# Water

[latexpage]

## Water in Biochemistry

Water is essential in biochemistry

• Nearly all biochemical reactions occur in water
• Reactants, products, nutrients, waste depend on water
• Water itself participates in chemical reactions:
• $\mathrm{H^+}$
• $\mathrm{OH^-}$
• Biomolecules adjust their shape (and therefore function) in response to physical and chemcial properties of water

Hydrogen bond = attractive force between hydrogen bonded to an electronegative atom (e.g., O, N, F) and an unshared electron pair on another electronegative atom

• In general, a hydrogen bond can be represented as D-H—-A
• DH = weaky acidic donor group (e.g., OH, NH, sometimes SH)
• A = weakly basic acceptor group (e.g., O, N, or sometimes S)
• The energy of an individual hydrogen bond is only approximately $20 \mathrm{kJ} / \mathrm{mol}$ BUT the number of hydrogen bonds in a given system can accumulate so that the overall effect of hydrogen bonds is very significant
• Note: hydrogen bonds are constantly forming and breaking in solution
• Water “networks” break up and reform every $2 \cdot 10^{-11} \mathrm{sec}$
• Thus, liquid water can be seen as a rapidly chaning 3D network of hydrogen bonds
• Importantly, weak molecular interactions dictate the structure of biomolecules
• This includes hydrogen bonds as well as ionic/electrostatic interactions and van der Waals interactions

Water affects solubility

• Hydrophilic = polar and ionic compounds
• Water is a good solvent for these types of molecules
• Hydrophobic = compounds that are virtually insoluble in water
• Most biomolecules will have both hydrophillic and hydrophobic groups (the technical term for these molecules is amphiphilic or amphipathic)
• Example: lipid bilayer of a cell membrane
• Hydrophobic effect = the tendency of water to minimize contacts with hydrophobic molecules

Water can participate in chemical reactions

• Water only has a slight tendency to minimize, but can form $\mathrm{H^+}$ and $\mathrm{OH^-}$ ions
• $\mathrm{H^+}$ associates with $\mathrm{H_2 O}$ to form $\mathrm{H_3 O^+}$
• Thus, $\mathrm{H^+}$ is never free in solution
• Proton jumping = when the $\mathrm{H^+}$ of $\mathrm{H_3 O^+}$ jumps to other molecules
• Explains why acid-base reactions occur most quicky in aqueous solutions

## Notes:

• high surface tension ⇒ concentrated into tiny droplets
• low surface tenson ⇒ spreads out

## Sources

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