2.08:_Enantiomers

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<p class="lt-bio-3786" style="background-color: unset;">In three-dimensional (3D) space, the four covalent bonds of carbon atoms point toward the corners of a regular tetrahedron. The molecule represented to the below is methane (<mjx-container class="MathJax CtxtMenu_Attached_0" jax="SVG" overflow="linebreak" tabindex="0" ctxtmenu_counter="96" style="font-size: 85%; position: relative;"><svg width="4.587ex" height="1.934ex" role="img" focusable="false" viewbox="0 -705 2027.6 855" aria-hidden="true" style="vertical-align: -0.339ex;"><defs><path id="MJX-97-NCM-I-1D436" d="M148 218C148 346 204 476 267 548C318 607 406 666 505 666C609 666 660 587 660 479C660 470 657 438 657 429C657 420 663 415 676 415C681 415 685 416 688 417C693 424 696 431 698 438L760 691C760 700 755 705 745 705C741 705 735 701 727 692L662 619C623 676 568 705 497 705C442 705 388 692 333 667C222 615 141 533 89 421C63 366 50 310 50 253C50 173 75 108 126 56C177 4 242-22 322-22C401-22 473 7 538 64C565 87 588 115 607 146C634 191 648 222 648 241C648 250 643 255 632 255C623 255 618 251 616 242C595 177 562 125 517 88C460 41 400 17 338 17C218 17 148 98 148 218Z"></path><path id="MJX-97-NCM-N-2062" 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transform="translate(760,0)"><g data-mml-node="mi" data-latex="H" data-semantic-type="identifier" data-semantic-role="simple function" data-semantic-font="italic" data-semantic-annotation="clearspeak:simple;depth:3" data-semantic-id="1" data-semantic-parent="3" data-semantic-attributes="latex:H" data-semantic-level-number="2" data-speech-node="true"><use data-c="1D43B" href="#MJX-97-NCM-I-1D43B"></use></g><g data-mml-node="mn" transform="translate(864,-150) scale(0.707)" data-latex="4" data-semantic-type="number" data-semantic-role="integer" data-semantic-font="normal" data-semantic-annotation="clearspeak:simple;depth:3" data-semantic-id="2" data-semantic-parent="3" data-semantic-attributes="latex:4" data-semantic-level-number="2" data-speech-node="true"><use data-c="34" href="#MJX-97-NCM-N-34"></use></g></g></g></g></svg><mjx-assistive-mml unselectable="on" display="inline"><math data-latex="CH_4" data-semantic-type="infixop" data-semantic-role="implicit" data-semantic-annotation="clearspeak:unit;depth:1" data-semantic-="" data-semantic-children="0,3" data-semantic-content="4" data-semantic-attributes="latex:CH_4" data-semantic-owns="0 4 3" data-semantic-level-number="0" data-speech-node="true" data-semantic-structure="(5 0 4 (3 1 2))"><mi data-latex="C" data-semantic-type="identifier" data-semantic-role="latinletter" data-semantic-font="italic" data-semantic-annotation="clearspeak:simple;nemeth:number;depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-attributes="latex:C" data-semantic-level-number="1" data-speech-node="true">C</mi><mo data-semantic-type="operator" data-semantic-role="multiplication" data-semantic-annotation="depth:2" data-semantic-="" data-semantic-parent="5" data-semantic-added="true" data-semantic-operator="infixop,&#8290;" data-semantic-level-number="1" data-speech-node="true">&#8290;</mo><msub data-latex="H_4" data-semantic-type="subscript" data-semantic-role="simple function" 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<figure><img height="224" width="191" class="internal" alt="Ball-and-stick model of a methane (CH4) molecule, with a central black carbon atom connected to four white hydrogen atoms, spaced symmetrically." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/5562/770px-Methane-CRC-MW-3D-balls.png?revision=1" /><figcaption>Figure 2.8.1: 3-D representation of methane. Image used wtih permission (public domain; credit Ben Mills)</figcaption></figure>

<p class="lt-bio-3786" style="background-color: unset;">Whenever a carbon atom has four different structures bonded to it, two different molecules can be formed.</p>

<figure><img height="154" width="246" class="internal" alt="Chemical structure of an amino acid with an amino group in green (H\2N), a carboxyl group in yellow (COOH), and a variable side chain (R) in red." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/5560/amino_acid_structure.png?revision=1" /><figcaption>Figure 2.8.2: Basic Amino Acid Structure. <span class="credit">Photo Credit: Credit: Yassine Mrabet</span> </figcaption></figure>

<div class="box-example"><span id="Example_2.8.1:_Alanine"></span><span id="Example_2.8.1:_Alanine"></span><h2 class="box-legend"><span class="lt-icon-default">Example 2.8.1: Alanine</span></h2>

<p class="lt-bio-3786" style="background-color: unset;">The amino acid <strong>alanine</strong>.</p>

<p class="lt-bio-3786" style="background-color: unset;">Bonded to its alpha carbon atom are four different groups:</p>

<ul>
    <li class="lt-bio-3786" style="background-color: unset;">a <strong>carboxyl</strong> group (COO<sup><font size="2">&minus;</font></sup>)</li>
    <li class="lt-bio-3786" style="background-color: unset;">an <strong>amino</strong> group (NH<sub><font size="2">3</font></sub><sup><font size="2">+</font></sup>)</li>
    <li class="lt-bio-3786" style="background-color: unset;">a <strong>methyl</strong> group (CH<sub><font size="2">3</font></sub>)(its <strong>R group</strong>)</li>
    <li class="lt-bio-3786" style="background-color: unset;">a <strong>hydrogen</strong> atom</li>
</ul>

<p class="lt-bio-3786" style="background-color: unset;">If you orient the molecule so that you look along it <strong>from</strong> the COO<sup><font size="2">&minus;</font></sup> group <strong>to</strong> the NH<sub><font size="2">3</font></sub><sup><font size="2">+</font></sup> group, the methyl (R) group can extend out to the left, forming <strong><small><font size="2">L</font></small>-alanine</strong> (shown on the left) or to the right, forming <strong><small><font size="2">D</font></small>-alanine</strong> (on the right). Although they share the same chemical formula, they are not interchangeable any more than a left-hand glove is interchangeable with right-hand glove.</p>
</div>

<p class="lt-bio-3786" style="background-color: unset;">19 of the 20 amino acids used to synthesize proteins can exist as <strong><small><font size="2">L</font></small></strong>- or <strong><small><font size="2">D</font></small></strong>- enantiomorphs. The exception is glycine, which has two (indistinguishable) hydrogen atoms attached to its alpha carbon. <strong><small><font size="2">L</font></small></strong> amino acids are used exclusively for protein synthesis by all life on our planet. (Some <strong><small><font size="2">D</font></small></strong> amino acids are used for other purposes).</p>

<figure><img height="260" width="384" class="internal" alt="Diagram of two molecular structures representing chiral molecules with COOH, NH2, H, and R groups, overlaid on mirror-image hands to illustrate chirality." loading="lazy" src="https://bio.libretexts.org/@api/deki/files/5561/2000px-Chirality_with_hands.svg.png?revision=1" /><figcaption>Figure 2.8.3: Two enantiomers of a generic  <em><span class="mw-mmv-title"><strong>chiral</strong></span> </em> <em><span class="mw-mmv-title">amino acid. </span></em></figcaption></figure>

<p class="lt-bio-3786" style="background-color: unset;">Does chirality really matter? <strong>Yes</strong>. The function of a protein is determined by its shape. A protein with a D amino acid instead of L will have its R group sticking out in the wrong direction (Figure 2.8.3).</p>

<p class="lt-bio-3786" style="background-color: unset;">Many other kinds of organic molecules exist as enantiomers. Usually only one form is active in biological systems. For example, if one form binds to a receptor protein on the surface of a cell, the other probably cannot. With their protein catalysts (enzymes), cells usually synthesize only one form. However, chemical synthesis in the laboratory or pharmaceutical factory usually produces equal amounts of the two enantiomers &mdash; called a <a title="https://chem.libretexts.org/LibreTexts/Athabasca_University/Chemistry_350%3A_Organic_Chemistry_I/Chapter_5%3A_Stereochemistry_at_Tetrahedral_Centres/5.08_Racemic_Mixtures_and_the_Resolution_of_Enantiomers" href="https://chem.libretexts.org/LibreTexts/Athabasca_University/Chemistry_350%3A_Organic_Chemistry_I/Chapter_5%3A_Stereochemistry_at_Tetrahedral_Centres/5.08_Racemic_Mixtures_and_the_Resolution_of_Enantiomers" target="_blank" rel="external noopener nofollow" class="link-https">racemic mixture</a>.</p>

<div class="box-example"><span id="Example_2.8.2:_Albuterol"></span><span id="Example_2.8.2:_Albuterol"></span><h2 class="box-legend"><span class="lt-icon-default">Example 2.8.2: Albuterol</span></h2>

<p class="lt-bio-3786" style="background-color: unset;">The drug <strong>albuterol</strong> (e.g., Proventil&reg;) contains equal amounts of two enantiomers. Only one of them is effective, and the other may be responsible for the occasional unpleasant side-effects associated with the drug (which is used to dilate the bronchi, e.g, during an attack of asthma). The active form can now be synthesized pure, and &mdash; called <strong>levalbuterol</strong> (Xopenex&reg;) &mdash; is available by prescription.</p>

<p style="text-align: center; background-color: unset;" class="lt-bio-3786"><em><span class="mw-mmv-title">Two enantiomers of albuterol: (R)-(&minus;)-salbutamol (top) and (S)-(+)-salbutamol (bottom)</span> </em></p>
</div>

<p class="lt-bio-3786" style="background-color: unset;">Enantiomers are also called optical isomers because their solutions rotate the plane of polarized light passing through them. If one enantiomer rotates light in the clockwise direction, a solution of the other enantiomer will rotate it in the opposite direction.</p>

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<hr class="autoattribution-divider" /><div class="autoattribution"><p>This page titled <a target="_blank" class="internal mt-self-link" href="/Sandboxes/johnnyphung/biology/02:_The_Molecules_of_Life/2.08:_Enantiomers">2.8: Enantiomers</a> is shared under a <a rel="nofollow" href="https://creativecommons.org/licenses/by/3.0" target="_blank">CC BY  3.0</a>  license and was authored, remixed, and/or curated by <a rel="nofollow" target="_blank" href="http://www.biology-pages.info/">John W. Kimball</a> via <a rel="nofollow" href="https://www.biology-pages.info/" target="_blank">source content</a> that was edited to the style and standards of the LibreTexts platform.</p></div>

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