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. Author manuscript; available in PMC: 2010 Aug 19.
Published in final edited form as: J Am Chem Soc. 2009 Aug 19;131(32):11581–11589. doi: 10.1021/ja904318w

Figure 5.

Figure 5

pKa determination of conserved histidines in spliced intein (A) and in a precursor with an N-extein, NI (B). (A) Plot of the ring 15N chemical shift versus pH for conserved histidines, B-block H73, F-block H429 in ΔΔIhh-SM. The squares and circles represent the chemical shifts of Nδ1 and Nε2, respectively. The pKa of H439 in the spliced intein is shown in Table I, along with pKas of other histidines in different constructs. (B) HMQC spectrum for intein precursor NI at pH 7.1. The average chemical shift of of H73 Nδ1 and Nε2 is 188 ppm, midway between the values expected for neutral (204.3 ppm) and charged histidines (176 ppm), suggesting that H73 has a pKa near 7 in the precursor protein.