sentence: Pairs b or y ions ( marked * y3 / y4 and b7 / b8 for K1299 , y283 / y293 for K1310 , y6 / y7 and b4 / b5 for K1905 ) are indicative of acetylation as these fragmentation ions contain a 42 Da mass shift from the natural lysine ( Fig. 6g ) .