Table 1:

Calculated pK_a’s of blocked model compounds and alanine pentapeptides in solution

Residue	Compounda	Peptidea	Thurlkillb	Nozakic	Castanẽdad
Asp	3.7±0.07	3.8±0.05	3.67±0.04	4.0	3.90±0.01
Glu	4.2±0.11	4.1±0.05	4.25±0.05	4.4	4.36±0.01
His	6.4±0.03	6.4±0.03	6.54±0.04	6.3	n/d
HID	7.0±0.04	7.0±0.05	n/d	6.92e	n/d
HIE	6.5±0.05	6.5 0.02	n/d	6.53e	n/d

^aError bars of the calculated pK_a’s are the standard deviations based on five independent sets of REX titration runs, each lasting 1 ns per replica. Blocked model compound refers to Ac- X-NH₂, while pentapeptide refers to Ac-Ala-Ala-X-Ala-Ala-NH₂, where X is the titratable residue Asp, Glu or His.

^bpK_a’s of pentapeptides in 0.1 M salt solution determined by Thurlkill et al. using NMR.⁴⁴ These pK_a’s were used as the model pK_a’s in our simulations.

^cpK_a’s of blocked amino acids given by Nozaki and Tanford⁴⁶ using potentiometric titration. Ionic strength is unclear.

^dpK_a’s of tripeptides Ac-Ala-X-Ala-NH₂ in 0.1 M salt solution determined by Castañeda et al. using NMR.³⁹

^eHID refers to the titration HID ⇌ HIP. HIE refers to the titration HIP ⇌ HIE.

^epK_a’s of model compounds measured by Tanokura using NMR titration.⁴⁷