Effect of alkylation with different sized substituents on the conformation of ovomucoid, lysozyme and ovotransferrin

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Abstract
Conformational changes induced in ovomucoid, lysozyme and ovotransferrin on reductive addition of different sized substituents have been studied employing differential scanning calorimetry (DSC) and circular dichroic spectroscopy (CD). The thermograms obtained by DSC revealed that extensive introduction of methyl, isopropyl, cyclopentyl, cyclohexyl, benzyl or n-butyl groups has a detrimental effect on thermal stability (enthalpy of denaturation); the effect generally increases with the size of the substituent. Circular dichroic spectra were affected only to a very limited extent by the modifications, near-u.v. spectra remaining much the same while far-u.v. spectra displayed minor changes. The general conclusion drawn is that the modifications had only limited effects on the conformation of the proteins while, nonetheless, perturbing (or breaking) long-range intramolecular interactions so as to destabilize the structure. Derivatization of lysozyme and ovotransferrin with some of the larger groups has been reported to result in spontaneous precipitation of the proteins [Fretheim, K., Iwai, S. & Feeney, R.E. (1979) Int. J. Peptide Protein Res. 14, 451–456]. The present investigation indicates that precipitation was caused by (partial) denaturation (and ensuing aggregation) as a consequence of modification.