These all share a common B-lactam ring. The ring is very strained and the bond between the carbonyl and the nitrogen in the p-lactam ring is very labile and hence makes the molecule reactive. The r-group substitute of the penicillin nucleus can be changed to give the molecule different antibacterial properties. The two naturally occurring penicillins from Penicillium notatum are enicillin G,[Benzyl penicillin,R=CH] and Penicillin V, PHenoxymethyl penicillin,R=CH,O(C6H6)1 X H R NH H COOHThese all share a common β-lactam ring. The ring is very strained and the bond between the carbonyl and the nitrogen in the β-lactam ring is very labile and hence makes the molecule reactive. The R-group substitute of the penicillin nucleus can be changed to give the molecule different antibacterial properties. The two naturally occurring penicillins from Penicillium notatum are Penicillin G, [Benzyl penicillin, R = C6H6 ] and Penicillin V, [Phenoxymethyl penicillin, R = CH2O(C6H6 )] N S O H COOH X H R NH O