Stability of Chlorophyll <i>a</i> at the Gas−Water Interface in Pure and Mixed Monolayers. An Evaluation of Interfacial pH

Abstract

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 H 3 O + 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 .