Table 2. Site-Specific Infrared Labels Suitable for 2D IR Spectroscopy of Proteins, Including Molar Absorptivity ε_a and Vibrational Lifetime T₁.

Class	Unnatural Amino Acid	Isotopologue	Frequency (cm–1)c	εa (M–1 cm–1)	T1d (ps)	Ref
nitriles	4-cyanophenylalanine	Phe-12C14N	∼2237	∼180–220	∼4.1	(56−58)
		Phe-12C15N	∼2210		∼0.9–2.2	(56), (59)
		Phe-13C14N	∼2184		∼3.4–5.0	(56), (59)
		Phe-13C15Na	∼2156		∼8–11	(56), (58)
	4-cyanoselenophenylalanine	Phe-Se12C14Na	∼2160	∼65	∼76	(59)
	cyanocysteine	Cys-12C14N	∼2150–2180	∼35–120	∼30–60	(60−62)
	5-cyanotryptophan	Trp-12C14N	∼2210–2240	∼160	∼1.4	(63), (64)
azides	azidohomoalanine	Ala-14N3	∼2100–2140	∼300–400	∼1.0	(65)
	4-azidophenylalanine	Phe-14N3b	∼2120–2130	∼500–620	∼0.7	(66), (67)
	4-azidomethylphenylalanine	Phe-CH214N3a	∼2110	∼310–370	N/A	(67)

^aNotably more expensive than other UAAs on this table and/or requires considerable synthesis.

^bPhotoreactive with UVC light.

^cBold entries: May require purging of CO₂ to avoid chromatic dispersion.

^dBold entries: Short vibrational lifetime limits spectral diffusion measurements.