[prakash785]

i want to attach Au nanoparticle to curcumin.. i want to modify curcumin with thiol group for that.. how can i modify curcumin.. it is having 2 OH group and 2 ketone group.. pls help me..

[SVXX]

Technically one of the ketone groups exists as an enol due to chelation, but that's beside the point. You want to substitute the -OH group in the two aryl systems present in curcumin with -SH. Speaking from a purely theoretical point of view, doing by nucleophilic substitution would be exceedingly difficult as S_NAr requires deactivating groups. You could do by the benzyne mechanism, but that requires drastic conditions, so I don't know whether it would affect the other parts of curcumin. I'm no qualified professional.
What you could also try is, remove -OH by zinc dust and try to incorporate -SH group by electrophilic aromatic substitution. As OCH₃ is an activator in EAS, the electrophile will attack the ortho position - the position formerly taken by OH - and you will have the thiol. For more info on how to introduce a thiol group that way, http://pubs.acs.org/doi/abs/10.1021/jo034013h
These are just my 2 cents. I repeat, I'm no qualified professional.

[orgopete]

I agree with SVXX that an atom replacement would be more difficult. However, I don't see that your principle would be altered if you were to use other atoms in your ultimate goal. You should be able to link something to the phenolic OH groups. (You may need to protect the vinylogous acid between the two aromatic rings.)

I suggest trying to find a suitable linker for something like this.

RO-(X)-SH

[ardbeg]

Thioctic acid (lipoic acid) is a great ligand for attaching compounds to Au nanoparticles. 

http://en.wikipedia.org/wiki/Lipoic_acid

Esterication at one of the OH groups should do the trick.

[Agathiyar]

I assume you don't want to disturb the functionality between the two aryl groups ( loss of activity!), since the molecule is symmetrical, you can do lot of ether (stability) or ester reactions with aryl OH with suitable linking group having a terminal SH (as protected if necessary) (as others have suggested).