The surface activity and film-forming properties of 1,2-dipalmitoyl-sn-glycero-3-phosphoglycolipoate (DPPGL) with a pendant five-member ring containing a disulfide was studied. There was a clear difference in the isotherm characteristics and also in the stability of the phases present on the monolayer both above and below the crystal melt temperature (T-m) of 22.9 degreesC. Although the polar lipoic acid headgroup of DPPGL affected the molecular packing throughout the compression, it did not change the final mean molecular area of a compressed monolayer compared with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine. The DPPGL formed a homogeneous self-assembled monolayer on a gold substrate, which was the first evidence of a successful cleavage of the disulfide bond. A catalyst (dithiothreitol [DTT]) in a basic bulk solution of ethanol could also accelerate the cleavage of the disulfide bond. However, no DTT was needed to catalyze a dimerization process that took place in a Langmuir monolayer on a basic subphase.