Q:

Using redefined \c and Turkish characters in citation

I once redefined the command \c since I need a shortcut for some formula that I use quite frequently.  Unfortunately, I now need to cite a turkish author that has a \c{C} in his name.  So, what I did is to save the old definition of \c into a new command \turc by using \let\turc\c.  Consider the following MWE:
\documentclass{scrartcl}

\usepackage{natbib}

\let\turc\c

\renewcommand{\c}{\ensuremath{c^\pi}}

\begin{document}
    \citep{ABC}

    \bibliographystyle{plainnat}
    \bibliography{lit}
\end{document}

lit.bib:
@Book{ABC,
    Title="Test",
    Author="{\turc{C}}enk",
    Year="2015",
    Publisher="Me"
}

In the References, the name is shown correctly with the corresponding turkish character.  However, in the actual reference \citep, the turkish character is replaced by the new definition of \c, i.e., by c^\pi. 
I guess this has something to do with expansion problems, but I can't figure out how to solve the problem. 

A:

It is imho not a good idea to redefine such accent commands.  But in your case you can make \turc robust:
\documentclass{scrartcl}

\usepackage{natbib}

\let\turc\c
\usepackage{etoolbox}
\robustify\turc

\renewcommand{\c}{\ensuremath{c^\pi}}

\begin{document}
    \citep{ABC}

    \bibliographystyle{plainnat}
    \bibliography{lit}
\end{document}

