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 monolayers at the nitrogen?water interface has been measured using Langmuir trough and HPLC techniques. In neat monolayers, at subphase pH less than 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, spectroscopically similar to the parent compound. This latter process appears to involve alteration at the pentanone ring peripheral to the porphyrin structure. For mixed monolayer studies, four oleoyl-bearing lipids with headgroups differing in charge have been used to explore the influence of such headgroups on these two pH dependent processes. Consistent with a mechanism by which a negatively charged headgroup may enhance H₃O⁺ concentration in the monolayer region, it has been shown that l-α-dioleoylphosphatidyl-dl-glycerol markedly increases pheophytinization while eliminating the formation of Chl?a?_hyd. By contrast, positively charged dioleoyl-N-(3-trimethylammoniopropyl)carbamoylglycerol serves to generate formation of Chl?a?_hyd with no production of Phe?a. In neutral but polar 1,2-dioleoyl-sn-glycerol monolayers, product yields were observed that are consistent with the larger acidity in the region of Chl?a. Finally, l-α-dioleoylphosphatidylcholine, which bears a zwitterionic headgroup, significantly enhanced apparent Chl?a hydrolysis. These results are interpreted in terms of alterations by the host lipid in pH local to Chl?a.