to side chain characteristics. While this may seem insignificant it is important to remember that these bonds are flexible and dynamic in that they are constantly broken and reformed under a range of temperatures and physiological conditions. The energy associated with an H bond is 2–10 kcal/mol. This article gives general information about neutron crystallography and shows specific examples of how the method has contributed to structural biology, structure-based drug design and the understanding of fundamental questions of reaction mechanisms.įrom a theoretical chemistry point of view, the H bond interaction energy can essentially be described as being composed of a combination of covalent and weak interactions, e.g., van der Waals forces. Also, as it gives explicit H atom positions, it can be very valuable to computational chemistry where exact knowledge of protonation and solvent orientation can make a large difference in modeling. The method is complementary to X-ray crystallography and the dynamic data obtainable with NMR spectroscopy. Neutron protein crystallography provides a powerful, complementary tool that can give unambiguous information to structural biologists on solvent organization and coordination, the electrostatics of ligand binding, the protonation states of amino acid side chains and catalytic water species. The H bond by definition involves the light atom, H, and it is very difficult to study directly, especially with X-ray crystallographic techniques, due to the poor scattering power of H atoms. Beyond holding protein structures together, H bonds are also intimately involved in solvent coordination, ligand binding, and enzyme catalysis. The hydrogen bond (H bond) is one of the most important interactions that form the foundation of secondary and tertiary protein structure.
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