1.04:_Noncovalent_Bonding

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<span id="Noncovalent_Bonding"></span><span id="Noncovalent_Bonding"></span><h2 style="background-color: unset;" class="lt-bio-3744"> Noncovalent Bonding</h2>

<p class="lt-bio-3744" style="background-color: unset;">Noncovalent bonding does not involve sharing of electrons. Instead it:</p>

<ul>
    <li class="lt-bio-3744" style="background-color: unset;">holds the two strands of the DNA double helix together (hydrogen bonds)</li>
    <li class="lt-bio-3744" style="background-color: unset;">folds polypeptides into such secondary structures as the alpha helix and the beta conformation</li>
    <li class="lt-bio-3744" style="background-color: unset;">enables <strong>enzymes</strong> to bind to their <strong>substrate</strong></li>
    <li class="lt-bio-3744" style="background-color: unset;">enables <strong>antibodies</strong> to bind to their <strong>antigen</strong></li>
    <li class="lt-bio-3744" style="background-color: unset;">enables transcription factors to bind to each other</li>
    <li class="lt-bio-3744" style="background-color: unset;">enables transcription factors to bind to DNA</li>
    <li class="lt-bio-3744" style="background-color: unset;">enables proteins (e.g. some hormones) to bind to their receptor</li>
    <li class="lt-bio-3744" style="background-color: unset;">permits the assembly of such macromolecular machinery as
    <ul>
        <li class="lt-bio-3744" style="background-color: unset;">ribosomes</li>
        <li class="lt-bio-3744" style="background-color: unset;">actin filaments</li>
        <li class="lt-bio-3744" style="background-color: unset;">microtubules</li>
    </ul>
    </li>
    <li class="lt-bio-3744" style="background-color: unset;">and many more</li>
</ul>

<p class="lt-bio-3744" style="background-color: unset;">There are three principle kinds of noncovalent forces:</p>

<ul>
    <li class="lt-bio-3744" style="background-color: unset;">ionic interactions</li>
    <li class="lt-bio-3744" style="background-color: unset;">hydrophobic interactions</li>
    <li class="lt-bio-3744" style="background-color: unset;">hydrogen bonds</li>
</ul>

<span id="Ionic_Interactions"></span><span id="Ionic_Interactions"></span><h2 style="background-color: unset;" class="lt-bio-3744">Ionic Interactions</h2>

<p class="lt-bio-3744" style="background-color: unset;">At any given pH, proteins have charged groups that may participate in binding them to each other or to other types of molecules. For example, as the figure shows, negatively-charged carboxyl groups on aspartic acid (Asp) and glutamic acid (Glu) residues may be attracted by the positively-charged free amino groups on lysine (Lys) and arginine (Arg) residues.</p>

<p class="lt-bio-3744" style="background-color: unset;">Ionic interactions are highly sensitive to<img class="internal right" style="width: 204px; height: 262px; float: right;" alt="Diagram showing ionic interactions between amino acids and water. Arrows point from molecules like Asp or Glu (-COO&#8315;) and Lys or Arg (H&#8323;N&#8314;) to water molecules in the center." loading="lazy" width="204px" height="262px" src="https://bio.libretexts.org/@api/deki/files/5108/ionicinteractions.png?revision=1&amp;size=bestfit&amp;width=204&amp;height=262" /></p>

<ul>
    <li class="lt-bio-3744" style="background-color: unset;"><strong>changes in pH</strong>.

<p class="lt-bio-3744" style="background-color: unset;">As the pH <strong>drops</strong>,</p>

<ul>
        <li class="lt-bio-3744" style="background-color: unset;">H<sup><font size="2">+</font></sup> bind to the carboxyl groups (COO<sup><font size="2">-</font></sup>) of aspartic acid (Asp) and glutamic acid (Glu), neutralizing their negative charge, and</li>
        <li class="lt-bio-3744" style="background-color: unset;">H<sup><font size="2">+</font></sup> bind to the unoccupied pair of electrons on the N atom of the amino (NH<sub><font size="2">2</font></sub>) groups of lysine (Lys) and arginine (Arg) giving them a positive charge</li>
    </ul>

<p class="lt-bio-3744" style="background-color: unset;"><strong>The result:</strong> Not only does the net charge on the molecule change (it becomes more positive) but many of the opportunities that its R groups have for ionic (electrostatic) interactions with other molecules and ions are altered.</p>
    </li>
</ul>

<p class="lt-bio-3744" style="background-color: unset;">As the pH <strong>rises</strong>,</p>

<ul>
    <li class="lt-bio-3744" style="background-color: unset;">H<sup><font size="2">+</font></sup> are removed from the COOH groups of Asp and Glu, giving them a negative charge (COO<sup><font size="2">&minus;</font></sup>), and</li>
    <li class="lt-bio-3744" style="background-color: unset;">H<sup><font size="2">+</font></sup> are removed from the NH<sub><font size="2">3</font></sub><sup><font size="2">+</font></sup> groups of Lys and Arg removing their positive charge</li>
</ul>

<p class="lt-bio-3744" style="background-color: unset;"><strong>The result:</strong> Again the net charge on the molecule changes (it becomes more negative) and, again, many of the opportunities its R groups have for electrostatic interactions with other molecules or ions are altered.</p>

<ul>
    <li class="lt-bio-3744" style="background-color: unset;"><strong>salt concentration</strong></li>
</ul>

<p class="lt-bio-3744" style="background-color: unset;">    Increasing salt concentration reduces the strength of ionic binding by providing competing ions for the charged residues.</p>

<span id="Hydrophobic_Interactions"></span><span id="Hydrophobic_Interactions"></span><h2 style="background-color: unset;" class="lt-bio-3744">Hydrophobic Interactions</h2>

<p class="lt-bio-3744" style="background-color: unset;"><img class="internal right" style="width: 211px; height: 140px; float: right;" alt="Diagram illustrating hydrophobic interactions and hydrogen bonds between molecules: C=O and H-N on one side, O-H and O=C on the other, with dotted lines representing bonds." loading="lazy" width="211px" height="140px" src="https://bio.libretexts.org/@api/deki/files/5109/hydrophobicinteractions.png?revision=1&amp;size=bestfit&amp;width=211&amp;height=140" />The side chains (R groups) of such amino acids as phenylalanine and leucine are nonpolar and hence interact poorly with polar molecules like water. For this reason, most of the nonpolar residues in globular proteins are directed toward the interior of the molecule whereas such polar groups as aspartic acid and lysine are on the surface exposed to the solvent. When nonpolar residues are exposed at the surface of two different molecules, it is energetically more favorable for their two &quot;oily&quot; nonpolar surfaces to approach each other closely displacing the polar water molecules from between them.</p>

<p class="lt-bio-3744" style="background-color: unset;">The strength of hydrophobic interactions is not appreciably affected by changes in pH or in salt concentration.</p>

<span id="Hydrogen_Bonds"></span><span id="Hydrogen_Bonds"></span><h2 style="background-color: unset;" class="lt-bio-3744">Hydrogen Bonds</h2>

<p class="lt-bio-3744" style="background-color: unset;">Hydrogen bonds can form whenever</p>

<ul>
    <li class="lt-bio-3744" style="background-color: unset;">a strongly electronegative atom (e.g., oxygen, nitrogen) approaches</li>
    <li class="lt-bio-3744" style="background-color: unset;">a hydrogen atom which is <strong>covalently</strong> attached to a second strongly-electronegative atom</li>
</ul>

<p class="lt-bio-3744" style="background-color: unset;">Some common examples:</p>

<ul>
    <li class="lt-bio-3744" style="background-color: unset;">between the &minus;C=O group and the H-N&minus; group of nearby peptide bonds in proteins giving rise to the alpha helix and beta configuration</li>
</ul>

<ul>
    <li class="lt-bio-3744" style="background-color: unset;">Between &minus;C=O groups and hydroxyl (H-O&minus;) groups in
    <ul>
        <li class="lt-bio-3744" style="background-color: unset;">serine and threonine residues of proteins and            <img style="width: 373px; height: 155px;" class="internal" alt="Chemical structures of the amino acids serine and threonine. Serine has an -OH group, while threonine has an -OH and an additional CH&#8323; group. Both have NH&#8323;&#8314; and COO&#8315; groups." loading="lazy" width="373px" height="155px" src="https://bio.libretexts.org/@api/deki/files/5112/Ser_thr.gif?revision=1&amp;size=bestfit&amp;width=373&amp;height=155" /></li>
        <li class="lt-bio-3744" style="background-color: unset;">sugars</li>
    </ul>
    </li>
</ul>

<p class="lt-bio-3744" style="background-color: unset;"><span class="class"><strong>Noncovalent interactions are individually weak but collectively strong.</strong></span></p>

<p class="lt-bio-3744" style="background-color: unset;">All three forms of noncovalent interactions are individually weak (on the order of 5 kcal/mole) as compared with a covalent bond (with its 90&ndash;100 kcal/mole of bond energy). And what strength these interactions do have requires that the interacting groups can approach each other closely (an angstrom or less). So we can conclude that all the examples given at the top of the page require:</p>

<ul>
    <li class="lt-bio-3744" style="background-color: unset;">a substantial number of noncovalent interactions working together to hold the structures together</li>
    <li class="lt-bio-3744" style="background-color: unset;">a surface topography that enables substantial areas of two interacting surfaces to approach each other closely; that is, they must fit each other</li>
</ul>

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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/01:_The_Chemical_Basis_of_Life/1.04:_Noncovalent_Bonding">1.4: Noncovalent Bonding</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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