che359_sp10_hw3_soln - Solutions prepared by Chris Singer...

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Page 1 of 3 ChE 359 Homework #3 Solutions prepared by Chris Singer 1. (15 points) A protein may contain several cysteines, which may pair together to form disulfide bonds. If there is an even number n of cysteines, n/2 disulfide bonds can form. Derive an expression for the number of different possible disulfide pairing arrangements. We first number the individual cysteines along the chain. The first cysteine along the chain (doesn’t matter if you start with #1) can bond to any of the other n 1 . Two cysteines are then taken out of consideration for future bonding. The third cysteine can then bond to any of the other n 2 ( ) 1 . The fifth cysteine can then bond to any of the other n 4 ( ) 1 , and so on until all bonds are formed. Thus, the total possible number of disulfide pairing arrangements is given by: W n ( ) = n 1 ( ) × n 3 ( ) × n 5 ( ) × × 1 . You can also derive this result by taking all the possible sequences of the (distinguishable) cysteines. There are n ! , where each pair represents a bond. To account for the fact that the order of the pair (1-2 vs 2-1) doesn't matter and that it doesn't matter where in the sequence the pairs occur, you divide by 2 n / 2 and n 2 Λ Ν Μ Ξ Π Ο ! respectively. This would give the expression: W n ( ) = n !
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This note was uploaded on 02/09/2011 for the course CHE 359 taught by Professor Cynthialo during the Spring '10 term at Washington University in St. Louis.

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che359_sp10_hw3_soln - Solutions prepared by Chris Singer...

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