Alpha acids (α acids) are a class of chemical compounds found in the resin glands of the flowers of the hop plant and are the main source of hop's bitterness contribution to beer. The Alpha Acids are isomerized during the boil, and it is the iso-alpha acids that contribute bitterness to the final product.
Common alpha acids include:
- humulone => degrades to cis- and trans-isohumulone during the boil;
Iso-alpha acids have an anti-bacterial effect on Gram-positive bacteria, but no effect on Gram-negative bacteria.
A good read for isomerization rates is this paper: http://www.ipcbee.com/vol41/003-ICEBB2012-E005.pdf