Structure of Amino Acids | MCAT Organic Chemistry Prep скачать в хорошем качестве

Structure of Amino Acids | MCAT Organic Chemistry Prep

Need help preparing for the Organic Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about structure of amino acids. Watch this video to get all the MCAT study tips you need to do well on this section of the exam! Importance of Amino Acids Everything about amino acids, particularly the 20 proteinogenic α-amino acids of eukaryotes, is fundamental material for the MCAT. For this reason alone, the study of their properties and structure should take up a fair amount of your content review time. Those amino acids that are either non-proteinogenic, such as ornithine, or non-α, such as β-alanine, or otherwise exceptions to the canonical set of 20 amino acids (such as selenocysteine), still deserve attention as structures you are ready to learn more about in a given passage, but do not need to be memorized and understood as thoroughly as the set of 20 amino acids that we will be focusing our discussion on. Backbone Structure Every amino acid contains a structure called a backbone. This backbone consists of a carboxylic acid group, an α-carbon (Cα) adjacent to this carboxylic acid group (thereby earning it the α designation), an amine, and an R-group attached to the α-carbon. We can conceptualize this as a central α-carbon bonded to four groups. These 4 groups are the carboxyl group, the amino group, a hydrogen and one of 20 possible side chains (of which one is simply another hydrogen). Now, the fact that amino acids are bound to four groups should make you suspect that most amino acids may be chiral. This is true. And as you may also be able to extrapolate, if the R-group is simply a hydrogen, then the molecule loses its chirality, because now two identical substituents are bound to the α-carbon. This is the case with glycine. While glycine clearly lacks a chiral center, the remaining 19 of the 20 amino acids we care about are all chiral. While threonine and isoleucine have two chiral centers, only the configuration at the α -carbon is likely to be talked about on the MCAT. Since we now know that most amino acids are enantiomeric, we need to discuss the different styles of classification for compounds with a single stereocenter. Enantiomers can be classified in three ways: 1. Absolute configuration 2. Relative configuration 3. Optical properties Absolute configurations are those you are likely most familiar with. These are those that use Cahn-Ingold-Prelog priority assignment for substituents to assign each group a ranked priority and then determine a handed-ness (R or S) on the basis of the ranked order for substituents 1-3, provided that the lowest-ranked substituent has been placed in the back. More on this in our section on stereochemistry. Absolute configurations have the advantage that they are unambiguous, clear, consistent, and recognizable in any sort of projection. They can however be somewhat clumsy to figure out, and for historical reasons, are not always the go-to method of identifying amino acids or monosaccharides. Most α-amino acids are in the S configuration – however, since sulfur has a high CIP priority, cysteine proves to be an exception and appears in the R configuration. Relative configurations are those which determine configurations of small chiral molecules by their homology to glyceraldehyde, something called the Fischer convention. This is a very common way of naming α-amino acids (and sugars), but it can feel somewhat unusual and unsystematic to students. Note that all amino acids are synthesized in the L configuration, although on extremely rare occasions enzymes called racemases may convert some amino acids to the D configuration. Also take heed, there is no systematic relation between absolute and relative configuration – a vast majority of L amino acids are also S, but there is no guarantee that this is the case. Always verify! As a final method of classification, we have optical activity, denoted with lowercase d and l (or + and -), which stands for dextrorotatory versus levorotatory for clockwise and counterclockwise rotation of plane polarized light. This property is determined experimentally and cannot be predicted on hand of the structure alone. MEDSCHOOLCOACH To watch more MCAT video tutorials like this and have access to study scheduling, progress tracking, flashcard and question bank, download MCAT Prep by MedSchoolCoach IOS Link: https://play.google.com/store/apps/de... Apple Link: https://apps.apple.com/us/app/mcat-pr... #medschoolcoach #MCATprep #MCATstudytools