Question: How to set the boundary conditions

How can I set  the boundary conditions? I need to specify diff(ui(e, t), e) and diff(ur(e,t),e) depending on e.  I write the rules:

> D1(e,t):=diff(ui(e,t),e);

> D12(e,t):=diff(ur(e,t),e);

 in order to use D1 and D12 in boundary conditions. 

I get error
> sol := pdsolve(sys, {D1(-500, t), D1(500, t) = 0, D12(-500, t) = 0, D12(500, t) = 0, ui(-500, t) = 0, ui(0, t) = 0, ui(500, t) = 0, ur(-500, t) = 0, ur(500, t) = 0}, {u(e, t), ui(e, t), ur(e, t)}, type = numeric);

Error, (in D1) invalid input: diff received -500, which is not valid for its 2nd argument

 as I understand, I can't use the function diff, but how can I set such conditions by another way?

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