Probing the Surface Polarity of Poly(α-amino acids) and α-Amino Acid Crystals with Genuine Solvatochromic Dyes

Reichardt's E T (30) and Kamlet−Taft's α (hydrogen bond donor acidity), β (hydrogen bond acceptor basicity), and π* (dipolarity/polarizability) values for various poly(α-amino acids) [−NH−CH*(R)−CO−] n (PAs) and α-amino acid crystals [H 3 N + −CH*(R)−COO−] n (AACs) are presented. The E T (30) values of the solid poly(α-amino acids) are directly measured by UV/vis spectroscopy using Reichardt's dyes 1a and 1b as surface polarity indicators. Kamlet−Taft's α, β, and π* parameters for the various solid PAs and AACs are analyzed by means of Fe(phen) 2 (CN) 2 [ cis -dicyanobis(1,10-phenanthroline)iron(II)] ( 2 ), Michler's Ketone [4,4‘-bis( N, N -dimethylamino)benzophenone] ( 3 ), 4-aminobenzophenone ( 4 ), and an aminobenzofurandione dye ( 5 ) as solvatochromic surface polarity indicators. The surface polarity of AACs is determined by a quite strong hydrogen-bond-donating capacity (α = 0.6−0.96) and moderate dipolarity/polarizability (π* = 0.3−0.7). For the same substituent R, in general for PAs the α value is lower than the π* value, whereas for AACs the result is opposite. Increasing the size of the alkyl substituent R of PAs or AACs decreases the α and β values. It can be shown that the β value of solid PAs is a function of Taft's sterical parameter ( E s ) if the size of R decreases, the β value increases indicating a strong influence of the steric effects of R on the accessibility of the peptide bond to hydrogen-bond-accepting probes. Theoretical E T (30) values for AACs are calculated by applying linear solvation energy relationships using the independently measured α and π* values of the solid acids according to E T (30) = [ E T (30)] o + a α + s π*, because E T (30) values cannot be directly determined for AACs.