STABILITY OF CHLOROPHYLL-A AT THE GAS-WATER INTERFACE IN PURE AND MIXED MONOLAYERS - AN EVALUATION OF INTERFACIAL PH

The stability of chlorophyll a, Chl a, in neat and mixed lipid monolay ers at the nitrogen-water interface has been measured using Langmuir t rough and HPLC techniques. In neat monolayers, at subphase pH less tha n 8, Chl a degrades rapidly to produce pheophytin a, Phe a. Above this pH, Chl a appears to hydrolyze, giving a new product, Chl a (hyd), sp ectroscopically similar to the parent compound. This latter process ap pears to involve alteration at the pentanone ring peripheral to the po rphyrin structure. For mixed monolayer studies, four oleoyl-bearing li pids with headgroups differing in charge have been used to explore the influence of such headgroups on these two pH dependent processes. Con sistent with a mechanism by which a negatively charged headgroup may e nhance H3O+ concentration in the monolayer region, it has been shown t hat L-alpha-dioleoylphosphatidyl-DL-glycerol markedly increases pheoph ytinization while eliminating the formation of Chl a (hyd). By contras t, positively charged eoyl-N-(3-trimethylammoniopropyl)carbamoylglycer ol serves to generate formation of Chl a (hyd) with no production of P he a. In neutral but polar 1,2-dioleoyl-sn-glycerol monolayers, produc t yields were observed that are consistent with the larger acidity in the region of Chl a. Finally, L-alpha-dioleoylphosphatidylcholine, whi ch bears a zwitterionic headgroup, significantly enhanced apparent Chl a hydrolysis. These results are interpreted in terms of alterations b y the host lipid in pH local to Chl a.