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admin
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Joined: 08 Dec 2003
Posts: 2168
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 Posted: Mon Aug 06, 2007 9:53 pm Post subject: |
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Don't worry about this question. You did great on this test! Normally on the MCAT, they will just ask about the isoelectric point of an amino acid with 2 ionizable groups (common). In that case, you must memorize the fact that the pI (isoelectric point) is the average of the pKa's of the 2 ionizable groups.
If you really want to understand why it depends on the top 2 pKa's when there are 3 groups (very very rare on the MCAT), then this was discussed in detail regarding a previous problem: www.mcat-prep.com/forum/viewtopic.php?t=773 |
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jvuofm1709
Joined: 06 May 2010
Posts: 18
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| Posted: Thu May 20, 2010 3:30 am Post subject: |
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| ....the iso electric point of tyrosine is pH=5.66. 2.2+9.11/2=5.66. It is the point where the CHARGES ARE NEUTRAL, and the neutral zwitterion is at its highest concentration. the alcohol on tyrosine is NEUTRAL, so once the carboxylic acid is deprotonated, it has a net neutral charge, further deprotonation of the tyrosine -OH would yield a net negative charge. |
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dltkdgn896512
Joined: 26 Jun 2010
Posts: 25
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| Posted: Fri Sep 02, 2011 5:13 pm Post subject: |
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it's not acids for sure cuz they have pI of less than 7
it's either tyrosine or cysteine then, but tyrosine has higher pKa at eveyr point |
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