Water Flashcards
Water Is the Medium for Life- why?
- Life evolved in water due to the protection it provides from UV light.
- Organisms typically contain 70– 90% water.
- Chemical reactions occur in aqueous milieu.
- Water is a critical determinant of the structure and function of proteins, nucleic acids, and membranes.
Structure of the water molecule
The octet rule dictates that there are four electron pairs around an oxygen atom in water. Two remaining pairs remain nonbonding (lone pairs).
Water geometry is a distorted tetrahedron.
The electronegativity of the oxygen atom induces a net dipole moment. Because of the dipole moment, water can serve as both a hydrogen bond donor and acceptor.
Hydrogen bonding- what is it?
Hydrogen bonds are strong dipole-dipole or charge-dipole interactions that arise between a covalently bound hydrogen and lone pair of electrons.
They typically involve two electronegative atoms (frequently nitrogen and oxygen).
Hydrogen bonds between neighboring molecules are weak (20 kJ/mol) relative to the H–O covalent bonds (420 kJ/mol). –> mas: efeito aditivo
Hydrogen bonding in water
• Water can serve as both:
– an H donor
– an H acceptor
• Up to four H-bonds per water molecule gives water its: – anomalously high boiling point – anomalously high melting point – unusually large surface tension
Hydrogen bonding in water is cooperative
Directionality of the hydrogen bond
Hydrogen bonds are strongest when the bonded molecules allow for linear bonding patterns. Ideally, the three atoms involved are in a line.
Ice: Water in a solid state
- many different crystal forms; the hexagonal ice is the most common.
- Hexagonal ice forms an organized lattice and thus has a low entropy.
- Hexagonal ice contains maximal hydrogen bonds/ water molecules, forcing the water molecules into equidistant arrangement.
• Thus:
- ice has lower density than liquid water
- -> ice floats
Importance of hydrogen bonds in biochemistry
“I believe (…) it will be found that the significance of the hydrogen bond for physiology is greater than that of any other single structural feature.”
- Linus Pauling,
The Nature of the Chemical Bond, 1939
Structure and function of
• Proteins
• DNA
• Polysaccharides
- Binding of substrates to enzymes
- Binding of hormones to receptors
- Matching of mRNA and tRNA
Water as a solvent
Water is a poor solvent for nonpolar substances (nonpolar gases, aromatic moieties, aliphatic chains)
Water is a good solvent for charged and polar substances
(proteins, aa, peptides, small alcohols, carbohydrates)
Dissolving Salts Involves Breaking Ionic Interactions (note the nonrandom orientation of the water molecules)
Water solubility of biologically important gases
-the nonpolar nature of some gases (Nitrogen, Oxygen & Carbon dioxide)
+ the decrease in entropy when they enter solution (The movement of molecules from the disordered gas phase into aqueous solution constrains their motion and the motion of water molecules)
–> combine to make them very poorly soluble in water.
Some organisms have water-soluble carrier proteins (hemoglobin and myoglobin, for example) that facilitate the transport of O2. Carbon dioxide forms carbonic acid (H2CO3) in aqueous solution and is transported as the HCO3 (bicarbonate) ion, either free— bicarbonate is very soluble in water (~100 g/L at 25 C)— or bound to hemoglobin.
Two other gases, NH3 and H2S, also have biological roles in some organisms; these gases are polar and dissolve readily in water.
The hydrophobic effect
Refers to the association or interaction of nonpolar molecules or components of molecules in the aqueous solution.
main factors behind: • protein folding • protein-protein association • formation of lipid micelles • binding of steroid hormones to their receptors
Does not arise because of some attractive direct force between two nonpolar molecules. Rather, it results from the system’s achieving the greatest thermodynamic stability by minimizing the number of ordered water molecules required to surround hydrophobic portions of the solute molecules.
(Water surrounding nonpolar solutes has lower entropy, which is thermodynamically unfavorable, thus hydrophobic solutes have low solubility).
Origin of the hydrophobic effect
- Consider amphipathic lipids in water.
- Lipid molecules disperse in the solution; nonpolar tails of lipid molecules are surrounded by ordered water molecules.
- Entropy of the system decreases.
- The system is now in an unfavorable state.
–> • Nonpolar portions of the amphipathic molecule aggregate so that fewer water molecules are ordered and entropy increases.
• All nonpolar groups are sequestered from water, and the released water molecules increase the entropy further.
• Only polar “head groups” are exposed.
• With high enough concentration of amphipathic molecules, complete aggregation into micelles is possible.
Hydrophobic Effect Favors Ligand Binding
• Binding sites in enzymes and receptors are often hydrophobic.
• Such sites can bind hydrophobic substrates and ligands, such as steroid hormones, which displace water and increase entropy of the system.
The aggregation of nonpolar amino acids in protein interiors, driven by the hydrophobic effect, also stabilizes the three-dimensional structures of proteins.
• Many drugs are designed to take advantage of the hydrophobic effect.
Water as a reagent
- Condensation–water elimination (endergonic)
- Hydrolysis(opposite to condensation, exergonic)
- Oxidation/reduction
Condensation reaction- example
The formation of ATP from ADP and inorganic phosphate is an example of a condensation reaction in which the elements of water are eliminated
Hydrolysis reactions- when do they occur?
Cleavage accompanied by the addition of the elements of water, catalysed by hydrolases
Proteinas, AA,
Poliósidos, oses, ácidos nucleicos, nucleótidos
