Amino acids molar refractions

The molar refractions of the amino acids were determined by measurements on their aqueous solutions and the expanded Lorenz-Lorentz equation. The refractive indices of a number of proteins were calculated from their amino acid compositions and the values for the refraction of the amino acid residues. These calculated results are in good agreement with those experimentally determined, demonstrating that refractive index is a unique characteristic of a protein. A comparison of the refractive index of heat denatured /3-lactoglobulin with the native protein demonstrated that changes in structure produced a small change in refractive index, not associated with a change in volume. 

Calculations. Refractive Indices and Molar Refractions of Solutions of Amino Acids and Proteins. The mean refractive indices of the amino acids and proteins were calculated by means of the following expanded Lorenz-Lorentz equation as given by Doty and Geiduschek (11) ... 

Table I. Refractive Indices, Molar Refractions of Amino Acids,...

Amino Acids. The values for the refractive indices and molar refractions of the amino acids calculated from the refractive indices by Lorenz-Lorentz Equations 1 and 2 are recorded in Table I. Values for molar refractions of the aliphatic amino acids are in good agreement with values calculated from atomic refraction factors. However, the molar refractions of tryptophan, tyrosine, phenylalanine, and histidine are larger than those calculated from atomic refraction factors and larger than might be expected from their comparative specific volumes. 

The importance of specific volume or density in determining refractive index is apparent in Equations 1 and 2, which are used in calculating refractive index and molar refraction. This inverse relationship between specific volume and refractive index is illustrated in Table n (cf. columns 2 and 4). The necessity of obtaining an accurate value for the specific volume of a protein in order to obtain agreement between its refractive index calculated from the amino acid composition and the determined value can be illustrated in the case of ribonu-... 

Refractive Indices of Peptides. To determine the effect of peptide formation on refractive index, the refractive indices of several peptides were determined (Table HI). The average molar refraction of water produced in peptide formation can be estimated, empirically, by subtracting the observed molar refraction of the peptide from the sum of molar refractions of its constituent amino acids. 

Table IV. Comparison of Molar Refractions of Amino Acids and...

Effect of Ionization on the Refractive Index and Molar Refraction of Amino Acids and Proteins. Since the electrostriction produced by an amino acid does not affect its molar refraction, the ionization of an amino acid might be expected to produce no significant change in molar refraction. Table V indicates that this is the case, provided the large change in the volume of the amino acid as a result of ionization, found by Kauzmann, Bodanszky, and Rasper (23), is used in calculating molar refraction. The refractive index of an equivalent concentration of hy-... 

An application of continuum solvation calculations that has not been extensively studied is the effect of temperature. A straightforward way to determine the solvation free energy at different temperatures is to use the known temperature dependence of the solvent properties (dielectric constant, ionization potential, refractive index, and density of the solvent) and do an ab initio solvation calculation at each temperature. Elcock and McCammon (1997) studied the solvation of amino acids in water from 5 to 100°C and found that the scale factor a should increase with temperature to describe correctly the temperature dependence of the solvation free energy. Tawa and Pratt (1995) examined the equilibrium ionization of liquid water and drew similar conclusions. An alternative way to study temperature effect is through the enthalpy of solvation. The temperature dependence of is related to the partial molar excess enthalpy at infinite dilution,... 

While the equations look the same at a first glance, some striking differences can be seen on a closer inspection. First, the vertebrate, but not the bacterial DHFR equations contain an electronic parameter in addition to lipophilicity and molar refractivity terms. Second, in the case of L. casei (eq. 137) the 5-position of the benzyl group does not at all contribute to biological activities. An explanation could be derived by a comparison of the 3D structure of L. casei DHFR with the E. coli DHFR structure. The active sites of both enzymes are more or less identical in the geometries of the protein backbone and the amino acid side chains. However, there is one significant difference E. coli DHFR contains a methionine side chain in the area where the 5-substituents bind, while there is a relatively rigid leucine side chain in the L. casei DHFR which obviously interferes with the 5-substituents. Therefore, the active site of L. casei DHFR is sterically more constrained and the positive lipophilicity and polarizability contributions of the 5-substituents are counterbalanced by their steric hindrance [432, 682]. 

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