string value of "n" to the property indicates that the key combination of alt+n can be used as a shortcut key.
I am not sure but I think property called "AssociatedControlId" is must.Please give a try
Try out for : Shift+Alt+n
Ex:
<asp:button id="btnNew" runat="server" text="New" tooltip="New Button" onclick="btnNew_Click" accesskey="n" associatedcontrolid="btnNew" xmlns:asp="#unknown" />
Regards,
Praveen Nelge