[SNRPS]

An enzyme (cyto p450) plays a role in stabilizing this intermediate, but I am not sure what the mechanism is. My guess was that the intermediate simply has a carbocation that get's attacked by the lone pair from the guanine nitrogen. The mechanism doesn't require an enzyme to be drawn, but this is definitely a stereospecific reaction.

Thanks!

[Babcock_Hall]

Do you have a reference?  This is different from reactions of Cyt P450 with which I am familiar.

[SNRPS]

This is the alkylation of guanine by aflatoxin.

[Babcock_Hall]

OK, now I can at least give you an educated guess.  Cytochrome C adds oxygen to various substrates, but the reactions are not all identical.  Cyt P450 may make an epoxide out of the double bond, whereupon the epoxide leaves the active site and gets attacked by a guanine base.

[Babcock_Hall]

Cytochrome C adds oxygen to various substrates, but the reactions are not all identical.

Correction, that was supposed to read "Cytochrome P 450."  The book "Enzymatic Reaction Mechanisms" by Christopher Walsh has a good section on this enzyme.  This book is a bit out of date now (published in 1979?), but it may still be a good starting point.

[SNRPS]

Alright, I think the question was just getting at why you see the stereospecificity in the product. The answer I'm going to put is that the intermediate shows epoxide-like coordination with oxygen, forcing the attack from guanine's nitrogen to come at the opposite face from where the oxygen will end up. I was thrown off because i though cytochrome p450 only formed epoxides some of the time.

[Babcock_Hall]

This PNAS article may help:  http://www.pnas.org/content/94/12/6121.abstract
This is a review:  http://carcin.oxfordjournals.org/content/22/4/535.full.pdf+html

It sounds as if the exo-isomer is the epoxide form that reacts with DNA.