Langmuir-Blodgett films prepared from purple membranes of the D96N mutant and two of its chromophore analogs (4-keto-retinal and 9-demethyl-retinal) were characterized for M-state kinetics and photocurrent production. These studies indicated that unlike the wild-type (WT) and its identical chromophore analogs, the photocurrent transients were unusual, showing proton release at pH 3.0. Several possible explanations for these results are considered. (C) 1997 Elsevier Science S.A.