The Professor Obligado serial was a back-up storyline published infrequently throughout the first two years of Tales of the TMNT Volume 2. The storyline was written by Steve Murphy, although he left it on a cliffhanger.
As the chapters popped up randomly throughout those first 25 issues, I've collected them here in an index for the sake of convenience.
Prologue:
*"The Grape"
*"The Raisin'"
*"The Risen"
Obligado's adventures:
*"The Question"
*"First Mud"
*"Apocalypse Vow"
*"Altered Fates"
*"The Doors of Deception"