Generation of tunneling spectra was completed through the procedure described byTurin26,43 and outlined within the SupplementaryMaterial. This procedure is an adaptation of earlier inelastic tunnelingliterature44,45 and similarly uses arbitrary units (a.u.)for the tunneling intensity. Our spectral procedure was validated by comparison ofthe spectra of the formate ion, which is prevalent throughout experimental andtheoretical literature in IETS43. These a.u. are proportionalto the conductance enhancement, as well as representative of an enhancement to themagnitude of electrostatic charge-dipole interaction an electron experiences duringtunneling. Necessary information for implementing the calculations - outlined in theSupplementary Material - was collected through quantumchemical calculations. Computations were performed using Density Functional Theoryat the 6-311G basis-level, utilizing the B3LYP functional which serves well fororganic hydrocarbons; in contrast to similar previous works26,43.Expanded pseudopotential correlation consistent 5-zeta basis was used for largeatoms where necessary46. DFT was employed both due to its highaccuracy in transition dipole frequencies and due to a desire to avoid encroachingerrors associated with dissimilarities between analyte and parameter molecules insemi-empirical methods. Initial applications of Hartree-Fock theory displayed thecharacteristic 0.8 factor shift to the vibrational frequencies, which is less thandesirable for ease of interpretation of the vibrational spectra. Vibrationalcalculations utilize reduced modal displacements, μ; proportionalto the Cartesian displacement through the modal center-of-mass factor, .This factor arises due to use ofcenter-of-mass coordinates within the classical theory after application of theharmonic approximation during calculations of the normal modes. Natural bond ordercalculations were performed to yield the partial charges, q i attributed to each atom constituting the agonist. Scaled Kronecker delta functionsare plotted at the on-resonance absorbance frequency of the mode; these functionswere convolved with Gaussian functions possessing a conservative FWHM of25 cm−1, representing a very narrow thermaldistribution. The spectral width was introduced to allow for peak additions, while25 cm−1 was selected to avoid overestimations of peak breadth.

Assessment of vibrational bands from the 5-HT 2A agonists whichcould facilitate the inelastic transfer of electrons within the protein environmentis of primary import. Agonists of a particular protein would share a single spectralfeature associated with the inelastic transfer, as the same amino acid side-chainswould be responsible for the electron donation and acceptance for a specificprotein. Tunneling spectrum of several selected 5-HT 2A agonistshave been generated. LSD, was selected as it possesses a high potency as well asactivity at this particular serotonin receptor within the cortical interneurons47. DOI (2,5-dimethoxy-4-Iodo-amphetamine) was selected due to its highselective for the 2A-subtype receptor48. The remaining selectedmolecules are members of the 2C-X (4-X-2,5-dimethoxyphenethylamine) class ofpsychedelic phenethylamines. All compounds selected are known hallucinogens49,50,51 some first characterized by Alexander Shulgin in thecompendia works PiHKAL and TiHKAL52,53.

Figure 1 shows the tunneling spectra of select agonists (abovethe axis). The selection of candidate peaks, possibly responsible for facilitatinginelastic transfer, was performed using the Spectral Similarity Index (SI), similarto that used for comparison of mass spectra54. The SI was taken overthe entire spectral region and repeated for a scan of the local regions with anoverlapping step of 500 cm−1 with a width of1000 cm−1. The SI is given by:

Figure 1 The tunneling spectrum of several known 5-HT 2A agonists aswell as the square of the tunneling PDF reflected below the energy axes; thesquare is used to highlight major spectral aspects. The Spectral Similarity Index of each plot is given in the Supplemental Materials over several ranges and regions, notingthat these similarity indices allude to good spectral agreement with thereference spectrum, LSD. More detailed information is provided within theSupplemental Material. Full size image

Where, within the above equation, N is the normalization constant (thenumerator performed for spectra b and a = b); b i isthe value of the spectra being analyzed at discrete location i while ais a reference spectra. Being the most potent agonist, LSD was selected as thereference spectra for our SI calculations. The SIs, both global and local scans,associated with each of the tunneling spectra can be found within a table providedin the Supplementary Material. To highlight major aspects of thetunneling PDF, we squared the function, exaggerating aspects which exhibit largetunneling amplitudes within the spectra (Figure 1 reflectedbelow energy axis). The only broadly shared spectral aspects were those at1500 cm−1. For a more thorough discussionof the spectral aspects, isotopic effects at functional groups and density of statesfor these systems, please see the provided SupplementaryMaterials.

The integral of the tunneling probability density was taken around the 1500± 35 cm−1 region and compared toknown EC50 data for compounds shown to be agonists of the 5-HT 2A receptor. The EC50 used within this paper is taken from Parrish et al42 who determined the elevated levels of phosphoinositides associated with theactivation of the 5-HT 2A receptors of the human A20 cell lineemployed within the experiement across a collection of compounds from thephenethylamine (PEA, or 2C-X) and phenylisopropylamine (PIA, or DOX) classes. Thedetection procedure was replicated from Kurrasch-Orbaugh et al.55.The selection of data used for our comparison is provided within our Supplemental Material. Data was selected from a single source, helping toassure uniformity in both collection and determination, while selecting andcomparing members of specific families of molecules (i.e. PIA/PEAs) helps tominimize drastic changes in their docking configurations which may affect potency.The similarities granted by selecting compounds from families may not allow forsubstantive prediction in the relative potency, beyond docking affinities; this canbe seen by noting that two PIAs, DOI and DOB, have similar docking affinities at the5-HT 2A receptor56, while possessing greatdifferences in their potency57.

The effective concentrations of several phenethylamines were taken from42 and compared to the local integrals of the tunneling PDF. Thiscomparison exposes a possible correlation to the inverse of the EC50 data, taken tobe representative of the potency for each species at the receptor. Results for the1500 cm−1 region are shown in Figures 2 and 3 for the DOX class and 2C-Xclass molecules computed, respectively. Figures 2a and 3a give the tunneling spectra for each molecule, Fig. 2b and 3b compare the integral values to theknown EC50s.

Figure 2 (a) The tunneling spectra of several DOX class agonists as well as theirmolecular structures. (b) The inverse of the median effective concentration forthe DOX class agonists plotted against the tunneling probability within theregion at 1500 ± 35 cm−1. Thetrend of tunneling intensity follows roughly the trend of theagonist’s potency at the 5-HT 2A receptor. Full size image

Figure 3 (a) The tunneling spectra of several 2C-X class agonists as well as theirmolecular structures. (b) The inverse of the median effective concentration forthe 2C-X class agonists plotted against the tunneling probability within theregion at 1500 ± 35 cm−1. Thetrend of tunneling intensity follows roughly the trend of theagonist’s potency at the 5-HT 2A receptor. Full size image

As inelastic tunneling facilitated by a charge-dipole interaction is a highly localprocess where the interaction potential falls-off as r−3for non-parallel displacements. Modes not local to the electron donor/acceptor siteswill not maximally contribute to the electron transfer, which is proposed to beresponsible for protein activation. Particular modes in 2C-T-2 and in Aleph-2 residewithin the thioether (roughly 5 angstrom from the ring system); due to thenon-locality of these oscillators, tunneling probability should be examined afterhaving removed their contributions from the spectra. Figures2a and 3a present the tunneling spectrum of 2C-T-2and Aleph-2 disregarding these contributions. After correction for non-localmotions, the integrals are in good qualitative agreement with the inverse EC50. Thispreliminary information supports a possible quantum mechanical origin for theactivation of sensory proteins. We shall propose a possible experimental validationof the theory within the following section.