matlab converting a string to variable name and assign a string to it

Hi , I have a problem ,I really appreciate if someone can help
 I tried to write a code to do this this assignment automatically
"tree.Rightkids=SE1"
here tree.Rightkids is a variable and SE1 is string .
 this is my code
    string1 =['tree.' ,'Rightkids']
    str ='A';
    eval(sprintf('%s=%c',string1,str))
 but I got the following error

 Error using eval
Undefined function or variable 'A'.
if anyone know How can I do this assignmnet.