always monospace in pdf: @, {, and }

[Mark Polesky]

The three special characters @@ @{ and @} are always
displayed in a fixed-width font (in PDF), even when they are
outside of any @code (or similar) block.  As far as I can
tell, there's no way to print these three characters in the
serif font that the rest of the text uses.  So the following
two lines print the same, and I'd like the first one to
print in the normal serif font.

@@@address@hidden

@code{@@@address@hidden

Thanks.
- Mark