Table 1. Mean Amide I Frequency Shifts (cm^–1) due to Protein, Water, and K⁺ Ions in the Selectivity Filter and the Total Frequency Shift from These Contributions^a.

configuration	Val76		Gly77
contribution	[W,S2,W,S4]	[S1,W,S3,W]	[W,S2,W,S4]	[S1,W,S3,W]
protein	86.2	57.5	65.0	82.7
water	–8.25	10.7	0.47	–11.0
K+ ions	–109	–90.2	–90.9	–103.6
total frequency shift	–31.1	–22.0	–25.4	–31.9
peak frequency	1587	1596	1592	1586
experimental peak frequency	1590		1585

^aThe frequency shifts due to protein are the sum of the nearest neighboring frequency shifts and the shifts due to electrostatic contributions from all atoms constituting the local chemical environment of the protein (excluding the nearest neighbors). Peak frequencies are determined by adding the frequency shifts to the ¹³C¹⁸O-labeled amide I frequency with no external field.⁷⁰