East germans were Ost Goths (lived on territory of today Ukraine), West Goths (lived on territory of today Romania and Moldova), Wandals and Burgunds (lived on territory of today Poland). Slavs at those days lived on territory of today Ukraine as well and were under strong Gothic political and cultural influence. There are many gothic words in slavic language borrowed at those times: polk, bukva, tyn, skot etc. Then all east germanic tribes migrated to the west: Ost Goths to Italia, West Goths to Spain, Burgunds to France, Wandals to North Africa. Their former lands were occupied by Slavs. Probably some amont of east germans which did not migrate was assimilated by Slavs.