Edit in JSFiddle

<div ng-app="">
<input id="setbtn" type="button" value="set" ng-click="myVar='my-class'">
<input id="clearbtn" type="button" value="clear" ng-click="myVar=''">
<br>
<span class="base-class" ng-class="myVar">Sample Text</span>
</div>    

.base-class {
  -webkit-transition:all cubic-bezier(0.250, 0.460, 0.450, 0.940) 0.5s;
  transition:all cubic-bezier(0.250, 0.460, 0.450, 0.940) 0.5s;
}

.base-class.my-class {
  color: red;
  font-size:3em;
}