Colorimetric analysis of surface reactive amino groups on poly(lactic acid- co-lysine):poly(lactic acid) blends

Abstract

The quantification of functional amino (NH 2) groups on poly(lactic acid- co-lysine):(poly( l-lactic acid (PLAL:PLA) blends was performed using a colorimetric assay based on the reaction of sulphosuccinimidyl-4-O-(4,4′-dimethoxytrityl)-butyrate (sulpho-SDTB) with primary amino groups. The colorimetric assay was used to assess the available reactive sites for coupling of biologically active species to PLAL. Blends were created that contained from 10 to 70 wt% poly(lactic acid- co-lysine). Bulk lysine contents within the blends were determined by amino acid analysis and ranged from 9.1 μmol g −1 to 52.9 μmol g −1 for blends created using PLA of 100 000g mol −1 molecular weight. Surface amino group concentrations on the same set of blends ranged from 0.23 to 1.45 nmol cm −2. Similar surface amino groups concentrations were measured on blends using 50 000, 200 000 and 300 000g mol −1 poly(lactic acid). Non-specific interactions of the colorimetric assay reagents with the PLAL-containing blends were measured on blends prepared from ε-amino protected PLAL and 100 000g mol −1 PLA. The presence of amino groups within the top 50 Å was confirmed by X-ray photoelectron spectroscopy.