Syntheses and monolayer properties of vitamin B-12 derivatives with seven alkyl chains

The heptapropyl and heptaoctyl esters of vitamin B-12 derivatives with a Co (II) or Co(III) center have been synthesized and their monolayer properties have been investigated. The effect of the esters' chain length on stabilit y and orientation of the vitamin B-12 in a lipid monolayer is discussed on the basis of their surface pressure molecular area (pi-A) isotherms. Their isotherms significantly depend on the length of the side chains regardless of the oxidation state of the center cobalt. The isotherm of the heptaoctyl derivatives has a relatively steep shape with a molecular area of ca. 3 nm (2) which is fairly close to the molecular area estimated by molecular mode ling for face-on orientation. In contrast, the heptapropyl derivatives show ed unstable characteristics with collapsing even at low pressures. The hept aoctyl derivatives can also be stably incorporated in the DPPC matrix monol ayer, while the heptapropyl derivatives were squeezed out from the same mat rix upon compression at high pressures. The effect of coordination of the c enter cobalt to the matrix lipid was also investigated. Mixing of the hepta propyl Co(II) derivatives with a lysine-functionalized lipid significantly improved the preservation of the vitamin B-12 function in the monolayer due to coordination of the lysine amino group to the open axial position of Co (II). The obtained results in this study indicate that stable accommodation of the vitamin B-12 functions in the lipid assembly can be achieved by the introduction of a long chain and/or coordination to the matrix lipid. The present study is the first example of a monolayer system of vitamin B-12 mi mics with a core structurally identical to the naturally occurring one. (C) 2000 Elsevier Science B.V. All rights reserved.