Hydrogen bonding phospholipid monolayers

The applications of PM-IRRAS also include fatty acids, phospholipids, and protein conformations. Desbat and co-workers reported on the variation of the dissociation of a Langmuir monolayer of arachidic acid at the air-water interface as a function of the subphase pH and for several cations (Cd2+, Ca2 +, Mg2 +, and Na+) with the help of the PM-IRRAS method [92]. Fig. 14 shows the PM-IRRAS spectra of Langmuir monolayer of deuterated arachidic acid in the presence of CdCb as a function of the subphase pH. At low subphase pH (pH = 3.5), the spectrum only presents absorption bands related to the acid form, i.e., the C = O stretching vibration (v(C = O)) and the OH bending (<5(0-H)) located at 1720 and 1270 cm respectively. The frequency position of the v(C = O) is characteristic of a hydrogen-bonded carbonyl group. As the subphase pH is increased, the arachidic acid is progressively deprotonated to... 

A consideration often overlooked in BAM studies is the possible influence of the compression rate on the domain structures. In the case of A-acylamino acid monolayers, the associations due to amide-amide hydrogen bonding are very strong and promote rapid domain growth and also make it unlikely that relaxation to an equilibrium domain shape can occur on any realistic experimental timeframe. Domain shape relaxation kinetics are noted to be dependent on the strength of intermolecular forces for example, dendritic condensed-phase domains formed in a phospholipid monolayer required 5 h to relax to equilibrium shapes and, for the phospholipid DMPE, compression rates as slow as 0.2 per molecule per minute were needed to observe equilibrium domain shapes. Examination of the variation of domain structure with time after then-formation or with compression rates are not commonly reported however, it is advisable to consider examining these variables when carrying out BAM experiments. 

Besides calcium carbonate and phosphate, only a few minerals have been studied with bio-inspired monolayers. Letellier et al. investigated the formation of calcium oxalate monohydrate at phospholipid monolayers [161]. The authors claim that lattice matching and hydrogen bonding cannot be a dominant factor in the control of crystal orientation. Rather, non-specific electrostatic interactions, similar to those reported for calcium carbonate [162], seem to control the crystallization of calcium oxalate monohydrate. 

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