1. The reaction between avidin and biotin was found to be exothermic, DeltaH being -20.3kcal./mole of biotin bound. The corresponding value of DeltaH for streptavidin was -23kcal./mole. 2. The heat evolved was independent of the pH (between 5 and 9), of the buffer (borate or ammonia) and of the fractional saturation of the avidin with biotin. 3. The entropy change for the reaction was zero, and it is suggested that the entropy increase to be expected from hydrophobic interactions was counterbalanced by a decrease in entropy accompanying the formation of buried hydrogen bonds. 4. Modification of the potential hydrogen-bonding sites of the imidazolidone ring led to a decreased heat output and a positive entropy of reaction.

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