No SIMM 11:21 AM Problem Sets Question 2 (12 marks) Tyrosine is a triprotic amin

Question

No SIMM 11:21 AM Problem Sets Question 2 (12 marks) Tyrosine is a triprotic amino acid with pKa of2.17, 9.19. and 10.47The first proton is removed from the carboxylic acid (-COOH), the second from the protonated amine group NHs ).The third, with a pKa of 10.47. is the phenolic proton (-OH) on the aromatic ring. At low pH, the molecule is a monocation. At high pH values, it is a dianion. (Recall: you did this on Problem Set 2) Tyrosine The capacity factor of tyrosine changes with pH on a reverse-phase support. With a phosphate buffer of 0.02 M and solvent comprised of 5:95 (vlv)ethanolwater, the pH values and the associated k values were determined to be: pH 15 4.59 2.70 2.88 1.90 4.40 5.80 1.86 1.80 7.22 8.83 0.88 a) If the mechanism of retention is strictly a reversed phase one, which form of the molecule would have the highest k ?Briefly explain (max. 50 words) b) What type(s) of interaction(s) is/are occurring on this reversed phase column between stationary phase and analyte? eg. H-bonding (adsorption), reversed-phase, hydrophobic ion exchange, exclusion...? c) If each of the ionized forms of tyrosine was analyzed separately using the same stationary reverse-phase and solvent, arrange these tyrosine forms in the order of increasing retention times (ie from the shortest retention time to the longest). Page 2 of 5 20%

Explanation / Answer

It is a simple theroy of retention factor. As you increase the pH , the retention factor or capacity factor (K) of acidic compound will decrease. Acidic compound then loose the Proton. If the pH of a solution is too low or in neutral form and it would display large retention time. If pH is too high, then the solution becomes more polar, then the retention of acidic solution will be decresed.

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