Molar-mass analysis of dendrigraft poly(L-lysine) (DGL) polyelectrolytes by SEC-MALLS: The "cornerstone" refractive index increment

Abstract

Dendrigraft poly(L-lysine) (DGL) polyelectrolytes, obtained by iterative polycondensation of N-trifluoroacetyl-L-lysine-N-carboxyanhydride, constitute very promising candidates in many biomedical applications. In order to get a better understanding of their structure-property relationships in these applications, their absolute average molecular weights have to be accurately measured. Size-exclusion chromatography coupled to a multi-angle laser-lightscattering detector (SEC-MALLS) is known to be the most appropriate analytical tool. These measurements require the determination of the refractive index increment, d n/dc, of these highly branched polycationic macromolecules in aqueous solution. This optical property has to be measured in the same aqueous conditions as SEC-MALLS eluents. Consequently, data are determined and discussed as a function of different aqueous SEC-MALLS eluents, as well as different counter-ions of the many ammonium groups of DGL (generation 3, DGL-3, used as a model herein). The resulting number-average molecular weights, M¯n, are found to be very dissimilar when the measured d n/dc values are directly considered. In contrast, very close Mn values are obtained (average M¯n = 18 700, standard error of 1110 g mol-1) with a low coefficient of variation for such data (ca. 6% for six analyses), when the d n/dc are corrected by the exact lysine amount (measured by the total Kjeldahl nitrogen method).