Bis102- MT1- W'03 KEY- Gasser

Only between hydrogen atoms are usually stronger than

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: ydrogen atoms are usually stronger than hydrogen bonds X can lead to attraction of any uncharged molecules are stronger between smaller molecules than between larger molecules Name KEY Last, First BIS102-03, Winter '03, Page 6 of 7 C. S. Gasser 6. Answer the following questions concerning the dipeptide illustrated below: OH SH CH2 O (9.1) H+ N 3 C H C CH2 O N H C H C O (1.8) a) (6) Write the names and one letter abbreviations for the two amino acids represented in this dipeptide. Tyrosine Y, Cysteine C b) (6) What would the net charge on this peptide be at pH = 12 (to the nearest 0.1 charge unit)? pKa values for the N- and C-terminal groups are given above, obtain others values you might need from the table on the front page. pKa s for Tyr and Cys side chains are 10.0 and 8.3, respectively. Charges are: a-amino = 0; Tyr = -1; Cys = -1; a-carboxyl = -1. The sum = -3 Charge = -3 c) (14) What would the net charge on this peptide be at pH = 7.3 (to the nearest 0.1 charge unit)? Charges are: a-amino = +1; Tyr = 0; Cys = see below; a-carboxyl = -1. For Cys: 7.3 = 8.3 + log [Cys- ] [Cys0 ] [Cys- ] = 0.1; [Cys- ] = 0.1[Cys0 ] [Cys0 ] [Cys- ] + [Cys0 ] = 1 0.1[Cys0 ] + [Cys0 ] = 1 1.1[Cys0 ] = 1; [Cys0 ] = 0.91; [Cys- ] = 0.09 ª 0.1 Charge on Cys = -0.1; Total charge = +1 + 0 – 1 – 0.1 = -0.1 † Charge = -0.1 Name KEY Last, First BIS102-03, Winter '03, Page 7 of 7 C. S. Gasser 7. (16) A solution is buffered with threonine such that the net charge on the threonine in the solution is - 0.5. To the nearest 0.1 pH unit, determine the pH of the solution. Thr+1 <–> Thr0 <–> Thr-1 2.6 10.4 A charge of -0.5 means it is 50% Thr0 and 50% Thr-1. Therefore pH= pKa = 10.4 pH = 10.4 8. (15) A mixture containing Asp, Arg, His, Ser, and Asn at pH 9.2 is loaded onto a column containing DEAE cellulose (hydrophilic beads with many POSITIVELY charged groups on them). The column in then washed with several volumes of the pH 9.2 buffer. a) Which amino acid(s) will come out in the pH 9.2 wash? Why? Add up charges on a-amino, a-carboxyl and R-group (if any) and see if negative Asp = <+1, -1, -1 = -1 sticks on column Arg = ~+0.5, -1, +1 = +0.5 flows through column His = +0.5, -1, 0 = -0.5 sticks on column Ser = ~+0.5, -1 = -0.5 sticks on column Only the Arg will flow through the column, the others will be retained by the charge interaction with the positive resin. b) The column in then washed with several volumes of buffer at pH 5. Which amino acid(s) will elute (come out) in this wash? Why? Add up charges on a-amino, a-carboxyl and R-group (if any) and see if negative Asp = +1, -1, -1 = -1 sticks on column Arg = no Arg on column it all came through at pH 9.2 His = +1, -1, +0.9 = +0.9 elutes from column Ser = pI of Ser is (9.3+2.3)/2 = 5.8 so pH5 below pI and it has a weak positive charge so it elutes from column The His and Ser will elute from the column, the Asp will be retained by the charge interaction with the positive resin....
View Full Document

Ask a homework question - tutors are online