"Proper case" for last names is not something you actually know. I've mentioned this in a number of posts on the topic. Some cultures take their
nobiliary particles very seriously. In trying to fix up names, you may outright insult the lead, which is certainly worse than the disease (which is, in fact, error-free) of all caps.
If you still want to make these haphazard guesses, you can use Marketo's Email Scripting to capitalize (search the docs). But I would recommend that any automated attempt to clean such lists be done by a company that has experience in such matters, so their algorithm knows that VON HUMBOLDT is more likely "von Humboldt" than "Von Humboldt."