An analysis of a structural difference between beef and pork insulin detected by differences in the exchange rates of amide hydrogens as measured by infrared spectroscopy.

Abstract

The exchange of amide hydrogens of pork and beef insulin with solvent deuterium has been followed for 120 min by infrared spectroscopy. Solutions made by dissolving carefully dried insulin in dilute 2HCl were maintained at 40 °C, and pH of 2.35 throughout the period of observation. The number of unexchanged amide hydrogens at a given time has been calculated from the absorbance at 1541 cm−1. A computer programme has been used to calculate the best two-term exponential to fit these data. This gives the number of hydrogens and their rate constant for exchange for each of two different groups. The number of hydrogens which exchanged too fast to be observed has been calculated from the difference between the total number of amide hydrogens (48) and the sum of those found from the two-term exponential. Pork insulin appears to have 9 amide hydrogens in the slowest exchanging group, 14 or 15 in the intermediate group and 25 or 24 in the fast group. Beef insulin appears to have 8 or 9 in the slowest group and their rate of exchange is higher than for the corresponding group in pork insulin, 11–13 in the intermediate group and 29–26in the fast group. Attempts have been made to assign each amide hydrogen of pork insulin to a particular group.