Summary : The NH2 group and the adjacent ring nitrogen of adenosine and cytidine react with a-acetylenic esters by addition across the triple bond and formation of a lactam with the ester group. We have recently shown that chlorotetrolic (4-chloro-2-butynoic) esters', C1CH2-CzC-COOR, react under mild conditions with nucleophiles analogous to those present in proteins2. Tests with small model molecules and with proteins have demonstrated the ...