<h2>Test for Proteins</h2>
<p class="para">The <em class="emphasis">Biuret Test</em> for proteins using involves testing for the presence of the peptide bond. Biuret reagent is a copper-based reagent that turns purple when bound to protein in an alkaline solution (<a href="#attachment_98762">see figure below</a>). The more peptide <a data-lesson="hydrogen-bonds-and-van-der-waals-interactions" data-subject="biology" data-tutorial="chemical-foundation-of-life" href="https://app.ulearngo.com/biology/chemical-foundation-of-life/hydrogen-bonds-and-van-der-waals-interactions">bonds</a> present, the greater the intensity of the purple colour, indicating a higher protein concentration.</p>
<p class="para">The presence of protein can also be detected using <em class="emphasis">Millon's reagent</em>. Millon's reagent reacts with tyrosine amino acids, common to most proteins, and results in the formation of a reddish-brown precipitate when heated.</p>
<p class="para">The table below summarises the major tests and their expected results in the presence and absence of protein.</p>
<table>
<tbody valign="top">
<tr>
<td><b class="emphasis">Test reagent</b></td>
<td><b class="emphasis">Positive result</b></td>
<td><b class="emphasis">Negative result</b></td>
</tr>
<tr>
<td>Biuret reagent</td>
<td>Violet/purple colour</td>
<td>Blue colour</td>
</tr>
<tr>
<td>Millon's reagent</td>
<td>Red-brown colour</td>
<td>White colour</td>
</tr>
</tbody>
</table>
<div class="ns-school-emp">
<h3 class="example-header"><strong class="title">Investigation: </strong><strong class="title">Test for the Presence of Proteins</strong></h3>
<div class="ns-school-defn">
<p class="para"><b class="emphasis">Aim</b></p>
<p class="para">To use the Biuret test as well as Millon's reagent to test for the presence of proteins</p>
<p class="para"><b class="emphasis">Apparatus</b></p>
<div class="list">
<ol class="arabic">
<li class="item">bunsen burner and a beaker containing water</li>
<li class="item"><i class="emphasis">or</i> water bath with hot <a data-lesson="water-and-life-2" data-subject="biology" data-tutorial="biology-foundations" href="https://app.ulearngo.com/biology/biology-foundations/water-and-life-2">water</a> (50°C)</li>
<li class="item">Dropper or plastic pipette</li>
<li class="item">Test tubes:<br/>
<div class="list">
<ol class="bullet" style="list-style-type: lower-alpha;">
<li class="item">two with albumin solution (positive control)</li>
<li class="item">two with sugar water (negative control)</li>
<li class="item">test tubes with samples to be tested for the presence of protein</li>
<li class="item">test tube with Millon's Reagent</li>
<li class="item">test tube with solution for Biuret test</li>
</ol>
</div>
</li>
</ol>
</div>
<p class="para">(<em class="emphasis"> NOTE:</em> The Millon's Reagent and Biuret's solution in this experiment should be prepared for you by your teacher).</p>
<p class="para"><b class="emphasis">Method</b></p>
<p class="para"><em class="emphasis">Test for protein using Millon's reagent</em></p>
<p class="para"><em class="emphasis">WARNING! Millon's reagent is highly toxic! Avoid <a data-lesson="breathing" data-subject="biology" data-tutorial="human-biology" href="https://app.ulearngo.com/biology/human-biology/breathing">breathing</a> in its fumes.</em></p>
<div class="list">
<ol class="arabic">
<li class="item">Using the dropper or pipette, add a few drops of Millon's Reagent to the test-tube containing albumin.</li>
<li class="item">Using the dropper or pipette, add a few drops of Millon's Reagent to the test-tube containing sugar water.</li>
<li class="item">Using the dropper or pipette, add a few drops of Millon's Reagent to the test-tube containing samples of your <a data-lesson="ingestion" data-subject="biology" data-tutorial="digestive-system" href="https://app.ulearngo.com/biology/digestive-system/ingestion">food</a> to be tested.</li>
<li class="item">Heat the mixtures in boiling water for 5 minutes.</li>
<li class="item">Observe any colour changes.</li>
</ol>
</div>
<p class="para"><em class="emphasis">Test for protein using the Biuret test</em></p>
<div class="list">
<ol class="arabic">
<li class="item">Using the dropper or pipette, add a few drops of the Biuret solution to the test-tube containing albumin.</li>
<li class="item">Using the dropper or pipette, add a few drops of the Biuret solution to the test-tube containing sugar water.</li>
<li class="item">Using the dropper or pipette, add a few drops of the Biuret solution to the test-tube containing samples of your food to be tested.</li>
<li class="item">Observe any colour changes.</li>
</ol>
</div>
<div class="figure" id="figure:Biuret">
<div class="wp-caption alignnone" id="attachment_98762" style="width: 229px"><a href="https://data.ulearngo.com/assets/1cd4c713-566e-4119-a9f4-fb54fdbaa22f"><img alt="" class="wp-image-98762 size-full" height="357" src="https://data.ulearngo.com/assets/1cd4c713-566e-4119-a9f4-fb54fdbaa22f" width="229"/></a><p class="wp-caption-text">Image credit: Siyavula</p></div>
</div>
<p class="para"><b class="emphasis">Observations</b></p>
<p class="para">Record your observations, noting any key differences between the positive control, negative control and experimental samples.</p>
</div>
</div>
<h3>Video Demonstration</h3>
<p>See the video below showing Biuret test for proteins.</p>
<p><iframe allowfullscreen="allowfullscreen" frameborder="0" height="360" src="https://www.youtube.com/embed/ufec89A47uM?rel=0&amp;modestbranding=1&amp;cc_lang_pref=en&amp;cc_load_policy=1" width="640"></iframe></p>

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Test for Proteins

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Test for Proteins

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