![]() Water is especially facile at hydrogen bonding because it is both an acceptor and a donor. Hydrogen bonds often occur in networks-frequently with water mediating. The closer the hydrogen bond is to correct geometry, the stronger the bond. 43 The angle the bond forms is also important in determining the strength of the hydrogen bond. This distance is generally considered to be from 2.7 to 3.3 Å, with 3.0 Å being the most common value for protein and water hydrogen bonds. If a proper hydrogen bond acceptor–donor pair is within the correct distance, the bond is taken to be a hydrogen bond. Hydrogen bonds are found in protein crystallography indirectly. In the latter half of the century, strong hydrogen bonds were discovered in several classes of organic molecules in crystals and aprotic liquid phases and characterized by crystallographic, spectroscopic, and chemical methods. Such strong hydrogen bonds were thought to be limited to crystalline states. Very few strong hydrogen bonds had been documented before the mid-twentieth century, principally the very strong hydrogen bond in hydrogen difluoride − (40 kcal mol −1). Unlike covalent bonds, which vary in strength within a factor of ~4 (30–120 kcal mol −1), hydrogen bonds are much less constrained in their geometry and physical properties, and they vary in strength by a factor of at least 20-fold (2–40 kcal mol −1). By mid-twentieth century, the weak conventional hydrogen bonds were reasonably well understood and widely accepted. Hydrogen bonds were controversial throughout the twentieth century. ![]() Frey, in Encyclopedia of Biological Chemistry (Second Edition), 2013 The Nature of Hydrogen Bonds
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