Lecture5 - Lecture 5 1/14/09 Outline: Peptides...

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Lecture 5 1/14/09 Outline: Peptides Complementarity Neuropeptides Other peptides Sequencing of amino acids
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Peptides Polymers of amino acids. COO - CH 3 H 3 N + C H COO - H H 3 N + C H Alanine + Glycine O C CH 3 H 3 N + C H COO - H N C H H + H 2 O Alanylglycine AG The carboxyl and amino groups involved in the peptide linkage cannot ionize. This dipeptide has only two pka’s: pKa 1 = 3.16 pKa 2 = 8.24 pKa 1 pKa 2 Only the α-amino group of the alanine and the COOH group of glycine are capable of ionizing.
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Peptides can vary: 1) in kind of amino acid 2) amount of each amino acid 3) arrangement of amino acids - Sequence For the dipeptide containing alanine and glycine have only two possibilities in sequence: AG and GA Tripeptide – composed of three amino acids. If each of these amino acids is different from one another, have 6 possible sequences (3 factorial, 3!). ABC BCA CAB CBA BAC ACB A tetrapeptide containing 4 different amino acids would have 4! Possibilities = 24 If have polypeptide containing one of each of the 20 amino acids would have 20! Possibilities = 2 x 10 18 Since the approximate molecular weight of an amino acid is ~100, such a peptide would have a molecular weight of ~ 2000.
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Proteins are much longer polymers of amino acids than peptides. If had a protein with a Molecular weight of 34,000 containing 288 amino
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Lecture5 - Lecture 5 1/14/09 Outline: Peptides...

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