Tutor profile: Yao Q.
Subject: Organic Chemistry
How do you find stereocenters in a certain molecule?
The definition of stereocenter is "an atom at which interchange of two groups gives a stereoisomer." Following this definition, there are a few steps to figure out whether an atom is a stereocenter or not. First, if there are four groups connected to one atom, and those four groups are different, then it is for sure a stereocenter. You can then assign configurations according to the priorities of the groups. Second, if there are two or more identical groups connected to the atom, then it is not a stereocenter. Third, if the atom is connected by a double bond, keep in mind that the three groups attached to it are on a plane. Swapping the other two groups that are not connected to the double bond and see if it will change the molecule to a new stereoisomer. Remember stereoisomers cannot overlap each other. Finally, if you are not sure, bring out your model kits and build two of the same molecule. Swap two of the groups and see if the two are still identical after a few twisting. If not, then the atom is a stereocenter.
What is the difference between natural killer T cells and cytotoxic killer T cells (if any)?
The two are similar. Most T-cells (including cytotoxic killer T-cells) recognize peptide (protein) antigens presented by MHC molecules on other cells. Natural killer T cells recognize glycolipid antigens presented by a different type of molecule (the specifics of this molecule aren't important, just know that it isn't MHC). Once it recognizes an antigen that causes a reaction, natural killer T-cells can then produce cytotoxins and cytokines (effectively behaving as both cytotoxic killer T-cells and helper T-cells).
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