6 5 I H I G H L I G H T S 2 0 2 2
PRINCIPAL PUBLICATION AND AUTHORS
Influence of the porphyrin structure and linker length on the interfacial behavior of phospholipid- porphyrin conjugates, L.-G. Bronstein (a), P. Cressey (a), W. Abuillan (b), O. Konovalov (c), M. Jankowski (c), V. Rosilio (a), A. Makky (a), J. Colloid Interface Sci. 611, 441-450 (2022); https:/doi.org/10.1016/j.jcis.2021.12.114 (a) Institut Galien Paris-Saclay (IGPS), Châtenay-Malabry (France) (b) Physical Chemistry of Biosystems, Heidelberg (Germany) (c) ESRF
 J.F. Lovell et al., Nat. Mater. 10, 324-332 (2011).  L.-G. Bronstein et al., Nanoscale 14, 7387-7407 (2022).  P. Cressey et al., Int. J. Pharm. 623, 121915 (2022).  K.A. Carter et al., Nat. Commun. 5, 3546 (2014).
how these compounds interact with each other and what conformation they adopt at the interface, which, in turn, may give answers about the impact of the chemical structure on their assembling properties and, thus, their optical properties. To do so, the 2D phase behaviour of six newly synthesised Pl-Por conjugates (PhxLPC and PyrxLPC) (Figure 54a), bearing either pheophorbide-a (Pheo-a) or pyropheophorbide-a (Pyro-a), was assessed using Langmuir film balance coupled to various physical techniques.
Compared to unconjugated porphyrins (Pheo-a and Pyro-a), both PhxLPC and PyrxLPC compounds formed more expanded monolayers (Figure 54b). In addition, both conjugates exhibited a monotonic shift toward larger molecular areas as the length of the spacer separating the polar headgroup from the porphyrin core increased. However, while the PhxLPC compounds displayed a monotonic increase in the surface pressure during monolayer compression with expanded-like profile, a slight plateau-like region appeared for Pyr3LPC and Pyr4LPC isotherms.
By means of Brewster angle microscopy (BAM) and atomic force microscopy (AFM) on Langmuir-Blodgett transferred films (Figure 54c), it was demonstrated that the structure of the porphyrin significantly impacts the lateral packing of the conjugates and controls whether they can form domains or not. In addition, the results
revealed that changing the number of carbon atoms in the linker separating the polar headgroup from the porphyrin core controls the shape and structure of the organised domains of the conjugates (Figure 54c). It was expected from the chemical structures of the Pl-Por conjugates that porphyrins grafted to a longer chain should align with the sn1 C16 alkyl chain, improving the lateral packing properties due to the reduction of the length mismatch between the two chains . In order to assess this hypothesis, specular X-ray reflectivity (XRR) measurements were performed at beamline ID10 on the different monolayers compressed at a surface pressure of 30 mN/m (Figure 55a). XRR makes it possible to determine the fine structures perpendicular to the plane of the monolayers.
Contrary to expectations, the electron density profiles (r) along the z-axis reconstructed from the best fit results of XRR curves (Figures 55b and c), did not show evidence of an alignment between the sn1 and sn2 chains independently of the chain length and porphyrin moiety. Indeed, porphyrin molecules remain at the air/water interface when conjugated to the modified phospholipids. This induces an increase in the electron density in both polar and hydrocarbon regions. Such results shed light on the importance of p-p interactions between porphyrins and p-cations between the porphyrin and the choline group in controlling the organisation of the Pl-Por conjugates at the air/water interface.
Fig. 55: a) Schematic illustration of the XRR experiment. b) and (c) are the reconstructed electron density profiles (e refers to the number of electrons) along the Z-axis for DPPC, PhxLPC and PyrxLPC compounds.