Hydrogen bond
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hydrogen bonds or hydrogen bonds are chemical connections with a free enthalpy of 10-20kJ/mol. They are electrostatic nature however their binding energy are clearly under those to the kovalenten atomic bond and the ionischen connections.
Hydrogen bonds develop, if two molecules or two suitably far from each otherseparate sections of a macromolecule over hydrogen atoms (H) into reciprocal effect step. In addition the H must kovalent to a strongly electronegative atom (z. B. N, O, F and in some cases also Cl) bound its, which the H onepositive Partialladung and the connection partner of the H a negative Partialladung provides, because the more electronegative atom exercises a strong attraction on the common pair of electrons. One speaks of the training of a positive pole (with the H) and a negative pole (with the connection partner of theH), often also of a strong dipole. The electrostatic forces of the dipoles lead to an adjustment and a mutual attraction of the dipoles (+ pole of a dipole tightens - pole of another dipole). The hydrogen bond is formed. Or: The hydrogen bondnow by the H of the molecule, for the donor, to an atom with negative Partialladung of another molecule one forms, the acceptor. This process is similar ansatzweise to the procedure ( Protolyse ), leading to the dissociation of protons with acids.
- HIGH…O-H 2
hydrogen bonds are responsible for the special characteristics of many molecules extremely important for organisms:
- Water: liquid state of aggregation, coherence, high boiling point
- DNA: complementary base pairing within the Doppelhelix
- RNA: complementary base pairing within tRNA- Molecules or between RNA and DNA molecules
- of proteins: Stabilization of the secondary structures (alpha Helix or folding sheet) and the Tertiary period structure, connection to the quart acre structure (it other binding types step on) with proteins in addition.
If for example in the water between the H 2 O-moleculesno hydrogen bonds would exist, ice with 0 ° Celsius would not melt and with 100 °C would simmer, but both at temperatures from two to three-figure temperatures below freezing would already do, with the consequence that it on earth no frozen ice and noneliquid water, but only gaseous water vapour would give. The life on earth in the today's form could not exist under such conditions. Hydrogen bond causes also the anomaly of density of the water.
Straight one in water however leads the ability of each water molecule, fourTo train hydrogen bonds, as a complex three-dimensional network among themselves verbrückter water molecules. Certainly these structures are not rigid but fluctuate in the liquid on a very short time scale, since constantly individual hydrogen bonds are again again formed to break and. Only if water tooIce freezes solidifies itself the structure and it comes to the crystallization.
The idea of the weak reciprocal effect oxygen atom of the water molecule with the hydrogen atom of another comes from Maurice Huggins, a student Gilbert Newton Lewis ', that the expression hydrogen bondin the year 1923 coined/shaped.
