For part b), to get the tripeptide required, do I combine ser and pro with the 2 functional groups labelled a OR the 2 functional groups underlined at the bottom? And also why?
Because by agreed convention for polypeptides of proteins, we (ie. scientists, chemists, biologists, biochemists, etc) always start with the amino or N terminus on the left, and the carboxylic acid or C terminus on the right.
This means Ser-Pro-Phe is totally different from Phe-Pro-Ser.
Let the R groups of the amino acids Ser, Pro and Phe be R1, R2 and R3 respectively.
Then the tripeptide Ser-Pro-Phe would be :
H2N-CH(R1)-CON(R2)CH-CONH-CH(R3)-COOH (as unionized molecular form)
+H3N-CH(R1)-CON(R2)CH-CONH-CH(R3)-COO- (as zwitterionic form in neutral pH aqueous solution)
Note that Proline is a special case (the structure of the tripeptide as written above is admittedly misleading, due to the unusual cyclic R group of Proline, see diagram below), as its R group is directly bonded to its alpha amine, making it a secondary amine (while all other proteinogenic alpha amino acids have primary alpha amines).