can't use angular value within quotes

Why does this not work:

<ul class="dropdown-menu">
  <li ng-repeat="choice in dropDownItems">
    <a class="btn" ng-click="mnuClick('{{choice}}')">{{choice}}</a>
  </li>
</ul>

But this does work:

<ul class="dropdown-menu">
  <li ng-repeat="choice in dropDownItems">
    <a class="btn" ng-click="mnuClick('xxx')">{{choice}}</a>
  </li>
</ul>

In the top example, the mnuClick() routine never gets called, but in the bottom example, it does. When I do an 'inspect element' everything looks fine.