Table 1. Summary of 3°γ (HBN) Effect in VSr, HHr, Psr, and 8-17dz^a.

	productive hydrogen bonding (%)
system	M2+	no M2+	3°γ (HBN)
VSr	86	14	6.1
HHr	95	45	2.1
Psr	49	40	1.2
Psr*	78	40	2.0
8-17dz	86	8	10.8

	productive hydrogen bonding (%)
	oxo	RP thio	inv. thio
VSr DM	23	88	3.8
expt			6

^aPercentages of productive hydrogen bonding and corresponding 3°γ (HBN) factors for VSr, HHr, Psr, and 8-17dz (Figure 4). For Psr, the divalent metal ion is outer-sphere bound to the NPO as suggested by experiment²⁰ and theory;⁹ however, for discussion we also consider an inner-sphere binding mode indicated as “Psr*” (Figure S3). Also shown are results in Figure 3 for the inverse thio effect for oxo and R_P thio substrates interpreted as the 3°γ (HBN) for the VSr G638I/A756(3cP) double mutant (“VSr DM”, Figure S2) and compared with experiment (expt).