Glutamic acid–hydrogen phosphate hydrogen bonds. Proton polarizability and proton transfer as a function of the cations present and of the degree of hydration: infrared investigations

Abstract

Polyglutamic acid (L-glu)_n–hydrogen phosphate (P_i) systems have been studied by i.r. spectroscopy dry and hydrated (75% relative humidity) as a function of the P_i:glu residue ratio and as a function of the type of cations present. In all (L-glu)_n–P_i systems [graphic omitted] hydrogen bonds with a high degree of proton polarizability are formed. In the Li⁺ system the weight of limiting structure (II) is < 0.05, whereas for the K⁺ system it is 0.95. This difference is explained on the basis of the different electrostatic field strength, and thus the different polarizing power, of these cations with regard to P_i.

The Na⁺ systems show intermediate behaviour; at P_i:glu = 1 : 3 the system resembles the Li⁺ system, but since the polarizing power of Na⁺ is less than that of Li⁺, the proton polarizability of the COH ctdot; ^–OP ⇌ CO^–‥ HOP bond is much larger than with the Li⁺ system. Proton-limiting structure (II), however, has a very low weight. At higher P_i:glu the system resembles the K⁺ system. In the P_i:glu = 1.6 : 1 system these hydrogen bonds are completely formed and 75% of the glu protons have transferred to P_i. The proton polarizability of these hydrogen bonds increases in the series Li⁺ < Na⁺ < K⁺. The influence of hydration and the secondary structure of (L-glu)_n are discussed. All results taken together show that systems with glu residue–P_i hydrogen bonds can easily be controlled by the nature of cations present and by the degree of hydration of the systems.