Persuade or inspire an individual or group to reneg on their commitment to an original purpose in favor of a counterpurpose. Formerly used in conjunction with loyalty to one's country or political party/spying (i.e. 'turncoat'), it has come to be used more often with a misunderstanding of homosexuality (i.e. 'turn gay'), or disloyalty to affiliates in an illegal enterprise (drugs, organized crime, white collar crime).
"Travis Fimmel is so pretty, he could turn Hugh Hefner." / "Madonna wasn't really lesbian, but Sandra Bernhard did her best to turn her."
"Once vice got the bag man to turn, they finally had enough evidence to put away the head of the crime ring."