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Mass Spec of Derivatized (TBDMS) Proline and Isoleucine

  1. Mar 19, 2010 #1
    Hi all,

    NOTE: Images attached if things get confusing.

    I'm trying to work out the identity of some of the major mass spec peaks found for proline and isoleucine after derivativization with tert-butyl dimethyl silane. I know that TBDMS is added to the OH side of the carboxylic acids of each amino acid, and that it adds to the NH2 of isoleucine, though I'm not certain if it would attach to the NH of the fused ring found in proline via some ring opening mechanism or not.

    I assume if the ring did open, the NH would remain connected to the carbon alpha to the carboxyl group and that an alkyl chain would be left extending from this carbon. Does this sound reasonable?

    My issue with isoleucine is not in the derivatization chemistry, but in the fragmentations enacted by mass spec. My mass-histogram shows major species of the following weights: 302, 274, 200, and 147, all with small isotopic peaks adjacent to them. The fully derivatized species weighs 359 and each TBDMS group weighs 115. It's pretty common for one of the tert-butyl methyls to fragment leaving a radical cation behind. I know that sometimes the tert butyl group is completely removed leaving the Si as the radical cation, as well.

    It is also possible that only one derivative group is added, rather than two, which would lead to varying weights. I still can't reconcile some of the weights I've listed and I've tried mapping all possible fragmentation products. Is it possible for a methyl to be cleaved from the Si and a methyl cleaved from the tert-butyl, leaving two radicals which then form a double bond between the Si and the former tert carbon of the tert-butyl group?

    Check out my guesses so far for isoleucine and see what you think. I'm at a loss. I guess if this gets a good response I'll then try to work through the proline fragmentations. Note that a radical dot should be added to each cation in my images.

    Any help would be greatly appreciated!!!
     

    Attached Files:

  2. jcsd
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