pH and Buffering

• Why will injecting bicarbonate (HCO3-) at the same time as a local anaesthetic be an advantage?


  because

  HCO3- + BH+ -> B +H2CO3

  
  

  this forms more of the unionised local anesthetic which is better as it passes through the cell membrane.
• What are all local anaesthetic agents?

  Weak acids, existing in 2 forms: unionised (B) and ionised (BH+)

  
  

  
  

  The drug needs to enter the cell, so the unionised form is better as it passes through the cell membrane more easily (as it is not charged).
• what happens when you eat a food?

  when you eat something, the PH of the saliva lowers below critical PH 9 leaving you at risk of tooth decay)

  
  

  it takes approximately 10 minutes to return back to safe PH
• What does Stephan's curve assess?

  The cariogenicity of different foods ( the ability of different foods producing tooth decay)
• What is the disadvantage to increasing local concentrations of calcium and phosphate?

  If the concentrations of either ion are too high, then the calcium phosphate will precipitate out to form calculus ( calcified dental plaque).
• What is the effect of increasing local concentrations of calcium and phosphate on critical pH?

  Increasing it would reduce the critical pH, meaning that the tooth could withstand a lower pH before demineralising
• What does the critical pH depend on?

  The amount of calcium and phosphate in the saliva
• What is the critical pH?

  The highest pH at which there is a net loss of mineral from the teeth, about pH 5.5
• is oxyheamoglobin or deoxyhaemoglobin a better buffer for the hydrogen ions produced in metabolism and why?

  during metabolism, H+ ion conc increases ( eg shown in diagram).

  
  

  so we want the buffer that has more of the protonated form ( as more of the H+ is accepted and removed).

  
  

  since the PH of blood (7.4) is lower than the pka of deoxyhaemoglobin, there will be more undissociated acid ( protonated).

  
  

  This means deoxyhaemoglobin is a better buffer.

  
• How do the levels of dissociated acid vary above and below the pKa?

  Below pKa=more undissociated acid

  Above pKa=less undissociated acid
• What is the pKa of deoxyhaemoglobin?

  Deoxyhaemoglobin pKa=7.8

  
  

  ( when oxyhaemoglobin releases the oxygen, there are confirmational changes and the neighbouring groups around the histidine change causing the pka to change).
• what is the pka of oxyhemoglobin?

  Oxyhaemoglobin pKa=6.8
• Why is the pKa of histidine in haemoglobin different to that of free histidine?

  Because neighboring groups surrounding the histidine acid affect the pKa
• What is the pKa of free histidine?

  6
• What makes haemoglobin a good blood buffer?

  The presence of a large number of histidine residues
• do polar amino acid side chains dissociate?

  no.

  
  

  exception: cysteine can dissociate however in proteins its usually in a disulfide bond.
• do non polar amino acid side chains dissociate?


  no
• Why are the R groups important in buffering?

  in amino acids, the carboxyl group and amino group is involved in the peptide bond so cannot dissociate.

  amino acids that have R groups that can dissociate and can do physiological buffering if the pka is around ph 7.
• is glycine a good physiological buffer?

  no

  glycine has its pka outside the physiological range as it buffers best at ph 2.3 and 9.6
• are all amino acids involved in physiological buffering?

  no.

  - amino acids are bonded to each other by peptide bonds so they cannot dissociate and buffer.

  so, buffering will depend on the amino acid R groups.
• why is determining pKa using equivalence point better than estimating by finding the midpoint of the buffering region?

  it is more precise because it is difficult to know when buffering exactly starts and stops so finding the midpoint is inaccurate.
• in general what is the pka for the alpha amino group dissociation?

  9-10
• in general what is the pka for the alpha carboxyl group dissociation?

  2-3
• summary of arginine titration:

  - triprotic amino acid (the fully protonated form can

  donate three protons)

  
  

  - There are three pKa values:

  alpha carboxyl, R group amino group, alpha amino group

  
  

  - There are three regions of buffering
• summary of histidine titration:

  - triprotic amino acid (the fully protonated form can

  donate three protons)

  
  

  - There are three pKa values:

  alpha carboxyl, R group amino group, alpha amino group

  
  

  - There are three regions of buffering
• summary of aspartic acid titration:

  -triprotic amino acid (the fully protonated form can donate three protons)

  
  

  -There are three pKa values:

  alpha carboxyl, R group carboxyl group, alpha amino group

  
  

  -There are three regions of buffering

  
• how do you work out which group is responsible for the buffering at a certain point on the curve?

  you have to compare with the glycine data to identify where the alpha carboxyl group and alpha-amino group are buffering (The pKa for these groups are similar for both amino acids)
• do electrically charged amino acid side chains dissociate?

  yes :

  
  

  Asp and Glu carboxyl groups dissociate

  
  

  Lys and Arg's side chains dissociate at high PH's= but they are not in physiological range

  
  

  His's side chains dissociates.
• Glycine Titration:

  
  

  Is there any buffering by the zwitterion of glycine?

  No
• Glycine Titration:

  
  

  Explain what is happening at each section of the glycine titration curve?

  --->At low PH= fully protonated (has a positive charge)

  
  

  --->At first buffering region= there is the weak acid and its conjugated base (some of the amino acid would have lost the proton at the alpha carboxyl group)

  pka1: midpoint of buffering-when you have 50% of the weak acid and 50% of the conjugate base. (This is when the solution buffers best)

  
  

  As more base is added the PH increases dramatically

  
  

  --->At the first equivalence point= there is 0% of the weak acid as it has fully dissociated and there is 100% of the conjugate base

  
  

  --->At the second buffering region:

  the previous conjugate base acts as the weak acid and forms a new conjugate base (now the proton from the alpha amino group is removed the weak acid and its conjugated base is negatively charged)

  Pka2: midpoint of buffering-when you have 50% of the weak acid and 50% of the conjugate base. (This is when the solution buffers best)

  
  

  
  

  -----> It is the same idea for all the titration curves however some have acid groups in their side chains and therefore are triprotic or polyprotic, so there would be more pKa and equivalence points.
• Glycine Titration:

  
  

  Describe the results of the titration of glycine using a strong base.

  glycine has two acids:

  -COOH and -NH3+

  
  

  ---->COOH is the stronger acid (pKa=2.34)

  = so it dissociates first ( at lower PH)

  = best buffering at pka

  ---->NH3+ is the weaker acid (pKa=9.66)

  = dissociates after as it has a greater affinity for protons

  = best buffering at pka

  
  

  there is no buffering in between the two acids dissociations. zwitterion is formed

  
• what is the difference between metabolic acidosis and respiratory acidosis pH's?

  They both have the same blood PH ( they just require different treatments). This is because when using the Henderson- Hasselbalch equation, we ca see that the log ratio will remain the same and so PH is the same.

  pH = pKa + log [A-]/[HA]
• What is the difference between metabolic acidosis and respiratory acidosis?

  acidosis is caused due to two different reasons:

  
  

  Metabolic acidosis= [Conjugate base] conc is low

  Respiratory acidosis= [Acid] conc is high
• what is acidisis


  when ph of patient is too low ( H+ conc is high)
• How can proteins dissociate ( act as buffers)?

  Protein --> Protein-

  (neutral--> negatively charged)

  
  

  Protein+ --> Protein

  (positively charged--> neutral)
• can proteins act as buffers?

  yes
• Describe how phosphoric acid dissociates and what is its pKa value?

  H2PO4- --> HPO4 2- + H+

  (pKa=6.8)
• Describe how carbonic acid dissociates and what is its pKa value?

  H2CO3 --> HCO3- + H+

  (pKa=6.1)
• What are the physiologically important buffers in the blood, saliva and body fluids?

  Carbonic acid, phosphoric acid and proteins
• what is a physiological buffer?

  buffers that work in the body.

  -need to have pka around neutrality (PH7)