The brew wiki has some information on it.
Hop chemistry - Home Brewing Wiki
"When added to boiling wort, alpha acids go into solution almost immediately. However, the bitterness they impart to beer appears only gradually, as the alpha acids are slowly isomerized in the boil to form isomerized alpha acids or iso-alpha acids. This is why bittering hop additions are normally done at the beginning of a boil of 60 minutes or longer."