BibTeX: where to write Sir in author's name?

The following form of the author field will work even if your bibliography style is set to abbreviate first names down to their initials:

author = "{\relax Sir I}saac Newton",

The title Sir is always used with the first name so never Sir Newton. usually titles are omitted from bilbliographies I think, but if you want {Sir Isaac} to be get the initial I possibly the simplest is to hide the Sir part in a macro so mark it up as something like {\Sir I}saac with \Sir defined to put Sir back on full names and do nothing if just given I (or leave it as Sir I., which is simper).

This has been edited to get the BibTeX braces more correct, it seems to work as given if \Sir defined simply as \newcommand\Sir{Sir }