Dipole moments of random coil polypeptides

Abstract

The Rotational Isomeric States model is applied to calculate dipole moments of polypeptides of the twenty natural α-amino acids in the random coil state. Dipole moments of each repeat unit (μ i), are evaluated using a quantum mechanics procedure. Dipole moment ratios ( D x = 〈μ 2〉 xμ i 2, x = number of repeat units ) of homopolypeptides are calculated and extrapolated to x →⇔. With a few exceptions, D ⇔ = 0.36 ± 0.1. Ten actual proteins and three enzymes are also studied; their dipole ratios ( D x ′ =〈 μ〉/ x) range from 7.34 to 10.57 in 10 −59 C 2 m 2 (6.6–9.5 D 2). Diffferences in the values of D x′ are due mainly to the different contributions, μ i, of the amino acid residues contained in each polymer, whereas the sequence of amino acids has a very minor effect.