Taking selfies with conference speakers
Yes, it is acceptable. I heard a rumor that scientists are human beings. I have not been able to confirm it, but it seems like a plausible assumption, and if you are willing to accept this as a working hypothesis, it follows that it is acceptable to behave around them as you would with other human beings. 

In particular, most human beings are flattered if you approach them with excitement and admiration and ask to have your photo/selfie taken with them because you want to brag to your friends (or whoever) about meeting them, and would either happily agree or (less likely, but possible) politely refuse. Scientists - even protein chemists! - are no exception to this rule.