Does it matter if I present in a PhD interview a topic which is partially unrelated to the PhD one? I have a PhD interview coming up on organic chemistry in a few days. As part of the interview process I have to give a short presentation regarding a research which I am currently undertaking. However the current research is more related towards bio-organic (more biology). Does this matter if I still present a topic which isn't exactly in the organic field?

I should however stress that I am a chemist and done a lot of organic research for my undergraduate but for my masters I took a different turn but now wish to return to the organic side.
I was offered a job in a sub-discipline that I was not a specialist in.  Everyone knew this; I was quite truthful about the situation.  If this should happen, think carefully because much extra work will be expected on your part.  I take it as obvious that you should be and have been honest throughout all this.  However, you still must evaluate very carefully what to do if the job-offer is too far removed from your field-of-study.  From:  One who has been there.